Ok, to get right down to it, there is realism and there is realism.
Guns: Some guns will run quite well with a high amount of abuse. some start fowling up pretty quickly, especially if dirty/substandard ammo is used.
Instead of having a "gun health" like stalker, you perhaps should be able to do a field strip and clean, if you have a cleaning kit. This comes into the crafting territory. So for balancing game mechanics, having guns get gunked up due to dirty, possibly home made ammo after a few hundred rounds is fine, causing missfeeds and stovepipes, so long as it is not a case of throwing crazy money at a technician every time a gun needs a clean. Then on top of that, you have the useful service life of a gun. meaning parts are worn out, barrel is burned out and it needs to be scrapped this would be 10-50 thousand rounds, depending on the ammo used. So really you are looking at two separate "Health" values of a gun.
I have personally had feed issues with rifles due to quite minor dirt and damage. the main thing that tends to give problems is impact. Magazines are the worst, often giving problems. Then the mag release catch and then the gas system. The main thing here is; don't believe the hype. guns are machines. Most times they work fine. Other times they do not. When they do not, you better have a back-up plan.
Armour; Level 2-3 armour, meaning kevlar fabric, flexible armour is only any good against unjacketed rounds. So, 9mm, 45acp etc. handgun and sub machine gun ammo usually. It can stop a whole clip of the stuff just fine usually. multiple clips even. Against jacketed ammo, it does absolutely nothing. You might as well wrap yourself in tinfoil. 5.56, 7.62 etc walks through it, and would keep going through another three or four vests just like it.
Plate armour is either ceramic, which does break and become pretty useless after the first few rounds have hit it, or you can opt for thick sheets of solid hardened steel. This stuff you can shoot it all day and you will not go through with conventional ammo. The down-side is weight. It weighs a total ton. All that being said, being hit with a big bullet into a piece of plate armour will hurt a fair amount, and will likely knock you over.
Now, with all these real-world options, the trick comes down to which pieces to implement into game-play. Having armour which has a percentage of protection makes no sense at all. armour either works or it does not. Personally I think a mechanic where by, soft armour is lightest, enabling you to carry more in your pack, ceramic armour is second lightest, but breaks really quickly, and steel plate is the heaviest, using nearly all your weight allowance. The two plates stop everything. The soft armour only stops 9mm and 45acp, shotguns etc.
I also would like to see a knock-down mechanic if you get hit with a calibre above 5.56ish. A 7.62x51 will knock you on your ass and feel like you were hit by a truck, regardless of your armour rating.
Also, with plates, you can choose to have one on your back, one on your front, or both.
Also, plates usually do not protect from side-shots, or gut shots.
Ok, just some ideas I thought I would throw out there.