The power tool of choice for doing single file animations is Motion Builder.
With Motion Builder you can create separate takes based on a copy of the primary motion and once done as a set each take can be exported to a single file based on the name of the take.
The other option is to check if you weapon of choice supports animations layers but as Polyx stated individual files is the best way to go as far as as a workable source chain goes as anyone can added animations and corrections using the base source and the addition or fix re-imported from the asset pool.
Also working from one big file can add key frame ripple that is difficult to find once introduced and you don’t catch it until things turn funky in game.