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I have a player that armed his special Imp familiar with a light crossbow, sent him through a tunnel the party couldn't fit through, and had him shoot the artifact that was creating the magical wall blocking them.
I allowed it without proficiency and at disadvantage because the imp is tiny and the crossbow is built for medium humanoids.
It worked, and everyone had fun.
Are there any rules that would govern the use of a weapon by a familiar? I can't find anything that screams "No!".
So, when I searched the question history for "familiar" and "weapon", I found nothing.
But I found this portion that I copied from Sebkha's answer to another question about familiars using armor/shields:
Proficiency for monsters is addressed in the inset box "Armor Weapons, and Tool Proficiencies" on page 9 of the Monster Manual:
Assume that a creature is proficient with its armor, weapons, and tools. If you swap them out, you decide whether the creature is proficient with its new equipment.
The stat blocks for imps and quasits don't specify any equipment, so without DM dispensation they aren't proficient.
So, I like my ruling better than I did before. Yes they can use the crossbow, but they haven't been trained to use it. And it's unwieldy because it's so big.
There is nothing that says it doesn't work the way you described it (with no proficiency and disadvantage for a weapon that's a bit heavy for it's size). It couldn't use a heavy crossbow at all but, the light one with disadvantage is fine.
According to DMG p.278 (optional rules):
A creature has disadvantage on attack rolls with a weapon that is sized for a larger attacker. You can rule that a weapon sized for an attacker two or more sizes larger is too big for the creature to use at all.
There is no actual rule RAW that prohibits tiny creatures from using any weapons, but the only tiny creature I found using weapons is a sprite and it does 1 damage per attack that hit's with its longsword and its shortbow(+ poisoned), which probably means they are tiny weapons since they don't even have a damage die.
A small person has disadvantage when using heavy crossbows and a tiny person can carry half the amount according to PHB p. 176. So going by logic it wouldn't be able to even wield it, but by RAW it could use it since it can lift it with it's Strength of 6(Can Lift 6/2*30=90 lbs and carry 45 lbs). But i wouldn't allow it to load bolts.
Light crossbow is over 3 times lighter so it shouldn't be as difficult to use for the Imp.
