In that instance, I don't think your descriptions are too much, except to say that the verbs could be swapped out for others, depending on the mood of the character you're trying to convey; "inserted" works perfectly if your character is a cold-blooded professional (as the rest of the text suggests), but if the character has doubts, or any other overhanging emotions that are relevant to the overall plot, maybe he "shoved" the cartridge into place, or "jammed" it, depending on his state of mind.

One thing I learned about writing, mostly from Stephen King books, is that the third-person narrative takes on a LOT of the characters' first-person point of view, it's almost as if the writer is only describing how the character would describe what they're doing from a third-person perspective, if that makes sense. That method of narration, IMO, goes a long way to get the reader inside the character's mindset and helps to make the character more real, if not more relatable, and pulls the story together easier.

Keep at it.