Flippin' metric system... it took me a second to understand what you were talking about.

Anyway, what Pygmy said. So, if you look at your barrel, it'll say the gauge (bore), and the length of the chamber. Something like 12 gauge 3" chamber (or for you metric users, 12 bore 76mm chamber). This means that the chamber is long enough to accommodate 3" (76mm) shells including the extra length when the crimp opens. Essentially the chamber is a cylinder, and for 3" shells the chamber will be about 3.25-3.5" long before the forcing cone. After the chamber comes the forcing cone on barrels that are not back-bored. The Forcing cone is a gradual taper from chamber diameter to bore diameter which smoothly and safely funnels the shot into the bore. If the shell is shorter than the chamber, then there is just a longer "jump" between the shell and the forcing cone, but it is still perfectly safe.

The danger is in using shells LONGER than the chamber. In the case of your shotgun, it would be shooting 3.5" (89mm) shells in your shotgun. Because of the chamber length necessary to accommodate the opened crimp, a 3.5" shell might chamber in a 3" chamber. If this occurs and the gun is fired, the crimp will open against the forcing cone creating a greater than bore diameter constriction of the shot column/slug immediately upon ignition. This creates what is basically a partial bore obstruction, which can cause pressure levels to spike well above the levels the barrel can handle, resulting in the catastrophic failure of the shotgun. To put it in simple terms, you'd be holding onto a hand grenade when it goes boom. Not a great thing for you unless you really like the idea of spending the rest of your life being fed through a straw.

Mike