SM Ammo loads

[Eviscerator]

A regular division expects to go into battle with support and resupply available if required. Special Forces must carry in everything required for the mission.
While Marines do not require sleep, food, or a breathable atmosphere and can survive wounds that would kill a normal man, the one thing that they cannot and will not expect to find on the field are munitions to feed their weapons.

Information in Codex:Wargear and Salamander novels hint that even between loyal Chapters the ammo/magazines may not be suitable for replenishment. Wargear lists a Godwyn MkVb bolter that uses sickle mags, while the Marines Malevolent encountered in Salamander carry have lengthened stocks with extended shoulder, use drum mags and have sarissas attached. Further on the Salamanders comment that neither the drums nor the individual shells would be suited for their weapons.

Normal Missions in the Pre-heresy era might have been large scale army battles with the odd surgical strike but the Norm today is that any force from a squad all the way up to multi-company level can and must expect to run out.