The only reasonably accurate formula applies only to the theoretical maximum FPE for a PCP operating "wide open", which is seldom the case.... In that case, cutting a barrel in half (eg. 24" like a Disco down to 12") would reduce the available FPE by 50% as well (a 29% velocity drop).... In practice the loss in velocity is nowhere near that severe, because the gun is already "throttled down" by port restrictions and limited valve dwell (the valve is closing well before the pellet gets to the muzzle).... Taken to the extreme in the other direction, where the valve closes when the pellet has only moved an inch or so, and most of the power comes from the expansion of that tiny sip of air behind the pellet as it moves down the bore.... obviously doubling the barrel length will not double the FPE.... In fact, given a small enough sip of air, and a long enough barrel, the pellet friction will eventually make it slow down if the barrel is too long....Therefore, there is NO calculation, or even a rule of thumb, that can even remotely predict the velocity loss as you shorten a barrel, as there are simply too many other variables to consider.... About all you can say is that for an "average" PCP, shortening the barrel will lose velocity.... The more powerful the gun, the more you will lose.... while the "milder" the tune, the less you will lose by shortening the barrel....In your specific case, and by making a guess that your regulator is set at 1200 psi (and a bunch of other assumptions), cutting the barrel to 12" would lose you about 50 fps, according to Lloyd's Internal Ballistics Spreadsheet.... Incidently, it would roughly double the residual muzzle pressure, making the gun about twice as loud....Bob
So your 780fps could become 693FPS.
QuoteSo your 780fps could become 693FPS.Interesting.... Lloyd's spreadsheet predicts 692 (starting from a 24" Disco barrel, not the 22" guess by the OP).... The spreadsheet automatically takes into account the power level you started with (ie a gas hog or not).... and yes, gas hogs lose more than ones that merely "sip" their HPA.... I gave a "range" (from 690-700) because some of the assumptions I put into the spreadsheet may not be spot on....Bob
You might reconsider increasing the regulator pressure and using your 12" barrel.... According to Lloyd's spreadsheet, changing from a 24" barrel at 850 psi to a 12" barrel would only require a pressure increase to about 1050 psi to get back to your 780 fps.... However, you would still be facing about a 30% loss in efficiency (about 37% fewer shots).... You could also use 1200 psi (which would give you about 830 fps) and then reduce the valve dwell to drop back to 780 fps and your efficiency loss would only be about 15% (about 30% fewer shots).... If you reduced the dwell further, to get back to your original efficiency, at 1200 psi with the 12" barrel you would be at about 710 fps.... but with unchanged shot count.... Other combinations of power and efficiency are possible, of course.... You will never get the same results from a barrel half as long, you will need to sacrifice power or shot count, or some combination of the two.... but by varying the preload, and hence the dwell, you can taylor your results to the best compromise.... Also, you might consider using a standard 14.6" Crosman barrel from a 1389, I think they are still available in .177 cal.... or cutting one to around 15" if you can live with that length....Bob
Ray, if you don't increase the setpoint above 1400psi, the 1.8k burst disk should be reliable in my experience. I have had one rupture at that pressure but it seems to be an outlier. At 1500psi, I've had several rupture.
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