Scenario:
Bad things are happening. Adrenalin is pumping, and panic is not far away. It's twilight so vision is slightly impaired by the low light. You are stressed, but not useless. You are without your gun! A buddy's friend throws you a spare firearm because there are ZombieGoblin Terrorist Bad Guys coming right around the corner. Are you gonna be able to wield it at least semi-effectively? There is no time or light to look it over. You can assume it is already loaded or you would have been tossed ammo, too. Are you able to reload it from the ample supply after the initial rush?
If I got tossed a revolver? No problem. "Revolvers work as easily as a fork." Not much difference between them for 95% of the revolvers he might toss my way. If it's a Colt, I'd have less of half second hesitation realizing I should pull instead of push on the cylinder release.
If I got tossed a Garand or M1A? Or 1911? Also no problem. A hitch might be the non-ambi safety on the pistol, but, again, half a second to figure that out. It would have to be some weird configuration to throw me for longer that that.
I've used to those models often enough they are like old friends.
But what if it is something else?
I'd have trouble with any pump shotgun of any type. If it hangs up I don't know how to remedy it. I don't instinctive know the fiddly bit I need to push to reload it. And I am not super familiar with shotguns in general. If he threw me a Reminton Model 11 or a Browning Auto-5 I'd do better, but even those can have variation in the safety. And I don't instinctive know which position is safety on/off. I've shot one, but only a Mossberg. I have no idea is the fiddly bits are the same with Remingtons.
An AR? I could probably handle this. Again, with the safety, positioning is not instinctive for me yet, but Up and Down is pretty obvious. I could see screwing up safe with full-auto/burst as it is horizontal. I know how the mag and bolt release work and can work those under stress. Forward assist, too. It wouldn't be super smooth, but I could get it into the fight against the ZombieGoblin.
An AK? I'd never even held one in my hand. That said, it's a pretty simple platform. Port cover up, safe, down, is not safe. Work the bold to cycle a round. Mag release is like an M1A, sorta. I bet I could be ok. Better than with a pump shotty.
Glock/S&W/HK/Regular-SIG pistol? Yeah, no problem. Even with HK's mag release. That would only slow me down briefly. I'd have to be pretty stress to fail to find that. Other than that, there isn't much to them. Almost as simpe as the revolver.
Lever or Bolt gun? Again, no problem unless the safety is unfamiliar. Even the total lack of a safety can be a problem if you are expecting it and waste time trying to unengage it. Loading a Krag might delay me.
Luger? I've half field stripped one. Once. Poorly. I know where the safety is and sorta how the mag release works. I can cycle the action.
Other things? That might be a problem. I've never looked at an FAL. I have no idea where the stuff on it is. MP5s? Nope? Tommy gun, BAR? I've shot an MP40. It was simple enough, but not instinctive for me to use, obviously. And M2 Browning? No, but that might not take long. Yank the handle, try to the butterfly trigger. If it hangs up I have no idea how to get it back in the fight. I don't know the Beretta pistol.
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The point I'm getting at is it might be a good idea for me to get MORE familiar with likely 'found' weapons. At least of the more common varieties. Terrorist drops his AK in the the mall rampage... Cop is shot dead and I can get to his duty belt and retrieve is pistol to try to ward off the horde of bad guys... I escape the zombies by crawling into a tank and the dead soldier inside has a Beretta.... It'd be a good idea to not have to familiarize myself for 5 minutes before needing to use the random gun.
I need to shoot an AR more. I need to shoot a pump shotgun more. I need to at least try an AK model once.