This could, in my view, be implemented as either a trait, parallel to the ordinary traits for range weapons or instead of traits for range weapons.
I am thinking of stuff like; Amputator Shells, Bleeder Rounds, Dumdum Bullets, Expander Rounds, Man-stopper Bullets and so on.
The way I see it is that using special ammunition gives us more powerful shots against certain targets like how Bleeder Rounds causes bleeding damage, Dumdum Bullets makes a mess of targets without armour and so on. But in exchange we get a reduction in the ammo we can carry with us in the mission. The more powerful the shots we got, the less ammo we bring along.
The same can perhaps be done with las-weapons in that we can how powerful shots we want to use but in exchange, more powerful shots will use off the charge pack in the las-gun quicker?