Developing a bstaley Disco

[rsterne]

One of the most unique ways of tuning the MRod and PRod is the addition of a stack of O-rings to form a buffer between the valve and hammer, called the "bstaley mod" after it's inventor.... It requires a hammer with an adjustable striker extending past the face, such as found in the MRod, PRod, and Challenger.... so the first part of trying this on a Disco is to fit such a hammer.... I chose the one from a Challenger, along with the RVA (rear velocity adjuster aka power adjuster) from a Challenger as well.... You need an adjuster with a hole through the screw so that you can insert a 1/8" allen key to turn the striker to adjust it.... Full CCW provides the maximum hammer stroke before it hits the valve stem, and as you turn the adjuster "in" CW the stroke reduces by 1/28" (0.036") per turn.... Since the end of the stem is fixed (when closed), what actually happens is that the hammer slides back that much, increasing the preload on the hammer spring by the same amount.... However, since the outside face of the hammer is what catches on the sear, the spring length and therefore it's force when cocked stays constant.... These have to some extent a cancelling effect, and the first 3-4 turns adjusting the stroke (in a conventional installation, without the O-rings) seems to have little effect on the velocity.... Once you screw the adjuster out a long ways, drastically reducing the travel, that is no longer the case, and the velocity starts to drop more rapidly.... Here is a photo of the Challenger hammer sitting beside a stock Disco hammer.... Note that I haven't had to cut a notch in the top of the Challenger hammer to clear the 4-48 Disco breech screw because I'm using a 22XX breech with the forward screw location.... In a Disco, the hammer will have to be notched to miss the screw like the Disco hammer is....



Note that the main body of the hammers are the same length, but the striker sticks out 0.080" past the front of the hammer when fully in CCW (as shown).... This means that compared to a Disco hammer, the stroke will be reduced (and the preload increased) by (at least) that amount.... A stock Disco has a hammer travel of 0.58", so when you install a Challenger hammer, that drops to 0.50" (maximum) or less (by 0.036" per turn on the striker).... For lower powered (stock or less) guns that will not be an issue, but it may hamper power tunes, requiring more preload and/or a stiffer hammer spring than would be required if a longer (rather than shorter) stroke was available.... This can be addressed by shortening the valve stem to gain back the lost stroke, and I'll be looking at that much later in this thread as it develops.... There is one #113 O-ring sitting on the front of each hammer.... Note that the Challenger striker, when fully in, comes just flush with the top of a single O-ring (because of the taper on the edge of the hammer).... We will be starting with four of them.... and they are 0.103" thick, so the hammer will make contact with the O-ring stack when the striker (when fully in) is about 0.31" from the back of the valve.... Since a stock Disco valve stem protrudes 0.32" from the back of the valve, that will mean that the valve will only open about 0.010" before the hammer first contacts the O-rings....

At this point, we need a quick review of how unregulated PCPs work.... At the beginning of the shot string, the air pressure is at its highest, and the force holding the valve closed (and closing it once it is open) is at its highest.... Since the hammer strike is (for any given tune) a constant, the valve will get knocked open a tiny amount (the lift), typically about 1/16".... This releases a very small amount of very high pressure air for a very short time (the dwell) to accelerate the pellet.... As the pressure drops, so do the forces resisting the hammer, so the lift increases gradually, and with it the dwell.... The pulse of air gets longer in duration but less in pressure, with the mass of air released remaining relatively constant.... As the pressure continues to drop, the valve opens further and further, and eventually there is not enough pressure to maintain the same mass of air escaping, and the velocity drops, ending the useful shot string, and you have to refill the gun.... At that point, the lift is typically about 1/8", about double what it was when you started.... The velocity in a properly tuned PCP starts out a few percent below the peak, rises, plateaus a bit, and then falls again.... The trick in tuning a PCP is to maintain the velocity within an acceptable Extreme Spread (ES) while getting as many shots as possible.... I usually use 4% as my acceptable ES at 50 yards, narrowing that to only 2% for 100 yards....

I have had a few discussions with bstaley, trying to understand the principles involved, and the basic idea is that as the valve lift increases during the shot string, the hammer, at some point (depending on how you have the stroke adjusted) starts to impact the O-ring buffer.... This reduces the valve lift in the latter part of the shot string.... By adjusting the hammer spring preload and the length of the striker you can govern at what point in the shot string the buffer starts to limit the valve lift, and what the maximum lift at the end of the string is.... Here is a quote from the inventor....

The main thing to understand is that as the pressure in the tank decreases, more and more of the hammer's energy transfers to the buffer and less to opening the poppet further and for a longer duration. So the stiffness of the buffer will definitely have an effect on the resultant shot string....

Screwing the striker in CW (when viewed from the rear of the gun) will increase the valve lift before the hammer contacts the buffer.... causing it to become effective later in the shot string.... If you screw the striker in far enough, the hammer will no longer hit the buffer at all.... and the gun will operate "normally", albeit with much reduced hammer stroke.... This is the first principle of the bstaley tune....

AS YOU SCREW IN THE STRIKER CLOCKWISE, IT INCREASES THE VALVE LIFT, INCREASING THE POWER, AND THE BUFFER BECOMES EFFECTIVE LATER IN THE SHOT STRING IF AT ALL.

Start digesting that while I work on my next post....

Bob

[rsterne]

To continue, I first wondered how much "give" there was in the stack of O-rings.... Did it simply sit there like a rock and stop the hammer completely once it contacted it, or did it indeed act like a "cushion" and gradually slow the hammer by varying degrees depending on the hardness (durometer) of the O-rings, the height of the stack, and how hard the hammer hit it?.... I had to test out bstaley's assertion that the stiffness of the buffer was, in fact, important.... To do that, I used a trick I have done many times before to measure the valve lift in a PCP.... I made up a lightweight indicator that looks like a common nail with the head captured between the hammer spring and the inside of the hammer.... It is made from a piece of 1/8" aluminum rod and a thin 5-40 nut threaded onto one end and peened in place.... Once in place, it moves with the hammer.... It is long enough to protrude through the hole in the center of the RVA adjusting screw, and carries a 1/8" ID O-ring on it.... To use it, you simply cock the gun, slide the O-ring up against the back of the screw and then fire.... The O-ring slides back on the rod, stopping when the valve is at maximum lift.... By measuring the gap between the O-ring and the screw, you have a direct measurement of how far the valve opened.... Once fired, it looks like this....



I have used this device on enough PCPs, at enough pressures, to realize that the range of lift values (at least in .22 through .30 cal) is pretty constant for any USABLE tune.... This is because any tune that produces a proper bell-curve works pretty much the same, regardless of pressure.... At the beginning of a shot string, the lift is typically about 0.050" to .060".... and by the end it runs about 0.100" to 0.120".... The only time the lift seems to go over 1/8" (on the many guns I have tested) is when the pressure is very low (ie you are past the usable portion of the shot string), or you have so much hammer strike that the first shot is the fastest and you have no bell-curve at all (really the same thing, if you think about it).... Therefore, the RANGE of lift values from the beginning to the end of most shot strings runs about 0.050" to 0.060"....

So how much does a bstaley O-ring stack compress when struck by a hammer?.... To determine that, I removed the striker from the Challenger hammer so that it couldn't hit the valve stem.... Now the front outer face of the hammer sat right against the O-ring stack at rest.... When you cocked the gun and fired it, the O-ring gap to the RVA screw showed how much the stack compressed when struck by the hammer.... I must admit, I was quite surprised to find that in my very first test, that was 0.040", as shown in the above photo.... There is one other piece to the puzzle, however.... How much was the stack of O-rings compressed just from the hammer sitting against it with the preload of the hammer spring?.... To measure that, I measured from the end of the indicator rod to the back of the screw, then removed the hammer spring, and measured it again.... The difference was the static compression of the O-ring stack from the hammer spring preload.... I used four different setups to get a feel for what is happening, and the results are below....



I used a stack of four #113 O-rings.... The first 3 tests were with 70 Durometer, and I tested one setup with 90 D.... "Stock Disco" means a Disco hammer spring at stock preload (like with a stock end cap, and yes I allowed for the thickness of the nut on the indicator rod).... "Max. Disco" is the same spring, set at coil bind.... "Max. 0.040" is my heavy hammer spring (made from 0.040" wire) set at coil bind.... The "Total" number is the important one, it represents the total distance the O-ring stack compresses on hammer impact from a relaxed state.... I came to the following conclusions from this (limited) testing....

A 90D O-ring is roughly twice as stiff as a 70D O-ring when acting as a buffer....
The spring rate is not even close to linear, there is much more give as the O-ring first begins to distort from circular than once it has collapsed somewhat....
This progressive spring rate means that the initial compression is fairly easy (look at the static numbers) but the final amount of compression doesn't increase much even with a big increase in hammer energy....

When thinking about how the NUMBER of O-rings in the stack affects the travel and stiffness, I compared them to the way a Belleville spring washer works.... As you add disc springs in series, you gain travel and the (total) spring rate goes down.... I think stacking O-rings works in exactly the same way.... Each O-ring is doing an equal share of the work.... If, for example, you only used two O-rings instead of four, I would expect the deflection of the stack to be half the distance at the same load.... ie the above numbers would be cut in half for a stack of two.... This remains untested at the present time.... There may be a small friction component that could skew the results slightly, of course....

The important "discovery " I made was simply to confirm bstaley's statement that the stiffness of the buffer changes the way it responds to the hammer strike and how much of that strike energy is being absorbed by the buffer.... This opens a big can of worms from an experimental point of view, of course, drastically increasing the number of variables.... However, I think it can be summed up as follows:

THE STIFFNESS OF THE BUFFER IS PROPORTIONAL TO THE HARDNESS OF THE O-RINGS, AND INVERSELY PROPORTIONAL TO THEIR NUMBER (A TALLER STACK IS SOFTER AND HAS MORE TRAVEL).

THE BUFFER DEFINITELY GIVES IN A PROGRESSIVE MANNER, A SIGNIFICANT AMOUNT RELATIVE TO THE TOTAL LIFT, AND THEREFORE HAS THE ABILITY TO MODULATE THE LIFT, NOT JUST LIMIT IT.

I know that bstaley already has a feel for all of this, but I didn't, so I hope I'm not just stating the obvious. In the next post I hope to get into the "nuts and bolts" of what happens when you install a bstaley mod....

Bob

[rsterne]

Well, I learned a valuable lesson about the O-ring buffers, today.... If the hammer isn't hitting them, they can shift out of place and cause weird results (you were right, Eric!).... I began to suspect that last night, and bstaley suggested it this morning, but I had already confirmed it.... I think the slight air blast coming out around the stem of the valve pushes them back a bit, or maybe just makes a space between them if the hammer isn't already contacting them as it would be before that blast of air occurs if the buffer was even partially engaged.... In addition, I made a new Lift Indicator Rod, even lighter and with a thinner head than the original.... This one is made from 5/32" OD aluminum hobby tubing, and only weighs 1.4 grams and only requires 1/2 turn adjustment on the preload.... The larger diameter means it doesn't wiggle around in the hole in the RVA adjusting screw, and the larger O-ring tends to stay nice and straight, allowing much more accurate and consistent measurements.... I then redid my previous lift measurements, and took a bunch of new ones, with the following results....



I have confirmed that at 4 turns in, with either O-ring stack, the hammer does NOT contact the buffer (or so lightly as to have no effect), providing the O-rings haven't shifted.... Therefore, those curves on the previous graphs should be ignored.... Anything happening there is a result of the buffer not remaining properly in place.... In addition, the previous chart I posted about the lift numbers with the 90D buffer are incorrect, the numbers above are the accurate ones....

Note that the lift at 1000 psi with no buffer in place changes 0.022" between max. stroke (full CCW) and 4 turns in, or about 0.005" per turn, and that should be linear.... That means that at 3 turns in, with no buffer, the lift at 1000 psi should be about 0.086".... I made another test, with the 90D buffer at 3 turns in, and the lift was 0.084", so the buffer is either not in contact, or it is just barely touching it at 1000 psi.... It would NOT be producing any changes to the shot string, so again any difference between that string and the unbuffered one would be due to the stack moving around.... I also tested the 70D buffer with the backer ring at 3 turns in, and the lift was 0.075" at 1000 psi.... This indicates that the hammer is just making contact at that pressure, but it would not be making contact with the buffer at over 1200 psi.... so the data for that string (unfortunately, the best string I had initially) may be somewhat suspect.... Perhaps that is why I couldn't duplicate it last night.... On the other hand, if the hammer is contacting the buffer at 1000 psi and below (only), that may be the reason that first string extended down to 850 psi....

Now that I have a better indication of what range of adjustments will and will not work, I am going to go back and retest both the 90D and 70D (with backer) stacks at the appropriate settings, but this time with 1/2 turn increments.... I'm hoping for a lot more consistent results....

Bob

[rsterne]

Yesterday I got the chance to rip apart my test gun and mod the Disco valve.... I drilled the throat to 0.234" (still the stock 0.156" stem), and bored out the exhaust port with a 5/32" mill on a 20* angle.... By the time I was done, both had an equivalent diameter of 0.174".... I drilled the transfer port to 0.162" (about as big as you can go without danger of the wall kinking), but out of curiousity (and because I only have one .22 cal barrel) I left the barrel port stock at 0.134".... I was therefore quite surprised when the gun made 31 FPE with 14.3 gr. pellets.... I didn't record a shot string at maximum preload because the 2nd shot was the fastest at 988 fps, but I shot strings at 2 through 8 turns out on the RVA and here are the results....



The average power is 28.3, 25.7, 23.6 and 21.0 FPE.... and the efficiency is 1.02, 1.17, 1.19, and 1.12 FPE/CI respectively.... The pressure shown in the above graph is only approximate, just to give you an idea of the pressure range for each string.... the starting pressures are very close.... These tests are only to establish a baseline for the testing using a bstaley buffer consisting of three 70 Durometer #113 O-rings.... I am using a Challenger hammer with a 1/4-28 SHSS with the point ground flat as a striker, a Disco hammer spring, and a Challenger RVA.... The above tests were conducted with the set-screw flush with the front of the hammer, so the stroke was 0.58" which is stock for a Disco....Remember, for a full volume Disco reservoir, the above shot counts would double....

We're really busy in the Motel right now, so I have no idea when I will be able to get to doing the bstaley tests, unfortunately....

Bob

[AirGunEric]

Having had no prior readings on the "bstaley mod"- my question is; what is the point of such a modification in addition to an RVA and an adjustable hammer pre-load? It strikes me that by creating this buffer, the obvious result would be less hammer strike at lower reservoir pressures, less duration and then less propellant consumption and a weaker and weaker shot as the air pressure diminishes- i.e. shortening the usable shot string than would be the case if no such 'buffer' were in place.

[rsterne]

The idea is that in some guns (maybe not all), the self-regulation of the valve (ie increase in lift) as the air pressure reduces is too great.... I'm sure you have heard PCPs get louder as the shot string progresses?.... That is because the efficiency is less in the latter part of the string.... With this mod the idea is to "knock the top off" the bell curve, flattening it, and then retaining some of that air for later shots, which use progressively less volume of air, but at a slightly higher pressure, thereby producing more shots overall.... I've NEVER tried it, this experiment is a first for me.... however, the success that this method of tuning has produced in the MROds and PRods is very good....

It also makes the tuning process easier overall, as once the ideal hammer spring preload has been found for the fill pressure, backing out the striker reduces the lift and hence reduces the power.... Again, I've never tried it, but the claims are that strings detuned in this way are significantly flatter, and hence longer, than those achieved by strangling up the transfer port.... The efficiency is supposedly higher.... and that's what I want to explore.... There is defintely something going on here that needs invesigation....

Bob

[rsterne]

I completed the second set of tests, changing the striker setting by only a half turn at a time instead of a full turn.... I also eliminated the settings where I was sure the buffer was not playing a role.... All strings were started at 2000 psi, and the Disco hammer spring was set just shy of coil bind as before.... and only shots within a 4% ES are graphed.... The four 70 Durometer O-rings had a 90D Backer ring under the stack.... and here are the results....



Basically, once the striker is screwed in 2 turns CW or more from the fully CCW position, it is ineffective in altering the shot string to any significant degree.... These results are different for the 3 turns out curve (blue line) which I found in the first test.... My only explanation is that the buffer in that test was moving out of place slightly, and interfering with the hammer in just the right way to skew the results.... That curve was a lower velocity, longer, and flatter, but since I haven't been able to duplicate it in several tries, all I can say is that it was a tease, unfortunately.... The 2 and 3 turn in shot strings are virtually identical to the unbuffered results....

When we explore the striker adjustment range from maximum CCW (minimum lift) to 2 turns in, however, we have some very interesting things happening.... As the velocity drops, the efficiency increases, and the strings get longer.... At 1.5 turn in, the average velocity is 775 fps (at 1.18 FPE/CI), and a 1 turn in, it drops to 718 fps (at 1.36 FPE/CI).... In both cases, a large portion of the first part of the shot string appears to be missing, an indication that the hammer strike is too great for the 2000 psi fill pressure.... For both of those strings, further experimentation is needed.... At 1/2 turn in, the velocity is only 617 fps, but the gun got 32 shots within 4% ES, and the efficiency was 1.65 FPE/CI, very efficient for a 12 FPE gun.... If you remember from the first tests with the 70D buffer, at maximum CCW, the velocity was about 430 fps and the gun did 65 shots at 2.11 FPE/CI between 1650 psi and 1000.... This soft, tall buffer is excellent for drastically reducing the power of the gun and raising the shot count and efficiency.... If that is what you are after, this is something you should definitely consider....

Now to look at the stiffer buffer, consisting of four 90 Durometer O-rings.... Here are the results....



With this stiffer buffer, and without the extra height of the 0.055" thick backer ring, the buffer was basically ineffective once the adjuster was past 1 turn in.... The top group of curves pretty much mirror those of the unbuffered gun at the same adjustments, although for some reason they seem to peak at a slightly higher velocity, although no higher than the unbuffered gun did at maximum.... The efficiencies are similar as well, so IMO no advantage can be found operating in that adjustment range....

Once again, however, when we explore from maximum CCW (minimum lift) over the first part of the striker adjustment, we see more shots at increased efficiency, but at lower velocity.... The shot string at 1/2 turn in (791 fps average) started out with the first shot the fastest, so it definitely needs to be explored with less hammer spring preload.... With the striker at maximum CCW, the velocity averaged 671 fps, and it looked like it too, could benefit from a bit less hammer energy for the 2000 psi fill.... The efficiency of that 15 shot string was 1.30 FPE/CI, and I have a feeling that both can be increased a bit.... However, when you consider that the 1 turn in string with the 70D buffer was higher velocity AND more efficient, I have my doubts if the 90D buffer will show any advantages over the softer one....

There are a couple of general observations I have to make.... When the striker adjustment first starts to have a serious affect on the velocity, it appears that the shot string shifts to the left, ie it peaks at higher pressure.... In an unbuffered gun, that usually indicates too much hammer strike, and that is likely the same case here.... Only adjusting the preload to a lower value will let us find out if there is more shots and efficiency hiding there.... Once the velocity is reduced further, however, the shot string returns to the original peak pressure, and eventually, as the velocity is lowered enough, then the fill pressure must follow, just as in detuning by hammer energy alone.... While the basic idea of reducing lift directly by decreasing the striker engagement with the valve is simple in concept, and very effective.... it still needs that fine tuning of trying various combinations of hammer spring preload and/or fill (and refill) pressure to find the optimum setting....

When time permits, I plan to concentrate on those parts of the envelope where less hammer strike is needed to optimize the results.... In the meantime, there is certainly lots here to chew on and discuss....

Bob

[rsterne]

I finally got a chance to get back to working on this today.... A couple of days ago, I got a chance to fire a few shots with four 70 Durometer O-rings installed as a buffer, and it wasn't quite thick enough.... With the striker in the Challenger hammer fully CCW, it just touched the valve stem at the same time it hit the buffer.... Today I picked up a couple of #113 "Backer Rings", which are a 90 Durometer flat O-ring only 0.055" thick (about 1/2 as thick as a #113 O-ring), that are flat on one side and concave on the other.... They are used to decrease the clearance and improve the sealing when using a 70D O-ring where the tolerances are too big to seal at the expected pressure, and are placed on the low pressure side of the O-ring.... In this case, I placed one against the back of the valve as a spacer (I could have used a metal washer, but this was an item that is readily available at any hydraulic shop, and will be consistent in dimensions, so a good choice).... It slid in just like installing an O-ring, and then I slid four 70D # 113 O-rings down against it.... This made the total thickness of the buffer 0.467", and when the hammer was dropped in, it just went "thud" against the bumper.... I started my testing by setting the striker fully CCW, and with no O-rings in the gun I shot a string to find out what hammer spring preload was needed.... Well, it turned out that with the reduced hammer travel with the Challenger hammer, the string looked the best with the Disco spring at maximum, just shy of coil bind.... All the testing was conducted at that spring setting, and all testing was done with JSB Express 14.3 gr. pellets....

I shot a string with the hammer stroke set to maximum (full CCW), and then repeated, turning the stroke adjuster in one turn at a time, stopping at 4 turns in.... Here are the results, to be used as a baseline....



You will notice that decreasing the stroke causes a loss in velocity, and eventually the first shot at 96% requires less than a 2000 psi fill.... Shot #1 on the graphs was at 2000 psi in all cases, but only shots above 96% of the peak velocity are shown.... The efficiency was relatively constant at between 1.04 and 1.20 FPE/CI, acceptable but not stellar.... with the most efficient being at 3 turns In, peaking at 838 fps and averaging 21.6 FPE for 12 shots using the 65 cc tube (half what a Disco has).... I then pulled the gun apart and installed the buffer, consisting of four 70 Durometer O-rings plus a Backer, and I repeated the tests, starting from 4 turns in where I stopped before.... Here are the results from 4 turns in down to 1 turn in, plotted on the same vertical axis (velocity) as the above for easy comparison....



The first thing to note is that the orange line, at 4 turns in on the striker, showed virtually identical velocity to the curve above with no buffer installed for the first half of the shot string.... At that point, I assume the hammer started to make contact with the buffer, and the velocity dropped like a stone.... In fact, the first shot after the peak (820 fps) was down 50 fps, so that setting was completely useless.... I was pretty disheartened at this point.... However, when I backed the striker out a turn, to 3 turns in, things changed in a hurry.... First of all, note that instead of the velocity increasing as the striker is backed out, it DECREASES because the hammer starts hitting the buffer and the valve has less lift.... Although the velocity was 60 fps slower than the same setting without the buffer, THE SHOT COUNT DOUBLED.... Now I was equally shocked, but in a good way!.... When I calculated the efficiency, it was well up, at 1.39 FPE/CI, and that string started at 2000 psi and continued all the way down to 850 psi before the velocity dropped more than 4% below the peak.... I have NEVER seen that wide a usuable pressure range on an unregulated PCP before....

With the striker turned out another turn, at 2 turns in, the curve was similar but shorter, only 16 shots, ending at 1200 psi.... I would assume that intermediate settings, with varying degrees of buffer interaction with the hammer, would produce intermediate results.... When I backed the striker out another turn, to 1 turn in, the velocity dropped, and the gun was a LOT quieter.... By the shape of the curve, it would appear that there is too much hammer strike (or too low a fill pressure) to get the longest shot count at that velocity, and that may also be true of the previous string at 2 turns in.... All of these strings had efficiencies between 1.26 and 1.41 FPE/CI, which is very good, although the velocities are, admittedly, quite a bit less than the gun shot without the buffer in place.... That is consistent with what bstaley has always said about this system, that it is at its best for low to medium power tunes, not flat out power....

Now you may be wondering why I haven't shown the results for the striker set to maximum travel with the 70D buffer.... Well, the buffer was pretty well engaged fully, and I didn't even know whether the gun would fire.... Anyway, I filled it to 2000 psi, and started shooting.... The first shots were under 400 fps.... 40 shots later, at 1650 psi, the velocity had climbed to 430 fps.... After another 35 shots, it peaked at 447 fps at about 1300 psi.... Add another 30 shots, the pressure was down to 1000 psi, and the velocity had fallen to 96% of it's peak.... That meant I got 65 shots with a 4% ES from 1650 psi down to 1000, at an efficiency of 2.11 FPE/CI.... For those of you that aren't impressed, that is the HIGHEST EFFICIENCY I have personally ever seen in any PCP.... For those of you that like the other units, it is less than 7.8 barCC/FPE.... One other thing.... the gun was so quiet the loudest sound was the pellet striking the backstop....

For any of you (including myself) that doubted whether or not the bstaley O-ring buffer and striker tuning system can deliver results, I can certainly tell you that I no longer have any doubts.... For detuning a gun a bit and extending the shot string, it shows huge promise, IMO.... Whether or not it is possible to get "better" results at higher power I have no idea, although I hope to find out.... Bear in mind, this is only the first time I have played with this idea, I am by no means an expert at tuning with it.... On first blush, it would appear that there may be some very strange (and undesirable) characteristics if the buffer starts to engage too late in the shot string, as evidenced by the "4 turns in" curve with the buffer in place.... This might not be the case with a firmer buffer (ie either shorter or using harder O-rings).... and it might disappear with the striker adjusted just slightly differently.... However, once the buffer is working, it would appear that playing with the striker position and the spring preload could well provide some very efficient shot strings at medium power settings....

The thing I found quite shocking, was how well the bstaley mod performed at low power.... Anyone who is interesting in seriously detuning a gun for a ton of shots at low power should consider trying this system.... The basis of tuning with it is that you set the velocity you want with the striker position, and then fine tune the fill and refill pressures with the hammer spring preload just as you normally would, at least that is my assumption.... My next experiment will be with a stack of four 90 Durometer O-rings, which will provide a much stiffer buffer.... I have absolutely NO idea what to expect....

Bob

[rsterne]

I had a chance this evening to do some testing of preload changes on the 90 Durometer stack for the striker position that was 1/2 turn out.... That tune had a shot string that started high and declined, so I wanted to see what would happen if I reduced the hammer spring preload, if the gun would react in a "normal" fashion even with the buffer.... Well, as I expected, it did.... The colours have changed on this graph for clarity, so don't use them to refer to earlier graphs....



All the solid lines are at maximum hammer spring preload, as before.... The black line is with the striker full CCW (0 turns), the solid red line is 1/2 turn in on the striker, and the blue line is 1 turn in on the striker.... The other two red lines show what happens with the preload reduced two and four turns.... As you can see, the peak velocity drops and shifts to a lower pressure, reacting just like a conventional unregulated PCP, as the preload is reduced.... In addition, the efficiency increases, and the shot count increases, again as per usual.... The interesting thing about the efficiency is that it increases from top to bottom on that graph, starting at 1.06 FPE/CI for the blue line, and progressing to 1.30 FPE/CI for the black line.... I have no reason to believe that any "magic" will happen in between, although in between tunes are certainly possible.... As an example, if you wanted to tune for the flattest curve peaking at 800 fps, you would use a striker setting roughly 3/4 of a turn in CW, and a preload likely in the neighbourhood of 2 turns out from coil bind.... and you could expect about 12 shots at an efficiency of about 1.20 FPE/CI.... Using a full size Disco tube, that would be about 24-25 shots within a 4% ES.... The problem is, that is basically no better than you can get with a stock Disco.... From what I have seen so far (and I'm running out of ideas), the 90 Durometer buffer doesn't seem to offer a significant improvement over stock.... I detuned a Disco with just an RVA (preload adjuster) and achieved 33 shots at 18.6 FPE within 4% ES from 1800 psi down to 1000.... That is 614 FPE from 452 CI of air, which works out to 1.36 FPE/CI.... That is more power AND more efficiency than anything I've seen so far with the 90 Durometer buffer.... It is possible, of course, that shortening the valve stem and increasing the hammer stroke might provide better results.... I'm still hoping that the 70 Durometer buffer will provide greater efficiencies as well....

Bob

[rsterne]

Here is the string for the four 70 Durometer O-rings plus a Backer ring with the stroke set at maximum (full CCW).... With the bstaley method, backing the striker out produces the lowest velocity....



That string was from 1650 psi down to 1000, and represents the portion within 96% of the peak velocity.... I shot 40 shots starting from 2000 psi, where the velocity was just under 400 fps, just to get to the start of this string.... The efficiency was a staggering 2.11 FPE/CI.... Anyone wanting to set up a PCP for the Canadian non-PAL requirments (under 500 fps) should have a serious look at doing the bstaley mod.... If you were careful with the height of the O-ring stack and the length of the hammer spring, you could acheive a huge shot count with no adjustable components (ie using a stock hammer).... This rifle only has a 2260 tube, with 65 cc of volume.... Here is a photo....



This was just thrown together from parts to give me a test bed.... It uses a stock Disco valve, transfer port, and 24" barrel, a Crosman steel 22XX breech, 2260 tube and cut down stock, a PRod gauge port and a Disco fill fitting.... The adjustable hammer is from a Challenger / PRod, and the RVA / power adjuster is also from a Challenger, running a Disco spring.... The performance will mirror a stock Disco, but with only half the air volume and half the shot count.... That saves time and pellets during testing changes....

Bob