The hand angle and the sights just don't work for me. Put down the Glock and pick up a CZ or a Sig and it's a huge improvement.
When I said handguns are essentially the same I meant with respect to their function, i.e. safety, mag release etc. are generally laid out similarly according to a small number of patterns, But since you have a single point of contact then the grip will make a big difference. You'd think I'd be more sympathetic to this idea because these days I'm almost exclusively a wingshooter & in the shotgun world, gun fit is all important. I also use both single selective and double trigger guns, and switching can be a comedy of errors, I assure you.
But I'm just not sure this kind of thing is worth worrying about in game terms, because the primary military small arm is the assault rifle and if you can hit with one, you'll hit with another. Long ago in my misbegotten youth I went through the Canadian army's conversion from the FN/Sterling/Browning MG to the M16/M249/MAG 58, and although of course we went through conversion training, hitting the target
wasn't a big concern. Adjusting from an antediluvian machine gun to a modern one in terms of maintenance, stoppages, etc. certainly was.
To gain familiarity you need study it for a long action of an hour or two where you basically either field strip and reassemble or otherwise handle it to figure out how to clean unjam maintain etc. requires a successful tech role.
I think this is on the right track, but I don't think it should require a skill roll. Spending a shift messing around with the unfamiliar weapon should be sufficient with modern small arms.
you also need to fire it either at a range or in combat and get 4 or six successes over multiple shots at the -1 penalty. This would represent zeroing it in.
The game really doesn't care about zeroing, but there's a misconception here: all zeroing is doing is aligning the bore with the sights, so for most purposes if a rifle is zeroed for me then it's zeroed for you. If we really want to get into zeroing then we have to periodically zero our rifle and we need to worry about the rifle losing its zero. Do we need to roll at the outset of combat to see if your weapon has kept its zero? And the really bad news is, you're gonna need to find yourself a boresighting tool for the main gun on your tank, and periodically zero that, too.
To me, this just isn't worth getting into, because this is an RPG, not a simulator. Having the PC jump behind that NSV only to have it jam when he has no idea how to clear it is RPG fun; keeping track of which specific guns he has a -1 DM to hit on and how many hits he needs to achieve to overcome that limitation is IMO just too much trouble. YMMV of course.