.30 cal Disco Double

[Doc Sharptail]

...



Here is summary of the averages for the shot strings in the above graph....

Maximum: 1162 fps (76.2 FPE) for 7 shots at 0.65 FPE/CI (all supersonic)
4 turns out on RVA: 1097 fps (67.8 FPE) for 12 shots at 0.77 FPE/CI (just subsonic)
5 turns in on velocity screw: 991 fps (55.4 FPE) for 18 shots at 1.09 FPE/CI
6 turns in: 922 fps (47.9 FPE) for 19 shots at 1.03 FPE/CI

There are a few useful shots missing on the last string, backing the RVA out to 4 turns out like in the other strings (it was at 3) would have dropped the velocity slighty, added shots, and increased the efficiency.... Tunes in between any of these can be obtained by using adjustments finer than a full turn, of course.... One thing that I was very pleased about was how tight the groups were with the Kings.... I'm used to seeing ragged holes at 20' even when I'm not taking my time and just shooting through the Chrony, but here is what I was getting for 8 shots.... Yep, that's just a 1/4" hole!....




Bob

Well done indeed, Sir! This gives me the idea that there may be hope for me yet. I'm hoping that your 1/4" at 20 feet will stay that way out to 25-30 yds.

On sort of a related accuracy note, all that off-hand work on soda can pull ring tab rivets at 12 meters has paid off in spades. That's pretty well equal to a grouse head at 35 yds, at the same 6X on yet another 4-12X Banner- which I proved with the 52-R Sporting at the start of the season. Wish I had an air rifle that lined up off the shoulder as instinctively well as that Winchester...

Regards,

Doc Sharptail

[rsterne]

Today I tried several different tunes for the JSB Kings around the 55 FPE level.... The plan was to try adjusting the hammer preload weaker (and drop the fill and refill pressures) while opening up the barrel port by backing out the velocity adjusting screw.... I wasn't fussy about getting exactly the same velocities with each tune, I recorded all the shots within a 4% ES (~40 fps), and all the shots were between 965 fps and 1018, so the strings were within about 15 fps for the peak velocity.... I backed off the hammer spring preload a turn, dropped the fill pressure 200 psi, and opened up the barrel port to keep roughly the same velocity for each string.... Here are the results....



The numbers on the legend are the number of turns in from maximum power on the bolt travel (barrel port restriction) and the hammer spring preload, measured from the lock nut to the end of the adjuster (bigger distances mean less preload).... The -5T and 0.358" curve started at 3000 psi, and the last string shot, (-2T and 0.568") started at only 2000 psi.... I know it's a bit hard to see, with all the lines on top of each other, and the curves look silly with the drastically expanded vertical scale.... but here are my conslusions....

1. There was little to choose from for the efficiency.... All the strings were between 1.02 - 1.10 FPE/CI, with no real trend apparent.... The differences may have been more due to some strings being a few fps faster, or missing a shot or two on the front end of the string because the starting pressure I used wasn't quite high enough....

2. As the pressure range dropped and I compensated by opening up the porting to keep the same power, the shot strings got shorter and more curved.... The range of pressure between the ends decreased.... Once again, it is apparent that wide open ports can develop good power at low pressures, but the price you pay is a low shot count.... You can equate big ports to a hot cam in a racing engine.... More power for a given pressure, but a narrow power band....

3. All these tunes used about 50 psi per shot from the 250 cc Disco Double reservoir.... in fact the range was narrow, between 48-52 psi per shot.... Each vertical line on the graph therefore represents about 50 psi.... With the ports wide open, I was able to get 1000 fps (55 FPE) from only 1700 psi, which is pretty amazing, IMO....

4. The longest shot string was at about 2800 psi fill pressure, and ran 18 shots, but it was missing 1-2 shots off the front end of the string, and peaked at over 57 FPE.... I feel that with some fine tuning, I can achieve 20 shots at 55 FPE average from a fill, within a 4% ES.... I can already get 13 shots within a 2% ES, and 17 shots within a 3% ES, averaging closer to 56 FPE with the Kings....

While the purpose of this gun was to achieve a higher power level using cast bullets, I'm very pleased with the flexibility demonstrated to shoot JSB Kings at 1000 fps with a good shot count.... and once again I was provided with the opportunity to learn more about PCP tuning....

Bob

[rsterne]

My new Boattail bullet mold arrived a couple of days ago and I cast some bullets yesterday.... The new .300 cal bullets came out at 70.4 gr. and they function perfectly in the modded MRod magazines....



Here is what they look like....



Since these bullets are a lot heavier than the 50 gr. JSB pellets the gun was built for, I maxed out the hammer spring preload and the velocity adjuster and ran a string.... It peaked at 114 FPE and averaged 837 fps (110 FPE) for 9 shots.... so a little bit more hammer strike is required to get the maximum from one 7-shot clip.... I'm hoping to fix that issue by replacing the valve poppet with one made from PEEK which is a harder material and takes less energy to break loose from the seat.... I shot a couple of other strings with the velocity adjuster run in a bit (which restricts the barrel port with the nose of the bolt), and here are the test results....



The last string was two full clips (14 shots) averaging 86 FPE at an efficiency of 1.11 FPE/CI, which is the same energy as I got with the 50 gr. pellets when the gun was set for 14 shots at fractionally better efficiency.... The way I see it, the best use for these bullets will be with the gun adjusted for a single clip at maximum power.... The bullets were all going into ragged holes between 3/8" and 1/2" OD during Chrony testing, which is pretty good for cast bullets.... Proper accuracy testing will have to wait until the spring, however....

Bob

[rsterne]

Well you knew I had to do it !!! .... My .30 cal Disco Double is one of the best documented guns I have, and with the new 70 gr. bullets it's now limited out on performance due to insufficient hammer strike.... or rather it WAS, until today when I made a new poppet out of PEEK for it.... All I can say is WOW !!!

Before I tore the gun apart, I made some measurements of valve lift, using an O-ring on the rear cocking shaft.... This is an easy procedure, you slide the O-ring up against the adjusting screw on the RVA, cock the gun, fire it, and measure the gap.... Voila!, instant measurement of how far the valve opened.... I set the preload to maximum (as near coil bind as will safely cock), opened the bolt stop to maximum (this gun has the ability to choke off the barrel port with the bolt nose), loaded a 50 gr. JSB, and checked the valve lift at 2800 psi and again at 2000.... I did 2 shots to confirm the measurement, and I was pleased that they were the same within 0.001" or so.... I then repeated the test with no pellet loaded (open bore), and then I moved the bolt nose forward with the adjuster to close off the barrel port, loaded a pellet to block the bore even more, and did another set of lift measurements.... These three conditions change the flow around the head of the poppet, from "normal" to as much flow as possible (barrel at atmospheric with no pellet), to virtually no flow where the pressure on both sides of the poppet is nearly the launch pressure (the velocity drops to about 200 fps, you can see the pellet arc through the air to the backstop).... This gives us a way to somewhat quantify the aerodynamic drag trying to close the poppet, relative to the major closing force, which is the air pressure acting on the 5/32" valve stem.... Anyway, I then tore apart the gun and made a new poppet out of PEEK, reassembled it, and repeated the tests.... Here are the lift measurements....



As you can see, the PEEK valve had about a third more lift at 2800 psi, with the same hammer strike, so it is obviously a LOT easier to unstick from the seat.... There can no longer be ANY doubt of that.... The lift of the PEEK valve at 2000 psi was limited by the physical room between the face of the hammer and the back of the valve, which is 0.162".... You can also see that compared to the "normal" case of firing a pellet, with no load (and therefore a bigger pressure difference across the head of the poppet) the lift is slightly less.... while with the barrel nearly blocked (and therefore very little pressure difference across the head of the poppet) the lift is slightly greater.... That difference in lift is due to the aerodynamic drag across the head of the poppet, which shows up as a pressure difference between the valve housing and the exhaust port, which in turn increases the closing force during the valve cycle.... The differences are smaller with the PEEK valve, likely because the head is a smaller OD.... Here are the two poppets....



You wll notice that the Disco poppet, which has seen maybe 500-1000 pellets during testing, is pretty badly pounded.... While it is not extruded like the one in my Hayabusa was (which had a larger throat), it would appear that 3000 psi on a 1/4" throat is really more than it should see.... The ID of the Disco valve, BTW, was bored out to 5/8" ID so there is lots of room for flow around the head.... When I made the new PEEK poppet, I made the OD the same size as the valve spring.... The stem is made from 5/32" drill rod with the end threaded 8-32 for 1/4" of length and then the stem is slimmed behind that to 7/64", the same as I did on the Disco stem.... The PEEK valve head is drilled and tapped 8-32 and glued onto the stem with medium viscosity CA glue before final finishing.... Thanks, Lloyd, for the idea of threading it on for extra strength !!! ....

Of course my testing didn't finish with just measuring the lift, I had to see if the new PEEK valve increased the performance.... Well, the first thing I noticed was that the gun was louder and a lot thirstier for air.... My SCUBA tank was down to 2800 psi, but that was enough to drive a 50 gr. JSB at 1033 fps (119 FPE) compared to a previous best of 1011.... The 70 gr. Boattail, which I could only get to 852 fps yesterday screamed across the Chrony at 918 fpr (131 FPE).... I'm sure those numbers will increase further at 3000 psi.... I then started backing off the preload, with the pressure in the SCUBA tank gradually dropping, and it was obvious that even with the heavy, 70 gr. bullets, the gun, rather than being limited by hammer strike, was operating on the velocity plateau for the pressure available.... I had to back the preload down 5 turns before the velocity started to drop significantly.... At 6 turns out, the velocity pretty much matched the way the gun shot with the Disco poppet at maximum preload, starting at 830 fps and climbing gradually.... There is one problem, however.... The gun is suffering from a LOT of air robbing hammer bounce.... It appears that the PEEK valve is so easy to open that the returning hammer is burping it a couple of times, even at velocities where I was getting a decent string yesterday....

My ShoeBox is currently chugging away topping up my SCUBA tank, so further testing will have to wait.... I'm guessing that I'll have to use a shorter, stiffer hammer spring to get rid of virtually all of the preload to eliminate the hammer bounce.... I know that something will work, because my Hayabusa doesn't have this issue, nor does Sean's .30 cal QB.... It looks like a whole new learning curve is required to find the best combinations to work with the new PEEK valve.... There can be no question, however, that by using this material for the poppet (or seat) you can immediately reduce the hammer strike required for a given pressure and valve throat size.... Can lighter hammers be in my future?.... certainly the gun just got a whole lot easier to cock....

Bob

[rsterne]

I had a chance tonight to find out how the .30 cal Disco Double responds to hammer spring preload with the new PEEK valve.... Previously there wasn't any point as the gun couldn't reach its maximum potential as the hammer strike was the limiting factor, not the porting.... With the new valve, and tethering the gun at 3000 psi, I could now explore the guns power curve.... Here it is, plotted for both 50 gr. pellets and 70 gr. bullets....



You can see the same velocity plateau as the gun maxes out as you get on a regulated PCP, where additional hammer strike just wastes air.... The "knee" of the curve is where the plateau starts, and there is no point in running more hammer spring preload than that.... This occurs at about 4 turns out from coil bind with the pellets and 3 turns out with the bullets.... Going past that point gains only about 20 fps and doubles the amount of air used.... Even at that setting, the first shot will basically be the fastest, normal unregulated bell-curves start just below the knee of the curve.... at maybe about 5 turns out with the bullets and 6 turns out with the pellets....

The absolute maximum values obtained with this gun were 117 FPE with the 50 gr. pellets and 135 FPE with the 70 gr. bullets....

Bob

[rsterne]

I was a bit worried about the excessive air consumption with the PEEK valve due to air-wasting hammer bounce.... The only string I recorded yesterday was 6 shots, 855-864-833 with the 70 gr. bullets, and that used 1100 psi for a horrible efficiency of 0.59 FPE/CI.... Something obviously had to be done !!!

As a cure, I decided to try a shorter, stiffer hammer spring to get rid of the significant preload with the QB spring.... Lloyd was the first to notice an efficiency increase from eliminating the preload, and both Sean and I have observed it too, so it seemed like a logical place to start.... I had to shorten a 2.5" x 0.059" wire spring that I had to 2.0" by cutting off 4 coils.... The spring was a snug fit in the hammer (so it would bind on compression), so I had to drill it out one size as well, to 3/8" ID.... I was still able to use the 7/32" spring guide / rear cocking rod.... I had the preload set for 5 turns out with the QB spring, and to my surprise, with the new spring, that put me right at zero preload but no slack as well, exactly where I wanted to start.... The first shot across the Chrony was at 1000 fps with the Daystate 50 gr. pellets, so I knew I was close....

The new stiffer spring is a LOT more sensitive to preload adjustment, of course, at 1 and 2 turns in the velocity maxed out at 1021 fps, which showed me that zero preload was right where the plateau started.... Since I wanted to be on the knee of the power curve, that meant I would have a slight amount of free play in the spring, which I hoped would eliminate, or at least severely curtail, hammer bounce.... At 1/2 turn out the velocity was 972 fps, at 1 turn out it was 956, 1.5 out it was 935, and at 2 turns out is was 904 fps, so I chose 1 turn out for my first string.... My SCUBA tank was down to 2980 psi, so the first string started there, and I got 7 shots (1 clip) within just a 7 fps range (0.7% ES), averaging 100 FPE, and with an efficiency of 0.96 FPE/CI.... WOW ! I've never seen anything that flat before unless regulated.... Maybe this PEEK valve DOES produce tighter strings?....



I backed the preload down another turn, and cranked the velocity adjuster in 2 turns (which does very little) and shot another string.... This time I got 13 shots, starting from 2920 psi, 904-940-906, averaging 94 FPE at an efficiency of 1.09 FPE/CI.... I cranked the velocity restrictor in one more turn and repeated the string, starting at 2900.... I got 14 shots, 878-897-864, averaging 86 FPE at 1.10 FPE/CI, and the string should have started 1 shot earlier if I had a full 3000 psi in the tank.... Even so, this 2-clip string spanned less than a 4% ES, and over 14 shots will be a bit tighter with a 3000 psi fill.... The string stayed within a 4% ES down to 1860 psi, which is 100 psi lower than with a similar tune using the Disco poppet.... The PEEK poppet was truly showing its stuff !!!

I'm very pleased with the new PEEK valve, now that I have the proper hammer spring to compliment it.... The valve is easier to knock off the seat, reducing the hammer energy required to get this gun to full power from over 1.12 FPE (over, because I never quite got there with the Disco poppet) to 0.90 FPE.... or putting it another way, the valve gets a third more lift from a 1.12 FPE hammer strike.... The PEEK valve opens so easily, however, that it is prone to being reopened by hammer bounce.... I cured that by using a shorter, stiffer spring, that instead of preload, had a bit of free space for the spring to rattle around when uncocked.... This DRASTICALLY reduced the report of the gun from reduced air wastage.... I had thought previously that this was the case, but today's experiment proved that it can and does work.... The only downside is that the new spring is harder to cock.... The ultimate solution would be more hammer travel and going back to a QB spring without preload, but adding that much hammer travel is not an easy thing to do when I've already increased the travel from 0.58" to 0.75".... One thing I know for sure, there will be more PEEK poppets in my future, particularly on high pressure, large throat valves, where driving them open is difficult anyway....

Bob

[sholo]

For a non-regulated gun those are some pretty sweet strings...

This PEEK material you speak of has really grabbed my attention and got the wheels in my head turnin'... Same stuff I'm guessing?...http://www.lairdplastics.ca/product/brands/peek-1000

Quick question for you - I'm assuming PEEK is quite hard seeing as it takes less effort to knock open compared to a Disco poppet, and obviously works well with pressures in the 2000-3000 psi range as you've shown, but do you think it would still seal properly with pressures in 1500-1800 psi range (or even less) or does it need the higher pressure to seal properly?

I noticed that Laird Plastics in Ottawa (where I buy my Delrin/Acetol rod from) also has PEEK rods listed in several sizes so I think I may get some on my next order...http://www.lairdplastics.ca/product/...Page=1#results

[rsterne]

That's it all right!.... Sean said he noticed a reluctance to seal at under 1500 psi, but I filled my gun to 1000 and went and had lunch and it held fine.... I chuck the stem in a portable electric drill, pull on it gently, and run it for a couple seconds, reverse it, repeat 4 times (twice each direction).... Pull the stem out, and if it's got a completely polished ring on both the poppet and valve seat you should be good to go.... I'm not so sure about CO2, but certainly at 1500 psi I've seen no problem.... Mind you, not much advantage over Delrin, either, below 2000 psi or so, unless you're running MONSTER valve throats....

Bob

[sholo]

.... Mind you, not much advantage over Delrin, either, below 2000 psi or so, unless you're running MONSTER valve throats....

Bob

I'm just starting the process of slowly switching my 2260 over to .25 cal, so upgrading the valve a bit: .234" throat, .188" exhaust/transfer and barrel ports. Gun currently is reg'd @ 1500 psi but will be bumping it up 1800 psi output...

I also like the idea of the PEEK poppet's smaller diameter, allowing for a bit better airflow into the throat...

[rsterne]

If you make the PEEK poppet the same OD as the spring (~0.35") you probably don't need to bother boring out the inside of the valve....
Stock Disco ID and poppet: equivalent hole = 0.29".... clearance = 0.045"
Stock Disco ID and 0.35" valve: equiv. = 0.38".... clearance = 0.085"
0.60" ID and Disco poppet: equiv. = 0.42".... clearance = 0.085"
When you figure that about the biggest you can do on a Disco valve is a 0.25" throat, even a stock valve is larger in area between the poppet and valve body.... and a 0.35" PEEK poppet has as much clearance between the head and the body as a bored out Disco valve with a Disco poppet.... With boring the valve body being a PITA, I think I'd spend my time elsewhere....

Bob