If you try to pick up an M24, get ammo from a crate then try to reload it, it never actually reloads. If I recall correctly the shotguns used to have this bug but this has since been fixed, I haven't tried it for field-acquired Mosins however since I use the M24 over the Mosin.

This only seems to occur as Insurgents - if playing as Security, picking up a dead teammate's M24 can be reloaded as normal.

Seems like this occurs for bolt action guns belonging to the opposing team.