At the moment, it seems muzzle velocity is only defined by the type of ammo used. I.E. all 6.5mm weapons fire at 800m/s. Barrel lenght is not taken into account at all.

This is a bit sad considering how good bullet physics we have in Arma3. It also means that a short carbine type weapon will be just as good as a designated marksman weapon at long range if fitted with same optics. That is not realistic.

All 5.56 except underwater ammo fires at 930m/s at the moment. To make that happen IRL you will need a fairly long barrel, longer than any of the ingame weapons.

This is how I think it should work:
Muzzle velocity should be defined in weapon config.
Magazine config should have a multiplier that impacts muzzle velocity, normally set to 1.0.

Example of real life muzzle velocitys:
Mk20 (FN2000): 900m/s (not 930 as it is ingame)
Carbine versions (short barrel): Hard to find info on these but it is realistic to take away 5 - 10% from the normal version becouse of short barrel.
TAR-21: 910m/s
CTAR-21: 890m/s

So why is this important to fix?
It gives the gameplay some variety. As the game stands now, every weapon is the same, it's just different graphics and sound effects. Arma tries hard to be authentic but fails when it comes to muzzle velocity.