Originally Posted by
SnowFella
Bit of a difference there between direct fire rifle rounds that generally are supersonic when they pass or hit (pass close enough and you get a supersonic crack) and arty rounds that generally never break the sound barrier as you don't want to deal with the transonic region that will destabilize the round.
Mortar rounds are usually subsonic but howitzers are nearly always supersonic at max charge. For example the US M777 155mm howitzer has an MV of 827m/s at max charge (8S) and the British L118 105mm howitzer has an MV of 708m/s at max charge (6). Depending on the number of charge increments, each weapon will have a number of discrete possible muzzle velocities (MVs). You are half right in that ordnance designers will seek to avoid an MV that is anywhere close to the speed of sound and the possibility of not knowing whether it will go supersonic or not. (Lots of variables there, variability in speed of sound due to air pressure, temp and variability in MV due to barrel wear, charge temp, shell weight and more).