I'm not sure if anyone can answer your question with precision and accuracy, just like one can never predict just what effect shortening your barrel by 2.5 inches will have on it's velocity and accuracy.
Velocity usually does decrease in relation to the barrel length, and there may only be a minimal loss of absolute attainable velocity caused by shortening your barrel if maximum charges were always being fired. But there's different powders with each having different velocity curves, so I'm not sure that there would be much of a measurable difference since the powder charge can be increased or the type of powder can be changed to compensate for it.
However, one can never tell what effect the barrel shortening would have on accuracy. There may be some shortened barrel length where the accuracy may actually start to become much more noticiable. For example, the barrel length that used to shoot accurately at 100 yards may now only shoot accurately out to 75 yards just because a couple of inches were taken off.
Once a barrel is altered, there just may be unintended consequences. Not necesarily, but possibly.
Let me ask why shortening the barrel by 2.5 inches is important?
If someone intends to use the gun for brush hunting then I don't think removing that length of barrel is very important. But if the gun is going to be used for shooting out past 75 yards, then maybe that additional length of rifling may become more important.
I ask myself why and I'm not totally sure. But I do remember reading about a famous BP barrel maker stating that it's the last few inches of the bore that affects accuracy the most.
Since most of the original barrel length is necessary to develop velocity, and the end is important for accuracy, then there's not much unecessary length left that can be chopped off without having some negative affect.
So unless the muzzle of your gun is damaged and you feel that it's necessary to repair it, then shortening it may carry at least a small risk of affecting its long range performance.
I think that when Knight designed the gun they made a decision about what the minimum barrel length should be to deliver the accuracy that it would be expected to deliver.
If you do shorten your barrel length, then maybe you should be prepared to accept sacrificing at least a token amount of accuracy at the fringe range that the gun was intended to shoot more accurately at. That amount may only represent 10 or 15 yards, but that's just a wild guess based on my gut instinct and nothing else.
If it was a round ball barrel that was that short, it might not accurately shoot anywhere near that far. But being a fast twist GM barrel, maybe it will still shoot slugs pretty decently much like a rifled shotgun barrel of a similar length. Not as much of a tack driver but maybe it will maintain pie plate accuracy which is good enough for most purposes.
The much easier answer would have been to just say that the rifle won't lose much if any velocity or accuracy. And while that might be true, I'd rather look at it from a critical point of view. My perspective is to ask, "What's the worst that could happen by doing it?"
