I'm pretty surprised, too, since 5.7 has such fast powder and so little of it, and a rifle primer, to boot. As small as it is, it seems hard to imagine it is very overbore. Has anyone actually plotted a pressure curve for 5.7? I looked into it, since I'm trying to design a gas-operated 5.7x28 carbine, and found that Quickload is apparently
horrible at predicting max pressures (and in the unsafe direction, too
). I think you
could back out the data using measured velocities for progressively-shorter barrels, but you'd still have a hard time capturing the peak of the wave which would be very near to or past the chamber throat.
What may be muddying the issue is that powder burn and pressure won't necessarily correlate, since the chamber volume is changing so radically relative to the case size. Powder
could still be burning, but the bullet already driven fast enough by the initial pressure spike that pressures continue to drop. I'm pretty sure you'd need super-delayed two-stage powders to both drive the bullet down the pipe,
then drive pressures higher (but that's not the same thing as 'burn rate'). I have no idea how burn rate is calculated or simulated, but I know it can't be measured as easily as pressure, which is I thought why most internal ballistics works on pressure figures.
That Longshot powder I was using, for instance, has a peak burn rate curve just beyond the length of the standard Five Seven barrel. It *would* - with no doubt - cause dramatically higher pressures at the muzzle in a 6" barrel than a 4.8" barrel. That extra 1.2" is all that is needed to realize the rest of the potential...
I thought for sure you originally meant
velocity rather than pressure. I fully expect that
velocity is most efficiently generated right around the length of the pistol that was designed with/around the cartridge. I also understand how the slower-burning round slated for the carbine would have trouble transferring sufficient energy in the 1/3rd length barrel of the pistol, drastically limiting its effectiveness.
Please forgive my ignorance on the matter but, if what propels a bullet is pressure and it's well known that the same bullet exits a rifle with significantly greater velocity that a pistol, how could the pressure in the rifle barrel be less?
In most (all?) other pistol cartridges, the powder is almost fully engulfed before the bullet has moved much at all, or is in the process of burning. Between the decomposition into gas components and super high temperatures also generated, this is when pressures peak. After this point, the chamber volume increases as the bullet moves away, and there is no more powder to burn and generate gas volume and heat; the result is pressures and temperatures drop rapidly. A shorter barrel can't drop as much as a longer rifle barrel before the muzzle opens, but that doesn't mean huge pressures aren't still accelerating the bullet. Even though pressure may have dropped to only a few thousand psi at a long muzzle vs. a short one, there's still hundreds of pounds bearing on the bullet (and rifling only saps about 100lb from that).
To correlate the pressure curve to velocity, you need the integral, or sum, of pressure times area (force) on the bullet for the duration it's in the bore. A rifle drives a bullet longer, with decreasing force, so the net energy imparted is more than a truncated section of the same curve you'd get from a pistol. Eventually, pressures drop enough that the bullet slows down (like 9mm from a 16" barrel, IIRC) but are still high enough to make noise once uncorked --not much though.
TCB
