Unless you rigged it correctly. For instance, if you, say, taped a 9mm Glock to your sleeve, and rigged a bit of wire and maybe wood or something to connect to the firing mechanism and then come up and around to the top of your arm, it COULD potentially save you the trouble of having to swap between your broadsword, which would cost you a valuable turn in the middle of a fight if it isn’t sheathed, and the Glock. Though it’d probably be prone to mechanical failure and you wouldn’t be able to do as good a job with both as you could with either individually.
It’d be situational and probably only useful in an already bleak circumstance either way, but it’d be kind of awesome for some higher-end professionally fabricated stuff. Arm-mounted fusion rifles instead of a fusion rifle mounded on your arm(stump).
Practical application of a fusion blaster CBM, anyone?