The story, as I understand it, is this. A pipe with an army/military bit is a pipe in which the stem does not fit into the briar itself, but inside a metal ring that is inserted into the shank. The thought behind this is that you can pull the pipe apart while it is still hot without worrying about damaging the wood inside the shank, since the metal that the stem fits into does not expand and grip the stem tightly like briar does when it is heated. It is named a military/army bit because these pipes were used by soldiers in the field during battle. If they would be in the middle of a smoke when the call to duty came, they could pop the pipe apart and throw it in their pocket/bag/whatever and not worry about damaging the wood by pulling the stem out.