The AAR-1 or Anti-Armor-Rifle-1 is a rare item that only the most skilled shooters use. The great power of it comes from its simplicity, making it easy to modify. P.S. this is mad for PCs.

Range: 120/650 (this can be changed with modifications)

It takes a bonus action to set up the AAR-1, you have to set up the gun before you can shoot (this can be changed with modifications).

Lvl. 3 Choose mod. 1/G.C.I. or 1.5/M.R.S.

Lvl. 7 Choose mod. 2/B.B.C. or 2.3/M.K.R. or 2.5/T.S.M. or 2.7/M.B.B.

Lvl. 11 Choose mod. 3/S.R.C. or 3.5/E.M.G.

Lvl. 15 Choose mod. Choose mod. 4/S.C.B. or 3-Bullet-Mag: 

Lvl. 20 Gain mod. X/V.V.S. and X/U.G.C.

Mod. 1/G.C.I. Gunpowder-Capacity-Increase: The gunpowder that your gun can hold is slightly increased. Increase damage by 1d6

Mod. 1.5/M.R.S. Multi-Range-Scope: You have added a toggleable scope. You now do not have disadvantage on attacks against enemies with half-cover.

Mod. 2/B.B.C. Double-Bullet-Chamber: You have added an extra chamber to your gun. You now have two shots before you have to reload.

Mod. 2.3/M.K.R. Muzzle-Kickback-Reducer: The muzzle has been suppressed. You can now skip the setting up stage of fire or stand up at the cost of 200ft. of range.

Mod. 2.5/T.S.M. Toggleable-Shotgun-Mode: You have added two side chambers that give out small metal balls in a spray. You can now shoot a 15ft. spray of smaller bullets instead of shooting one. This deals 2d6 damage and you must take a long rest before you can use it again.

Mod. 2.7/M.B.B. Machine-Bolt-Barrel: Fire all of you're bullets at the cost of 400ft. of range.

Mod. 3/S.R.C. Sound-Repulser-Cannon: You have put a special chamber on your gun that releases a magical sound pulse that pushes back enemies. As a bonus action, you can make any creature within a 10ft. radius make a Strength check equal to 8 + your prof. modifier + your Dex. modifier or be pushed back 20ft. You must take a short or longest before you can use this again.

Mod. 3.5/E.M.G. Even-More-Gunpowder: The gunpowder that your gun can hold is slightly increased. Increase damage by 1d6.

Mod. 4/S.C.B. Siege-Cannon-Blast: Your gun is now designed to destroy structures. Deal An extra 4d6 damage to buildings.

Mod. 4.5/3.B.M. 3-Bullet-Mag: You increase your bullet capacity by one. You now have three bullets before you have to reload.

Mod. X/V.V.S. Variable-Vision-Scope: As a bonus action you can activate a magical scope that enables you to see through walls in your gun range. You have to take a short or long rest before you can do this again.

Mod. X/U.G.C. Unrestrained-Gunpowder-Capacity: As a bonus action you can open up a special chamber that greatly increases the amount of gunpowder you can load. Increase all the damage you deal by 1d6 with the AAR-1 until your next turn. You must take a long rest before you can use this again.

It’s up to you to decide whether a character has proficiency with a firearm. Characters in most D&D worlds wouldn’t have such proficiency. During their downtime, characters can use the training rules in the Player’s Handbook to acquire proficiency, assuming that they have enough ammunition to keep the weapons working while mastering their use.

Notes: Damage: Piercing, Range, Two-Handed, Ammunition (Firearms), Reload, Slow