An often asked question is: What is the deal with all these different shotgun shell lengths?
The chamber in a shotgun is bored with a constant diameter for length. This is the length typically stamped on the gun. It is meant to be greater than or equal to the length of a fired shell.
Just in front of the chamber is an area called the forcing cone in which the diameter of the chamber is decreased over some distance to the diameter of the bore in the barrel. Its purpose is to quickly direct the shot and wad into the barrel bore. This was much more important in the days of paper shells as the thickness of the shell casing was significantly more than it is now and a minimum loss of gas pressure while the wad made the transition was desirable.
With advances in shell design, the forcing cone has lost some of it's significance. Plastic shells have much thinner walls and plastic wads are able to expand to match a much greater diameter than the old paper wads. Many new guns are being manufactured with larger than "spec." size bores and longer more gradual forcing cones. These both contribute to less deformed pellets and lower pressure peaks resulting in better patterns and lower recoil.
Now, about shooting shells longer (after fireing) than the chamber in you gun... DON'T DO IT! If the end of the shell extends into the forcing cone area when the gun is fired, it results in a constricted region and higher pressures. How much higher and whether it is safe or not is entirely dependent on the individual gun/ammo.