When you fire a shotshell, the part of the shell that is crimped expands so that it is flush against the chamber walls. This is why the chamber is long enough to allow the fired length of the shell.
If you fire a shell that is longer than the gun is designed for, then this extra shell length has to expand out into the bore. Since the wad and load has to squeeze through this constriction, it increases the pressure when firing.
Yes, you can get away with it, but for how long? The gun is designed to withstand a certain amount of pressure, and eventually this could cause a dangerous condition.
My guns are older.. I got them second hand. So I dont want to take the risk of exposing them to the higher pressures.
I am gonna do some patterning, like Timbr suggested, so I can figure out my maximum effective range. When I can afford it, I will go out and buy me a 'real' turkey gun.