There are 12 stats on weapon that are shown in inventory.

• Magazine
  
• Total ammo
  
• Rate of fire
  
• Damage
  
• Accuracy
  
• Stability
  
• Concealment
  
• Threat
  
• Reload time

• Damage
  
• Knockdown
  
• Charge time
  
• Range
  
• Concealment

The number of shots held in a single shot.
SMG, LMG and Assault rifle
Having Surefire - basic increases magazine size by 15 for SMG, LMG and Assault rifle.
Shotgun with magazine
Having Close by - aced increases magazine size by 15 for shotgun with magazine.
Non DLC shotgun with magazine:

• IZHMA 12G shotgun (primary)
  
• Goliath 12G shotgun (secondary)

Pistols
Having Gun nut - basic increases magazine size by 5 for each pistol. (Increases by 10 for akimbo pistols)

Maximum number or ammo you can carry for that weapon.
All weapons
Having Fully loaded - basic increases your total ammo by 25%.
Akimbo weapons
Having Akimbo - aced increases our total ammo by 50% for akimbo weapon.
Saw
Having Portable saw - aced increases your saw blade by 1 and increases its durability.

How constant you can fire your weapon.
Pistols
Having Gun nut - aced increase the rate of fire of your pistols by 50%.
Shotgun
Having Close by - aced increase the rate of fire of your shotgun by 50%

Damage of the weapon (pretty self-explanatory)
Shotgun
Having Shotgun impact - aced increase damage of shotgun by 15%
Having Overkill - basic increase damage of shotgun or saw by 75% (last for 20 sec) (after killing an enemy with shotgun / saw)
Pistols
Having One hand talent - aced increase damage of pistols by 15 (not percentage)
Having Trigger happy - basic increase damage of pistols by 20% every time you hit with pistols, stack for 4 times (last for 2 sec)
Silenced weapon
Having Specialized killing - aced increased damage of silenced weapon by 30%
All weapon
Having Overkill - aced increase damage of all ranged weapon (exclude rocket / grenade launcher) by 75% (last for 20 sec) (after killing an enemy with shotgun / saw)

How accurate will the shot be, higher means the shot will be more likely land in the same position when aimming at the same place and vice versa. Around 80 accuracy is already laser-accurate. (Except some grenade launchers)
All weapon
Having Stable shot - aced increase weapon accuracy by 16 (while standing still)
Having Steady grip - basic increase weapon accuracy by 8
Having Fire control - aced increase weapon accuracy by 12 (while hip fire) and reduce accuracy penalty by 20% (while moving)
SMG, Assault rifle and Sniper rifle
Having Rifle man - aced increase weapon accuracy by 16
Having Marksman - aced increase weapon accuracy by 8 (single fire mode) and 20% (aim down sight, single fire mode)
Silenced weapon
Having The professional - aced increase silenced weapon accuracy by 12
Pistols
Having Desperdao - basic increase pistol accuracy by 10% stack to 4 times (last 10 sec) (when you hit with pistols)
Having Equilibrium - aced increase pistol accuracy by 8
Shotgun
Having Far away - basic increase shotgun accuracy by 40% (aim down sight)

The recoil of your weapon when you spray, higher stability means lower recoil and vice versa.
All weapon
Having Stable shot - basic increase weapon stability by 8
Having Steady grip - aced increase weapon stability by 16
Silenced weapon
Having The professional - basic increase scilenced weapon stability by 8
Akimbo weapon
Having Akimbo - aced reduce akimbo weapon stability penalty by 16
Shotgun
Having Shotgun impact - basic increase shotgun stability by 8

Concealment affect your detection risk and dodge gain while using artful dodger tier 4 skills
Silenced weapon
Having Optical illusions - aced reduce silenced weapon conceal by 2 and increase concealment by 1

How likely enemy will crouch while fighting you or target you, higher threat means there is a higher chance and vice versa.

• Threat level can only be changed by using different weapon mod
  
• Silenced weapon usually have very low to none threat

Time needed for long reload (when you empty your entire magazine)
Reload time can be reduce by using some weapon magazine mod (E.g. speed pull magazine) or skills
All weapon
Having Bloodthirst - aced increase weapon reload speed by 50% (last for 10 sec)(trigger by killing an enemy using melee weapon)
Assault rifle, SMG and sniper rifle
Having Agressive reload - basic increase weapon reload speed by 15%
Having Agressive reload - aced increase weapon reload speed by 100% (last for 4 sec) (trigger by single fire mode of AR, SMG, SR)
Shotgun
Having Shotgun CQB - aced increase shotgun reload speed by 35%
Auto fire mode weapon
Having Lock n' load - aced increase next weapon reload speed by 100% (reduced by 1% for each bullet above 20) (trigger by killing 2 enemies)
Pistols
Having Desperado - aced reduce pistol reload speed by 50%

Damage deal to enemy
--------------------------
Having Pumping iron - aced increase melee weapon damaged by 100%
Having Berserker - basic increase melee weapon damage up to 250% (below 50% health)(around 30% health for maximum damage)
Having Bloodthirst - basic increase next melee weapon damage by 100% (stack for 16 times)(trigger by killing an enemy)

How likely you can knockdown your enemy on the floor, higher knockdown means higher chance and vice versa.
-----------------------------
Having Martial arts - aced increase melee weapon knockdown by 50%

Time needed to charge your mele weapon and deal highest damage
No skills can change the charge time

Melee weapon's attack range, higher range means farther it can reach and vice versa.
Highest range avaliable is 275
No skills can change melee weapon's range

Same as ranged weapon concealment, it affects the detection risk and dodge chance gained while using artful dodger tier 4 skills
Highest melee weapon concealment without skills is 30
--------------------------------------
Having Inner pockets - basic increase melee weapon concealment by 2
If you find this guide helpful, feel free to share with others
And you can find out more of my guide here!