Here is a brief description of all the kits:

[ 1911 Emissary Kit ]
Based on: Springfield Armories 1911 Emissary
Larger magazine, more damage. Makes it comparable in performance to the military pistols, but you get super slick and quick reloads.

[ Ace Bone Collector Kit ]
Based on: S&W Model 500
Conversion to a single action hand cannon. Significantly slower fire rate, but sniper-tier damage.

[ Augewehr Parabellum Kit ]
Based on: AUG Para
Conversion to a submachine gun. Lower damage, higher rate of fire, more controllable recoil.

[ Avenger Raffica Kit ]
Based on: Beretta M93R
Conversion to a 3-round-burst machine pistol.

[ Bulldog Extended Kit ]
Based on: Uzi
Conversion to the full-size Uzi carbine. Slower rate of fire, more controllable recoil, higher damage. Accepts tactical attachments.

[ Cobra Brace ]
Based on: MCK Glock Brace
Conversion to a Glock carbine. Slower weapon draw, but significantly improved recoil control and accuracy. Allows optic attachments.

[ Crossbow Magazine Mod ]
Based on: Crossbow (Call of Duty Black Ops II)
3-round magazine attachment for the crossbow that allows for quicker follow up shots.

[ Desert Falcon Hand Cannon Kit ]
Based on: Hand Cannon (Call of Duty Black Ops Cold War)
An excessively powerful pistol fitted with a high powered laser.

[ Dragonfang Low Caliber Military Magazine Adaptor ]
Based on: M249 SAW using a STANAG magazine
Makes the Dragonfang take Low Caliber Military Ammo. Reload speeds are significantly faster, recoil is much lower, and damage is much lower.

[ Eaglefire Singularity Kit, PDW Singularity Kit, Eaglefire/PDW Singularity Blackout Magazine ]
Based on: AR-15 MOD 3 (Girls Frontline)
The Singularity Kit grants the Eaglefire and PDW a faster fire rate, lower recoil, and the capability of firing in semi, burst, or full auto. The magazine increases recoil and damage dealt. When the kit and the magazine are equipped together, reloads are significantly faster.

[ Focus DMR Omega Kit ]
Based on: Ohm (Call of Duty Advanced Warfare)
Grants a shotgun fire mode. The shotgun blast consumes up to 5 energy cells per shot. Each cell yields 2 pellets, meaning 10 pellets are fired when 5 energy cells are consumed, the most of any shotgun weapon.

[ Focus Rifle Basilisk Kit]
Based on: LV8 Basilisk (Call of Duty Black Ops III)
Replaces the fire modes to a singular mode of charged full auto. The gun charges up before unleashing full auto fire at an exceptionally high fire rate.

[ Fusilaut Fire Selector ]
Based on: FAMAS G2 Fire Selector
Enables full auto fire. Full auto has more recoil and deals less damage than burst and semi fire modes.

[ Hawkhound/Rubicon/Timberwolf Straight Pull Bolt ]
Based on: Straight pull bolt action rifles
Enables faster and smoother bolting for an increased rate of fire, less scope shake when bolting, and slightly faster empty reloads.

[ Heartbreaker Eclipse Kit ]
Based on: SCAR-H Magazine
Increases damage and recoil substantially, turning the gun into a battle rifle.

[ Hell's Fury Extended Barrels ]
Based on: M134 Minigun
Increases accuracy and greatly increases fire rate.

[ Herstal Operator Kit ]
Based on: FAL Paratrooper
Reduces recoil, greatly speeds up weapon draw, and greatly increase reload speeds if a drum is not equipped.

[ Kestrel Fire Selector ]
Based on: M2 Carbine
Allows full auto fire. Full auto fire deals reduced damage per shot.

[ Kreigs Präzisionsschützen Barrel/Stock ]
Based on: PSG-1
Conversion into a DMR. The barrel removes full auto fire capability, slows down rate of fire, increases accuracy, reduces recoil, increases damage, and suppresses the gun. The stock greatly improves recoil control.

[ Kryzkarek Stitcher Kit]
Based on: Stetchkin
Conversion into a suppressed machine pistol. Reduces recoil and increases accuracy slightly, and enables full auto fire mode.

[ Luger Carbine Barrel + Luger Stock ]
Based on: Luger Carbine and Artillery Luger
Conversion into a pistol carbine. The carbine barrel significantly lengthens draw time but enables full auto fire and tactical attachments. The stock reduces recoil.

[ Maplestrike Harbinger Kit + High Caliber Magazine ]
Based on: AR-15 chambered in .50 Beowulf
Conversion into a high caliber battle rifle. The magazine make the gun take High Caliber military ammo, fire significantly slower, with substantially more recoil, with a massive damage boost. The Harbinger Kit greatly reduces recoil of the gun.

[ Masterkey Extended Barrels ]
Based on: Double barrel shotgun with long barrels
Greatly extends effectiveness at range with a much tighter spread and slightly higher damage.

[ Masterkey Rifle Drilling ]
Based on: M30 Luftwaffe Drilling
Adds a new fire mode that fires a High Caliber Civilian bullet with rifle-like performance. 

[ Matamorez Valiant Kit ]
Based on: AS Val
Reduces damage, but increases recoil control and fire rate.

[ Musket Tactical Kit ]
Based on: Tactical Musket
Removes the bayonet, but allows customization of modern weapons, including any sight, tactical gadget, and foregrip.

[ Nightraider Centaur Kit ]
Based on: G36
Increases damage and recoil control and increases rate of fire.

[ Nightraider Dual Scope + Reflex ]
Based on: G36 Hybrid Optic
Has a midrange optic and a reflex. The reflex sight goes in the attachment slot on the dual scope. Toggle between the optics with N.

[ Nykorev High Caliber Box ]
Based on: PKM Box
Converts the Nykorev into a high caliber machine gun, similar to the Dragonfang. Significantly elevates damage, but requires more expensive high caliber ranger ammo.

[ Scalar Goliath Suppressor ]
Based on: Kriss Vector Suppressor
A massive suppressor that substantially reduces recoil.

[ Scalar Karma Kit ]
Based on: Karma 45 (Call of Duty: Infinite Warfare)
Allows the Scalar to accept two magazines for increased capacity. The weapon also shoots slightly faster.

[ Schofield Resistance Kit ]
Based on: Obrez Pistol
Grants an extremely fast pistol draw time and slightly faster rate of fire, but the gun cannot equip optics and has reduced accuracy and higher recoil.

[ Shadowstalker Spartan Kit ]
Based on: Spartan Laser (Halo)
Converts the Shadowstalker into a high damaged charge laser weapon. Harder to use, but deals much more damage.

[ Single Shot Shotgun Showdown Kit ]
Based on: Alofs' Magazine Tube
Adds a magazine tube that holds 4 extra rounds, allowing for repeated firing.

[ Sportshot Magnum Magazine ]
Based on: Ruger 10/22 Magnum
Makes the gun take High Caliber Civilian ammo, but deals increased damage at a slower firerate. But is the Sportshot finally good?

[ Sportshot Suppressed Barrel ]
Based on: AWC Ultra II
Grants sound suppression and reduces recoil.

[ Swissgewehr Fire Selector ]
Based on: SG 550
Grants 3-shot burst and full auto fire modes.

[ Teklowvka Assault Barrel/Stock]
Based on: Interdynamic MP-9
Conversion to a submachine gun. The stock mitigates recoil and the barrel enables full auto fire at an extremely fast fire rate. The barrel also enables attaching underbarrels.

[ Viper Constrictor Magazine ]
Based on: MP5/10
A slightly higher caliber magazine. Increases damage dealt and recoil experienced.

[ Viper Shadow Kit ]
Based on: MP5 (Shadow the Hedgehog)
Converts the Viper to a pump action weapon. Increases damage dealt, and reloading makes you feel like an ultimate life form.

[ Zubeknakov Defy Kit ]
Based on: AK-15
Conversion to a battle rifle. Reduces fire rate and increases recoil, but substantially increases damage dealt.