It may or may not be the reason for the higher perceived recoil when shooting drastically longer shells than the chamber length, but here's my take on why it is. When the crimp opens and contacts the forcing cone, preventing complete opening, for a microsecond the entire load being propelled pushes against the forcing cone. Note that this is a FORWARD push. The load chokes itself down to get through the constricted case mouth as the gun is pushed ever so slightly forward. Then, after passing into the barrel the load passes without interference through the barrel and exits the muzzle where, and when, the laws of physics again cause gun movement. This time rearward. However, because of the slight forward push at the forcing cone the gun now has further to move as it recoils into your shoulder. This greater movement causes a harder "whack" when the recoiling gun meets resistance in the form of flesh and bone. This all occurs so quickly that we cannot perceive the forward movement from the short chamber, but I am convinced it occurs.
Some of us, being more recoil sensitive than others, can notice less difference than others. I can definitely feel the difference when a 3" shell is fired in a 2 3/4" chamber, but can't feel it when the difference is only 1/8".
One of the local idiots around here was, for a few days, fooling people into shooting a short barreled coach gun that he had gotten hold of. He was loading it with 3 1/2" magnums loads and betting them they wouldn't pull both triggers at the same time. I am not certain, but believe the gun has 2 3/4" chambers, certainly not over 3". He was forcing the shells into the gun and closing it before letting the (other) unsuspecting idiot shoot it. It is, amazingly, still sound. He never was.
