Basic Machine Gun

Quick facts

• Weapon Type: Martial, ranged (firearm style)
• Base D&D analogue: Heavy crossbow (range/weight/role).
• Damage (single shot): 1d10 piercing
• Properties: Ammunition, heavy, two-handed, special (see below)
• Range: 120 / 480 ft
• Weight: 15 lb
• Rarity / Attunement: Very Rare; requires attunement by a character with martial weapon proficiency (optionally: proficiency with firearms if your table uses that).
• Cost (craft/buy): 1,200 gp (craft: 800 gp + 10 days + Tinker’s tools DC checks), Repair parts/month: 5 gp
• Ammo: 1 bullet per single shot. 50 bullets = 5 gp, weight 1 lb per 20 rounds.

Attack & firing modes

You must load the weapon with ammunition (1 bullet per single shot). The machine gun has three firing modes. Switching modes is a free object interaction.

Single Fire

• Action: Attack with a ranged weapon attack (Dex or Strength as your table prefers; default: Dexterity).
• Cost: 1 ammo.
• Damage: 1d10 piercing + your Dex modifier on hit.
• Notes: Use normal critical rules (crit doubles weapon dice).

Three Round Burst

• Action: Make one ranged weapon attack (single attack roll).
• Cost: 3 ammo.
• Damage on hit: 3d10 piercing (roll once, add Dex mod once).
• Recoil: After using a burst, you take a 2 penalty to attack rolls with this weapon until the end of your next turn (represents recoil recovery).
• Balance note: Burst is a reliable single-target burst; it uses more ammo and causes recoil.

Full Auto (Suppressive / Area)

• Action: Choose either (A) a single creature or (B) a 30 ft line (suppressive fire).
  • (A) Single creature: make an attack; on hit, deal 5d10 piercing (cost 5 ammo).
  • (B) Line (30 ft long, 5 ft wide): make a single attack roll against a Dexterity saving throw DC equal to 8 + your proficiency + your Dex mod. Each creature in the line that fails the save takes 3d10 piercing; on success, they take half. Cost: 5 ammo.
• Overheat: Full Auto forces an immediate Overheat check (see below).
• Recoil: After Full Auto, you suffer disadvantage on your next attack with this weapon until you use your action to re-stabilize (Sleight of Hand DC 10, or use an object interaction to brace).

Special: Loading / Reload

• The weapon uses a box magazine-style reload. Reloading from empty takes 1 action (replace magazine).
• If you still have rounds, you can use a bonus action to swap magazines if you have a spare.
• Running out of ammo in the middle of a burst/full auto cancels the remaining bullets (no partial damage).

Jam, Overheat & Clearing (roll DCs & checks)

Weapons like this need maintenance fun for the table mechanics.

Overheat rule

• Track consecutive rounds fired this turn. If you fire 10 or more rounds cumulatively across actions without taking an action to “vent/brace” (see below), you must make a Constitution saving throw: DC 12 + # of consecutive rounds beyond 10 (e.g., 12 for 10, 13 for 11, etc.).
  • On fail: The weapon overheats. The weapon becomes jammed, and the wielder takes 1d6 fire damage; the weapon is unusable until cleared.
  • On success: The weapon works, but counts as “hot” next round, any Full Auto attempt automatically increases DC by +2.

Clearing a jam

• Action: To clear a jam, make a Sleight of Hand check (DC 15) or use Tinker’s Tools with Intelligence (Tinker’s Tools) DC 12.
  • On success: Weapon is cleared.
  • On failure: You take 1d6 fire damage, and the weapon remains jammed; you may retry (each retry risks more heat/damage).
• Alternative: A short rest with access to tools/parts clears the jam automatically.

Vent / Stabilize

• As an action, you can “vent and brace” the gun to cool it: make a Dexterity (Sleight of Hand) DC 10 check. On success, you reset the consecutive round counter to 0 and remove the recoil penalty. On a fail, you still cool 5 rounds off the counter but suffer no further effect.

Maintenance & Reliability

• Long-term maintenance: Each month without proper maintenance (Tinker’s Tools check DC 10), the weapon accumulates “wear”; after 3 failed monthly checks, it becomes unreliable (DM may impose +1 DC to clear jams, or reduce rarity).
• Tinkering: Using Tinker’s Tools allows a smith to reduce jam DCs by 2 or reduce overheat thresholds after a successful long repair (8 hours + 25 gp parts).

Suggested Rarity / Balancing Tips for DMs

• Rarity: Very Rare. It’s powerful and resource-intensive. Require attunement by a martial proficient user.
• Ammo scarcity: Keep ammo scarce/expensive in low magic settings to limit abuse.
• Environmental limits: In close dungeons or magic-suppressed zones, the DM can increase jam/overheat chance.
• Damage tuning: If this feels too strong, reduce burst multiplier (e.g., burst = 2d10) or make full auto 3d10 single target instead of 5d10.

Example stat lines (copy/paste for a stat block)

Basic Machine Gun (very rare, requires attunement by a creature with martial weapon proficiency.)

• Ranged weapon attack: +X to hit, range 120/480, one target.
• Single: 1d10 piercing.
• Burst (3 rounds): 3d10 piercing (cost 3 ammo), 2 to next attack with this weapon.
• Full Auto (single): 5d10 piercing (cost 5 ammo); Overheat check (Con save DC 12) after firing.
• Loading: action to reload the magazine. Overheat/jam rules apply.

Crafting & Cost breakdown (DM suggested)

• Buy: 1,200 gp.
• Craft: 800 gp in parts + Tinker’s Tools proficiency (takes 10 days; Smith’s Tools/ Tinker’s Tools DC 13 to complete).
• Ammo: 50 bullets = 5 gp (0.1 gp per bullet). Spare magazines: 10 rounds each (1 gp / mag).

Quick play tips for players

• Use Single for repeated, accurate shots.
• Use Burst to chew through a tough single target quickly.
• Use Full Auto to suppress groups, but watch your heat and ammo.
• Always keep at least one spare magazine and use Vent as an action when you anticipate sustained fire.

Proficiency with a Heavy Crossbow allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Ammunition, Heavy, Loading, Range, Two-Handed, Push