Pool balls ar ok, but you have to call your shots.
I can remember being stationed at Fort Devens, MA when in ASA. At some point one of the officer classes put a set of pool balls in the pack howitzer used to fire the morning salute for reveille. They were discovered (after the fact) and took up a collection for all the windows (buildings) and windshields that were taken out.
I'm sure that there is a standard size AND a commercial professional organization regulating that size - somewhere, as there is regulating the size of bowling balls.