"Less Lethal" (rubber) rounds are similar to regular rounds by dimensions and heft, and also if you don't READ THE LABEL. All "Less Lethal" (rubber) rounds that I have used were clearly marked as such, and it is also evident from looking at and feeling the end of the projectile.

A few years ago a Portland, OR police officer carelessly shot a suspect with buckshot (the guy lost his leg) instead of a rubber slug. The officer was rightfully fired.