Well not just one. They were smuggled into British forts by females below their pettiskirts, and then handed to the warriors when the attack began.
There are examples of Plains Indians having very short flintlock and caplock guns, which they used from horseback, riding close to an enemy or bison to use it. More akin in the second example to a "horse pistol" kept in a "bucket" on a dragoon's saddle than to a musket, but fashioned from a musket. They may have been fashioned from a damage gun..., the Indians didn't really keep records on them.
Well a lot of repro blanket guns are 16" barrels or less
Blunderbuss barrels I've seen available in 20" or less
Canoe Guns I've seen with 24" barrels or less.
I think that the TC White Mountain Carbine was also 24"..., I'd say under 24" for a barrel on a shoulder fired weapon is short. Other's opinions may vary.
Here is a 12" hunting rifle, from the mid 1700. Note that it has an adjustable rear sight. Other very innovative features on found on the lock, and it has a set trigger. It may be newer than it's date in the video. I wonder if this was uncommon, being so short I wonder if perhaps the owner might have been physically impaired and could only hold the rifle up with one hand? I've not seen any others this short, but..., maybe I've simply not looked much at European gun collections? This does show that short, shoulder fired, rifled pieces did exist.
12" Flintlock
LD