I'm in the process of setting up my game, and decided to come up with a way of actually stating what marines can carry.
House rule:
Marines have "slots" for the following:
Slots Item type
2x 3x grenades
3x "small" items (pistols, knives, small gear)
2x or 1x May carry 2 "medium" (basic weapons, or large melee), or 1 "heavy" (heavy weapons) items
Ammo:
5x Reloads for "small" items (shared across the all of them)
10x Reloads for "medium" items (shared across the all of them)
or
1x Reload for "large" item (basically just the ammo backpack, and only if they have the weapon)
Conversions:
3x small items = 1x medium item (effectively, they may carry 3 medium items, at the cost of no small items, or 6 small, and 1 medium, or even 9 small)
Small ammo = medium ammo (basically, 15 reloads without heavy, or 5 reloads with heavy)
3x grenades = small item
1x small item = 5 reloads
1x medium item = 15 reloads