after doing some testing I think ive narrowed down these mechanics enough to make a post on them, im going to go over what effects recoil, reload speed, bullet drop, muzzle velocity and penetration.
im going to go over each accessory type that effects each of these, as most of them affect more than one category. when i say something increases reload speed i mean it takes less time to reload, if i say it decreases reload speed it takes longer to reload. slight increases in muzzle velocity probably wont be noticed during gameplay.
Stocks effect recoil and reload speed.
Compensated stocks decrease reload speed, and decrease recoil. good for full auto or sniping from a distance.
stock buttstock / extended buttstock are identical on all guns except the ACR as far as ive found, and in this specific case the extended stock increases recoil and reload speed.
short stocks / folded stocks increase reload speed heavily but also increase recoil. unfortunately there are no guns that can equip both and im to lazy to deal with percentages, so im putting these as the same.
quite simply, the larger the magazine, the slower the reload.
standard magazines are good for reload speed
extended are bad for reload speed
large mags are worst for reload speed
grenade launchers increase reload speed and might increase horizontal recoil.
AFG/Short shift grip both increase vertical recoil and decrease horizontal recoil. the short shift grip has an increased effect for both of these. the AFG increases reload speed slightly, whereas the shift short grip deceases reload speed.
Vertical foregrip/ V2 / V3 / V4 all of these grips do the same thing, but are scaled with their level from 1 -4. all of these grips increase horizontal recoil, decrease vertical recoil and increase reload speed.
some of the side rail options seem to change the recoil patters on the weapon itself, causing the recoil of the weapon to pull to the right.
ATPIAL laser sight decrease horizontal recoil but increase vertical recoil
3-Dot laser sight seems to be a strictly better version of the APTIAL laser sight, decreasing horizontal recoil more and increasing vertical recoil less
Range finder will increase vertical recoil more than the ATPIAL and decrease horizontal recoil less than the ATPIAL, but will decrease the bullet drop for the weapon
so while this isnt a weapon accessory i feel like i need to clarify this before going onto the barrels and muzzles for your gun.
from my testing ive found that there are 5 levels of cover, and 4 levels of penetration.
regardless of what your guns penetration says, its still has one of 4 penetration levels, which are influenced by the barrel of your gun and whether or not there is a suppressor on it. each barrel gives you level 2, 3 or 4 penetration, but a suppressor will take one level away from the barrel. if your gun is level 1 penetration (short barrel + suppressor) you wont be able to penetrate any type of cover except windows regardless of the penetration stat on your gun
level 1 cover includes windows where any weapon can shoot through it.
level 2 cover includes wire fences where every weapon with level 2 penetration or higher can shoot through.
level 3 cover like sheet metal or the cover on railings can be shot through by anything with level 3 or higher penetration
level 4 cover includes the walls of some buildings, only guns with level 4 penetration can shoot through this
Level 5 cover includes closed off buildings, or anything thicker than an average wall, no guns in the game can shoot through this cover.
Short barrel decreases vertical recoil, increases horizontal recoil, increases your reload speed and gives your gun penetration level 2 slightly decreases muzzle velocity and increases bullet drop
Standard barrel is an in between for long and short barrels, its good for reload speed, horizontal recoil, vertical recoil and gives penetration level 3 average muzzle velocity
long barrel is the worst barrel for reload speed and vertical recoil, but is the best for hoizontal recoil and penetration, giving your gun level 4 penetration and sightly increases muzzle velocity aswell as decreasing bullet drop
Compensator reduces both horizontal and vertical recoil may have a slight effect on muzzle velocity
Compensator V2 does the same as the compensator but better, also slightly increases muzzle velocity
Suppressor heavily Reduces muzzle velocity and decreases penetration by one level
EDIT1: typos and formatting
EDIT:2 side rails findings