It’s projectile weight. The common sizes of 9x19mm ammunition (what we usually mean when we say 9mm), are 115 grain, 124 grain, and 147 grain. NATO standard is 124, but most people shoot 115 at the range because it has less weight and it cheaper.
Lighter projectiles, all else being equal, will have higher muzzle velocities, but will also slow down more quickly and are more prone to wind shift.
It’s projectile weight. The common sizes of 9x19mm ammunition (what we usually mean when we say 9mm), are 115 grain, 124 grain, and 147 grain. NATO standard is 124, but most people shoot 115 at the range because it has less weight and it cheaper.
Lighter projectiles, all else being equal, will have higher muzzle velocities, but will also slow down more quickly and are more prone to wind shift.
Mass, like a church? Like like the measures on the façade of the Torre Civica in Assisi?
/s, #anything but metric