.30 cal Disco Double

[rsterne]

I finally had a chance to get the gun out for some long range shooting today.... It is tuned for one 7-shot clip at 107 FPE with the 50 gr. Daystates (988 fps) or 105 FPE with the 46 gr. FX/JSB pellets (1012 fps).... I first shot a 3-shot group with each pellet at 50 yards, and they were all under a quarter, with this being the best....



That measures 1/2" C-T-C.... I then shot a 5-shot group with each pellet at 100 yards.... Here are the targets....

Daystate 50 gr.



JSB 45 gr. (higher aim point)



FX 46 gr.



There was a slight wind swirling around though the trees.... Four of the five shots in each group are within 1.0" - 1.2", and the average group size was under 2" except for the 1 shot I pulled.... I didn't do any tuning of the air stripper, nor did I check to see if the accuracy is any better at a slightly lower velocity.... To be frank, I wasn't shooting at my best in the cold weather, it was just above freezing today, and I know for a fact I pulled the one FX pellet up and to the left.... Overall, I'm very pleased with the groups for a first time out.... I stapled a pellet tin (2.5" D) to the 100 yards support and nailed it first shot.... All three pellets print at very nearly the same POI at 100 yards....



The development work on the .30 cal version is now complete, and it's time to start work on the .25 cal barrel.... This testing certainly proved that the .30 cal barrel Sean and I designed for the .300" head size JSB pellets does the job....

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

[sholo]

Slick idea being able to adjust the length of the probe - basically an adjustable TP...I like it way much!

So if I'm understanding the concept correctly, in a regulated gun for example I should be able to set the hammer spring preload to match the output pressure (1500 psi in this case), then fine tune the velocity I want by adjusting the probe length, correct?

Hmmm.....I can see another mini-project coming up!

[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