From a 10.5" barrel there are quite a lot of very hot particles under quite a lot of pressure. They exit the barrel and hit the first baffle in the can, the blast baffle. These particles damage the baffle. Over time the baffle gets eroded by these particles giving it an appearance that Phil Dater likened to the surface of the moon. In longer barrels (14" 16" etc.) there is a little more room for the pressure to drop and less damage is done to the baffle.
Many industry professionals say 10.5 is the shortest you should try to run with a can.
Perhaps you should read on some of the real suppressor boards.