I'm indebted to one of you for giving me this idea but I can't remember who it is. In any event thanks, and don't expect a small-frame 28-gauge in the mail.
Mostly, it's the shooter. If you'd like an illustration of this, go to a benchrest match where everyone's rifle can send five shots into .12245-inch (or something of that size) under ideal conditions. Yet if one of these guns is put in the hands of a non-benchrest competitor, you'll get half-inch or one-inch groups. The reason? Tiny groups come not only from the rifle, but from a shooter's ability to dope wind and mirage at a supernatural level.
A bad rifle in the hands of a good shooter will produce bad groups, but a good rifle fired by a poor shot will also turn in poor groups.
For shotguns, it's somewhat different. A really skillful hand can shoot just about anything well, but to turn in his best shooting, he has to have a gun that fits him perfectly and whose balance works for him. Uncle Robert Brister, who was the best I ever saw with a scattergun, could take anything and outshoot anyone (almost), but if he had his druthers, he would always go to a gun that particularly suited him.
A fellow named Dave Crosby, who worked for various gun publications and was a wonderful shotgunner, never used anything but a 30-inch-barreled Parker (I think) for everything. Skeet, trap, birds, it didn't matter. How he would have done with something else I do not know, but he was a wizard with that old side-by-side.