You simply can't, with today's capabilities, make guns balanced on where hit land on a target. We see it countless times where a sniper hits a guy in the foot for a OHK, a high powered AR unloads on a guys face but still only manages a 4 hit kill.
My idea is to change damage multiplier into chance for crit hit. As of now we have multpliers which grant extra damage to certain parts of the body. The issue I see is that a multiplier guarantees certain damage which thanks to inherent hit detection issues cause for unfavorable results.
My idea being a chance for crit damage does multiple things. The idea is that you simplify the areas of the body. Head, torso, and extremities. The head is 100% chance for crit damage, torso is 75%, and extremities is 20% (for instance).
Each weapon has a it's standard base damage and it has a critical damage chart that obviously deals more damage. This adds a new stat to the mix and allows for better classification to each weapon. Now SMGs would have a lower crit damage because typically SMGs use pistol rounds which don't deliver nearly as much kinetic energy and only probable chance. Snipers will have high, certain crit damage. ARs will be somewhere in the middle with moderate crit damage and probable chance . LMGs can be quite diverse as that's just the nature of the class with some weapons having high crit damage and low and some having low crit damage with a higher chance.
Over the past several titles certain weapons become given. But when you mix chance into a weapons stat you add a very much needed risk/reward system. This greatly opens up the weapon area and can easily make all weapons more interesting as you adding a stat that can completely change the way weapons are balanced and viewed.
Another thing this does is making sniping a bit more fair. When it comes to sniping only two things are factored in. RoF and accuracy. Range is obsolete and damage is obsolete because they're typically all the same minus that one sniper that's semi-auto with less damage.
To make snipers a more diversified class you can add crit damage and chance to open up the class for interpretation while at the same time simplifying the issue of "I shot him in the foot, why did I get a OHK?" Now we have a situation where it is possible to get a OHK or not get a OHK and it will actually make sense.
With sniping you change the dynamic of what a OHK is. The current system adds a multiplier which doesn't make sense as every sniper has infinite range and 98 damage (minus the outlier). With a crit damage system you can make all snipers have a 90%+ chance of getting crit damage and then play with the crit damage to make some snipers almost always OHK, some mostly always a OHK, or some rarely a OHK (like rifles designed for QSing). A shot to the head is 100% for a sniper, a shot to the chest is 90% , or a shot the extremities is 50%.
How the weapon balancing would work would be to divide the chances up into classes.
With "S" rank rifles and "A" rank rifles you rewarding the high power, high recoil, low RoF weapons with high degree of crit hits which give these weapons high chances for OHK or two-shot kill potential. Weapons with "S" and "A" ranks are limited to snipers and MRs.
With "B" rank you can reward burst and semi-auto ARs high recoil, low RoF weapons with a moderate chance. Still these weapons are limited to players who are willing to be a bit more mobile so to speak. The weapons with "B" rank will be burst, shotguns, low powered MRs, and QSing rifles.
The "C" rank is for your full-auto ARs of the more powerful variety. These ARs are typically the higher recoil or lower RoF. This would be your AK-like weapons, high powered LMG that have a lot of recoil, as well semi-auto low powered shotguns. These weapons can only earn a OHK for headshots or with shotguns that are ADS'd and in close proximity allowing their spread to be tightly delivered and accurately. Two-shot kills are the primary reward and are given to the head and, depending on the crit damage rating, long range 3-hit kills.
Your "D" rank weapons are high powered SMGs, average ARs and LMGs, and high powered pistols. This rank is for the more middle ground weapons that balance RoF, range, and base damage such as your ACR/Remington, MTARX or AK-74u, or run or the mill LMG. It's design is to never allow a OHK but give you an accuracy bonus that MAY give you a quick 2 shot kill to the head or a 3 shot kill to the chest.
Finally, "F" rank is for weapons that are designed to be sprayed. This is primarily for full-auto pistols, high RoF SMGs, and large mag high RoF ARs such as the FAD. These weapons are not designed to deal damage but rather overwhelm your opponent with a high RoF and large mag. Because the amount of rounds it fires it will still hit the crit total often even though it percentage is low. This means that if you can remain accurate you will get a faster kill most of the time.
I would also like to point out that crit damage is important too. Typically the bullet that is used determines crit damage. The higher caliber bullet the higher the crit damage. This runs, typically, in concordance with base damage. However, this can be exaggerated in a video game setting to make for more interesting weapons balancing.