9mm and .308 cal Disco Build

[airpuffhunter]

congratulations, that gun is really something.
Bob I wonder, can you brake the valve, if the hammer hit it too hard? (too much preload)

[airpuffhunter]

another thing, do you think that if you use the same weight bullet in 9 mm, will get a lot more power?

[rsterne]

I guess you might shorten the life of the valve if you hit it too hard with the hammer....

Generally, in the same gun, set up the same way.... if you keep the same bullet weight but in a larger caliber you will get higher velocity and energy.... This is because the base of the bullet (that the air pushes on) is larger and hence more force is available to accelerate it.... There are limiting factors, of course.... If the bore is too large in volume compared to the valve you may not have enough "oomph" for the larger bullet.... I would certainly expect that if I used a 120 gr. bullet in 9mm that the velocity would be higher than the same bullet in .308.... and the energy higher.... PCPs tend to favour heavy bullets and develop more FPE with them.... I have a 132 gr. bullet in 9mm but they are too large and I need to size them down to work properly.... The first trial (at 2000 psi) resulted in slightly less energy than the 78 gr. EunJin at the same pressure....

Bob

[rsterne]

After yesterdays success in chambering the .308 barrel I decided to rechamber the .357 barrel today for cast bullets.... The process went better than yesterday, and when I put the bigger barrel back on I got the following results....



As I expected, this combination gave the highest FPE levels I have achieved so far.... the peak level with the 132 gr. Lee Round Nose was 698 fps which is 143 FPE.... With the hammer spring at maximum preload I got four shots using 300 psi per shot.... The average was 687 fps (138 FPE) at an efficiency of 0.84 FPE/CI.... I backed the preload out 1.5 turns and got 5 shots starting at 2800 psi and ending at 1550 (250 psi per shot).... They averaged 650 fps (124 FPE) at an efficiency of 0.90 FPE/CI....

The new longer chamber cost me about 10 fps with the EunJin pellets at the same spring preload.... However, by winding in the spring 1/2 turn and increasing the fill pressure to 3000 psi I gained that back and still kept 5 shots at 250 psi per shot.... So the current results are 5 shots at an efficiency of 0.90 FPE/CI with either pellet/bullet at the same 1.5 turns out on the hammer spring.... The 77.8 gr. EunJin shoots 812 fps (114 FPE) using a 3000 psi fill, and the 132 gr. Lee shoots 650 fps (124 FPE) filled to 2800 psi....

I'm pretty darn pleased with the results, and I certainly learned a lot building this Big Bore Disco.... Who would have thought you could take a gun all the way from 22 FPE to 143 FPE....   

Bob

[lloyd-ss]

Bob,
Again, very nice.  Bullet fit has always been illusive to me, too.  I have actually taken a piece of 1/2" steel plate and drilled and and reamed a series of slightly undersized holes, and then gone back with a tapered pin reamer and opened up each hole slightly more than the previous.  That way I had a nice assortment of sizes for testing.  Then once I got the size figured out, I worked on a real sizing die to produce the same bullet diameter.

143 fpe from a Disco configuration is sweet!
Lloyd