One idea I saw in another game was that some weapons could keep up to 1 ton of ammo on-weapon. It saves a critical slot, but you still have to pay for mass. Since this could also get applied to quarter ton machine guns, I house-ruled it to be up to 1/4 the mass of the weapon, or 1 ton, whichever is less.
The advantage is that you have one less critical slot full of ammo that can go boom. The down side is if the weapon got hit, you have a chance of that location going boom. Also, if one weapon runs out of ammo (for whatever reason), ammo that is on-mount for another cannot be used to load it.
So assume you have a light Mech with 8 machine guns (2 per arm and 2 per side torso). CBT Machine guns are half a ton each, so they can each have up to 1/8 of a ton of ammo on-mount. So instead of needing a critical slots to store 1 ton of machine gun, the designer chooses that each machine gun will have 1/8 of a ton of ammo on-mount, or 25 shots per machine gun. Later in the game, the PC has been preferring to use the left arm machine guns and as a result they are out of ammo. Because the designer put all the ammo on-mount, that means the ammunition stored in the other machine guns cannot be used to load the machine guns in the left arm. Perhaps the designer should have 13 shots per gun on-mount, and left the rest of the ammo (96 shots) in a central ammo bin, risking the ammo critical hit?
This would be fairly useful for handheld weaponry, where you aren't going to be reloading the weapon during combat anyways