SCAR-H, which is a 7.62x51mm gun, should have 20-round default magazine size, not 30. I’m not 100% sure why but it seems most, if not all, 7.62x51mm real-life guns have 20-round magazines: M14, FN-FAL… SCAR-H is no exception. Weight issue? Feed issues if more than 20 rounds?
Also, it seems the game has “50-round drum magazines” for SCAR-H, and I’m fairly sure those don’t even exist.
Edit:
Anyway, it’s easy to fix: Edit the file \data\json\items\magazine\308.json and the SCAR-H mag entries are at the bottom.