.30 cal Disco Double

[Doc Sharptail]

And you haven't shot it?

Regards,

Doc Sharptail

[rsterne]

Just finishing up the stock and have to make a transfer port yet.... the next day or two and I'll blast a few upsized JSBs down the bore for a first test.... Find out if I'm in the ballpark....

Bob

[rsterne]

Just a couple of pics of the finished stock.... I'm very pleased with the results....



It's a Boyd's Blaster in Forest Camo (green, brown, black).... Inletting done by Norm at Discos-R-Us....



I finished it with 7 coats of Watco Danish Oil, wet sanded (2 x 220, 2 x 400, 2 x 600) and then a final coat of Minwax....

Bob

[sholo]

That came out beautiful, Bob!

Looking forward to the completed gun and subsequent testing...


Todd

[rsterne]

I got the gun assembled with the short .308 PB barrel and fired a few test shots tonight.... First the photo.... I know, it's ugly with the barrel shorter than the tubes.... and parts still needing bluing.... but you can see the potential there....



The gun was pretty hard to cock, I'll have to take a look to see if there is anything binding.... However, I was still able to cock it at coil bind, and the good news is that is way too much preload, even at 3000 psi.... The first shot was the fastest, the gun was very loud, and the 46 gr. JSB pellet screamed through the Chrony at 971 fps which is 96.3 FPE.... Four shots later it was down to 920 and those 4 shots used 700 psi.... I backed the adjuster out 2 turns, and got 6 shots starting at 953 and ending at 918, using 750 psi.... Out 2 more turns, and I was starting to get a string.... Starting at 2900 psi (my tank needs filling) I got 10 shots starting at 914, peaking at 929, and ending at 886, and it used 800 psi.... Backed out 2 more turns, starting at 2900, I got 13 shots starting at 861, peaking at 906, and ending at 860 fps at 2000 psi.... That string averaged 880 fps (79 FPE) at an efficiency of nearly 1.1 FPE/CI.... At 8 turns out the usuable pressure range is from about 2600 down to 1600, returning 14-16 shots in the mid 800s.... and at 10 turns, the string starts in the high 700s, peaks in the low 800s, and returns, but this is with running stock Disco pressures starting at 2000 psi and ending at about 1300.... and the gun is still shooting 65 FPE, three times what a stock Disco does....

These are just preliminary tests, using upsized JSB 46 gr. pellets in a 21" barrel.... I just couldn't stand the suspense of waiting until the new barrels get here.... The additional barrel length should add roughly 5 FPE.... but I have already surpassed my goal of 88 FPE in .30 cal.... so tonight's testing was a resounding success.... BTW, I was running a 3/16" transfer port, I can go to 7/32" if I want to....

Bob

[SeanMP]

Hmmmmm 971fps eh........see I knew calculating twist at a 1000fps was the right thing to do.

Bravo congrats That is simply epic.

[Doc Sharptail]

Looks good ,Bob! Can't wait for final everything to be done, and your results and opinions...

Now, some body come up up with a dual tube system for AR/QB :P

Regards,

Doc Sharptail

[rsterne]

I changed the gun over to a rear cocker last night, and also enlarged the transfer port to 7/32".... Here is the hammer, which now weighs 82 gr. with the cocking knob....



The gun is now very easy to cock.... I put a small O-ring on the shaft which can be used to directly measure the valve lift.... You simply slide the O-ring up against the back of the RVA screw, cock the gun, fire it, and the O-ring slides back exactly the amount the valve opens and you can measure the space with calipers.... I did that for all RVA settings from coil bound to 12T out, for the first shot, and the last shot before the velocity fell more than 4% below the peak.... The smallest lift measured was just under 0.06", and the largest was 0.13".... In virtually every case where there was a proper bell-curve, the lift at the first shot was about 1/16".... and at the last shot was less than 1/8".... This is a very valuable piece of information, and while it won't be the same for every gun, for this gun it indicates that limiting the valve lift to 0.15" (eg. by having the hammer hit the back of the valve at that point) would not have any affect.... That means I can lengthen the hammer stroke on this gun to a full 3/4" by grinding a bit more off the valve stem or counterboring the front of the hammer a bit more.... The fact that the valve lift was virtually the same regardless of pressure and preload.... once the gun was adjusted for a proper bell-curve.... was a bit of a revelation to me.... though in retrospect it makes perfect sense....

The larger transfer port increased the power a bit, made the shot strings more curved (and hence shortened them), and required about 3 turns less preload on the RVA for the same FPE level.... Possibly 1 turn of that is likely due to the slight increase in hammer weight with the longer guide and the cocking knob.... Here are the results of the testing, from coil bind to 12 turns out, by which time the fill pressure required was less than 2000 psi....



You can see that with a 3000 psi fill, anything within 6 turns of coil bound, while producing a bit more power, drastically shortens the shot string, and of course wastes a lot of air.... I saw 1000 fps (102 FPE) with everything maxed out at 3000 psi, however.... but the gun ws using nearly 200 psi per shot, and was down 4% in just 3 shots.... The best setting I could find was 7 turns out, starting from 2900 psi and ending at 1900.... It peaked at 929 fps (88 FPE), and averaged 913 fps (85 FPE) over 11 shots.... but the efficiency was only 0.88 FPE/CI.... This is the first gun I have tried where the area of the transfer and barrel ports (0.219"D) were larger than the area of the throat, which is the equvalent of a 0.203" hole.... I'm pretty sure that is why I couldn't get over 1.0 FPE/CI like I did with the 3/16" transfer port in the initial trials yesterday.... This leaves me with two choices, either drill the throat out to 1/4" (it is currently 15/64").... or refit the 3/16" transfer port.... The former should increase the power even further, and may increase the efficiency slightly.... The latter will drop the power a bit, but increase the shot count and efficiency.... I will wait until I get the new longer barrel to make the final decision.... but since I'm already pushing these pellets into the mid 900s more power may not be the answer.... so I'm leaning towards going back to the 3/16" transfer port with the 25" barrel....

Bob

[Doc Sharptail]

I like your thinking on the transfer ports.

It's becoming more apparent to me that efficiency, low e.s.'s and shot count are what matters in a hunting PCP. Velocity becomes sort of a redundant issue, after a certain point...

I'm so close to equalling the 29 gr CB cap (which is an efficient grouse harvester this side of 25 yds), that I'll leave the AR alone, until I can evaluate how it behaves on game. I had been hoping for low to mid 8's with it and the predator. After seeing how the predator groups, and expands at the mid 7's, I may not have that much more work to do...

Regards,

Doc Sharptail

[rsterne]

My Grouse gun shoots 18.1 gr. JSBs at just over 700 fps (20 FPE).... with a head shot, by far the majority fold or just do the funky chicken in the ditch.... A .22 cal at 12 FPE, while plenty of power to kill a Grouse, produces many fly-aways (with the same shots) by comparison.... The same thing with a .177.... plenty of power to kill, but not the "knock down" momentum of the heavier slug at higher power....

Bob