Originally Posted by Godiva
Title really says it all.
I realize that if the barrel is too short then the bullet leaves the barrel while the gas is still expanding, but how long is too long? I mean say the gas expands 20 inches (of a 24 inch barrel) down the barrel till it equals the external pressure. Why would be 24 inches be better than say 22, or 26 for example. What I'm really asking is how would you calculate optimum barrel length to maximize the bullet distance.
The effect of barrel length on range potential is, of course, related directly to the velocity which can be achieved. In general, the faster it goes, the further it goes. To a certain extent, long barrels produce higher velocities; although, as hasbeen pointed out, there is a point of diminishing returns dependent upon caliber, bullet weight, powder burning rate, primer strength, and a host of other factors. So, a 22 long rifle will achieve it's maximum velocity in about fifteen inches; most centerfires will require considerably more length to "max out".
The maximum length for the various cartridge/bullet combinations is, well, variable. The optimum length is often a matter of conjecture. I typically build all of my long range rifles with 26 inch barrels which is shorter than is preferred by many of my fellow shooters. I find this length is adequate to produce the velocity I desire while retaining stiffness. I have one 6.5x55 which I built with a 28" barrel and it seems just fine as well.
Popular wisdom contends that we want to start the bullet out fast enough that the bullet remains supersonic beyond the range at which we are shooting. With a 308 shooting 190 grain Matchkings, this can be achieved with a 22 inch barrel if we are shooting 1000 yards. The 26 inch barrel just gives a bit more "cushion".
In the end, longer barrels will be faster but the amount of gain may not be worth the effort. Also, if the longer barrel does not shoot as well, it is, in my opinion, a failure. It doesn't matter if the bullet is travelling faster if it misses the mark. GD