Powerful weapon originating from a different dimension to ours
This is fully automatic weapon, but for purposes of easier integrating into DND’s combat system, characters use it in bursts.
When making an attack with this weapon, character can shoot up to 3 bursts of bullets from the weapon. Due to recoil, every subsequent burst has additional -2 penalty to attack rolls (so first burst has 0 penalty, second -2, third -4). Every individual bursts deals 1d12 damage.
Maximum ammunition carry capactity for a single character is enough bullets for 70 bursts.
This weapon does not impose disadvantage on melee attacks.
Most creatures in Tookania do not know how to operate the weapon. Unless somebody shows them how to use it, creature must make three successful investigation checks with DC 16 to know how to operate it (but they are not yet proficient in it). If any investigation check is <= 9, investigator accidentally shoots random creature around them (including themselves possibly). If investigator fails on three investigation checks before succeeding at three, weapon is destroyed.