Hi bagrat,
the 20mm round is considerably bigger than the rounds that you normally see out of the american or british planes (altough the later spits, hurricanes tec have them too, British planes), The Americans at that time favoured the .50 calibre round (in itself a fair sized bb).
The difference though, is in the size of the round. A .303 is precisley .303 of an inch. A .50 Cal is .50- of an inch. 20 mil is . pretty close to 3/4 of a inch in diameter. It was a pretty devastating round, even then with no explosive charge.
So, In answer to your question, There are no "explosive" bullets here in AH. Only different Calibers (and by weight and type different trajectories).
Hope this makes sense to you. (I'm a bit beebled tonight, but welcome to the BBSa and Acesd HIgh
cheers,
RTR