Realistic weapons can hardly be fun, unless you design your game around high lethality (that is, give your game an easy chargen), or give your characters superhuman reflexes to dodge bullets, or insane armor/muscle to help them soak up damage.
Cyberpunk 2020 goes the armor way, and it is retarded. Any single weapon could practically kill an unarmored character, no matter their BODY (which is the equivalent of CON), but regardless of how buff your character is, you can put a Metal Gear on yourself, with 25 stopping power, and ricochet an entire 30-bullets cal 5.56 (5d6 damage) magazine like it's nothing. This leads to all PC wearing Metal Gears at all times "but muh social", even though in Night City everybody knows life is dangerous as shit, and being armored is just the smart thing to do, and all enemies wearing AP bullets even if it doesn't make any sense. Did I mention you can layer armor, with silly penalties and diminishing results that do not disuade any PC due to how nice armor is?
Anyway, OP, if you want a "realistic" guns system with high lethality, look up Cyberpunk 2020, then reduce most armor dramatically with the next rules:
>AP is now a flat value. Generally speaking, you want to have +4 or +8 AP for most average AP weapons and rounds, +15 for sniper rifles, and +20 to +25 to monoblades, monofilaments and vibroblades.
>Stopping Power for body armor is "capped" at +20. You can go slightly above for ACPA and other exotic armors, but really, something above that value has to be REALLY exceptional.
>Maximum values for the armor of each bodypart are: +20 to torso, +18 to head but usually only for PC (this is to stop random headshot deaths), and +12 for limbs. Average values for NPC should be +12 for torso, +8 for head and +6 for limbs.
>Skinweave only provides +6 to SP. Advanced skinweave (-1 ATTR) only provides +8 SP. Period.
>Custom armored clothing rules get their SP halved. You either gear for combat, or go incognito with backup plans, not both.
Now, the game is not exactly balanced this way. You will notice all fights still end within the turn they start, simply because the game is that fucking lethal. Apply the following rules if you want to be slightly less lethal.
>Notice how your char's health is represented by 10 groups of 4 tick boxes, distributed in two rows, in your char sheet? Scrap that shit. The amount of checkboxes each group has is now equal to your BODY. That is, if you have a BODY of 7, you have a total of 70 theoretical maximum health points, or 10 groups of 7 check boxes each. This means someone with BODY 10 theoretically has 100 HP. If you think that's a lot, we once rolled 76d6 to kill an NPC with a mounted turret. Points above 10 have diminishing returns, and only add one checkbox for each 2 points of BODY above 10.
>Rounds are now 1 second long. This means you have to divide all weapons' ROF by 3, except handguns which are already nerfed as they are.
>Specific bodypart aiming penalties are different now: aiming to a large bodypart (think torso, any part of the legs) is +2 to difficulty, aiming to a medium bodypart (think any part of the arms, any half of the leg) is +3, and small bodyparts (head, hands, feet, etc) is a +5.
>Successive action penalties during your turn get a +10 difficulty penalty, rather than the usual +3. Running as a last action is exempt of this, but running then shooting will give you that +10 to difficulty. Failing a consecutive roll ends your turn immediately, and kind of counts like a critfail. The GM chooses how to punish you; I recommend accidentally staying out of cover for too long, if they were trying to shoot from behind cover.
If you want to make stats that are not REF anything other than a dump stat:
>Semi-auto ROF for "uncapped" weapons (think fast-as-you-can-pull-the-trigger revolvers, and similar weapons with simple mechanisms) is equal to your TECH / 3, with a minimum of 1. This makes handguns slightly less underpowered.
>Long range targets can be shoot at by rolling with INT, at the player's choice. Ultra long range targets have to be rolled with INT, or TECH if we are talking about some weapon with an adjustable aiming mount, and you can add your Physics score to your weapons skill, alongside your INT or TECH and weapon skills. GMs, feel free to go way above 30 target difficulty for these shots, specially if they are not shooting a sniper rifle.
>Death and shock saves are now rolled via COOL for maximum anime feeling.