The problem with guns being deadly is usually people get tied up in The Myth of the Gun.
Oi. I played a homebrew system that my brother-in-law spent years perfecting. (He and his buddy are big The Forge followers). Lovingly researched with data compiled from CIA and FBI databases on GSW, etc etc, with detailed bell curve plotting and direct mathematical links between published bullet velocities and ranges, etc etc.
Totally unplayable. You'd spend more than an hour making up a character and then the first trouble or combat get shot or stabbed, incapacitated, and die. That was it. I call it: "Bang, yer dead" Gaming.
If you're going to make it mathematically improbable that anybody will get in a gunfight and survive ... just say: "Bang, yer dead" and be done with it. Don't bother with dice or MDTs or WP or anything.
I've been a big detractor of Myth Of The Gun game styles since then.
Now, it's possible to make guns PSYCHOLOGICALLY scary, without making the game unplayable. You can, say, have people in a gunfight make saves vs. being Shaken if your players are the kind of people that refuse to be scary. You can make things SEEM deadly so they don't want to get into things ... crank down the MDT to 10, make the save 10+(1/2 damage) ... that's "gritty" but it lets the characters get hit and have to make a save, but spend APs on those saves so it eats up other resources.
Here's the thing ... if you're making detailed special rules on guns, that means guns are going to be in the game ... they'll be "rare", but not so rare you're not going to be including them at every opportunity ... possibly more than they would be, otherwise, because now there's a FOCUS on the rare guns. Like gasoline in Mad Max ... everybody wants the gas, so gas is the whole darn focus of the movie. Gas may be "rare" but we're focused on it now so it's the most common thing in the movie. We've got tankers full.
If you're making deadly-rare guns, they're going to show up, and if they show up, each PLAYER CHARACTER will be on the recieving end of the hurt more often than any single NPC. The NPCs will show up and leave, dead or alive or whatever, and maybe only have to encounter the guns once, while the PCs are going to be experiencing them every time they turn around, and LOOKING for them and looking for the bullets. So anything that affects NPCs affects PCs tenfold.
Good gaming, the players FEEL LIKE their characters are mortal and guns are scary and they need to take cover ... but they're really not that fragile ... or else you're running through sheets and nobody cares about their disposable one-shot-killed characters and ... oddly ... guns become LESS scary. They just lead to a disconnection between the player and his character. It's like playing Call Of Cthulhu, where, really, the whole darn POINT of the game is your character getting taken out of the game in some bizzare way and not the character itself.
--fje