orphanedcowboy said:
I handload 2 3/4" steel loads and shoot them in 3" and 3 1/2" clambered guns and pattern test them over a chronograph and there is no degradation in velocity or pattern density doing so.
There is a slight loss in velocity for a given choke, and barrel length, based on the chamber size. Subtle but there. Certain powders and longer barrel length make it less perceptible.
It is certainly not significant, but he asked what it does.
It is simple math, the chamber is wider than the bore, it is typically slightly over .8 inches, while the barrel is .729 (plus or minus based on choke and possible backboring), and the gas has to fill the larger space of the chamber before it builds to the same pressure and starts imparting velocity to the payload at a faster pace in the barrel. The larger the chamber space the more gas it takes to reach the intended pressure.
It takes more gas to reach the same pressure the wider the tube, and the projectile is not as wide as that chamber so it does not have the added rear surface area for the pressure to act upon.
So in a 3 1/2 chamber firing a 2 3/4 load it is going to take a little longer to get up to the same pressure because the gas has to fill more unnecessary space. It is like starting with 3/4 of an inch of 9 gauge barrel, then choking it down to 12 gauge.
The shot also has to jump that gap, and while that does not effect the shot itself in most modern loads starting within a shot cup unless it doesn't stay centered, it certainly can effect shot that is not in a shot cup because it is an abrupt transition in bore diameter after the shot from a 2 3/4 shell has already been traveling for about .75-1 inches in a 3 1/2 chamber. It does not enter the forcing cone until shot not within a shotcup has been allowed to open slightly. This is especially true in many short abrupt factory forcing cones.
That can increase how much the shot deforms, which would impact the performance.