Actually there were “magazines in 1791” and in a context of greater than 10 ’rounds’ there were ‘high-capacity magazines.’

It was common for a person to have a supply ‘to carry’ an amount of power, and musket balls (’rounds’) of greater than 10, in a ‘container’ that was sometimes referred to as a ‘magazine’. There were also ‘repeating’ rifles that had a built in magazine, for example, the Girandoni had a built in 20 round magazine (sometimes it was referred to as a ‘hopper’) and a full magazine could be fired in less than 30 seconds.

“Assault Weapons” Before the Second Amendment – There were a lot more than muskets before the Second Amendment – one firearm even had 30 round magazines > https://www.youtube.com/watch?v=ffMNksrki1E

and…

A close look at even more high capacity repeating arms from before the Second Amendment was written > https://www.youtube.com/watch?v=Sr99YCyp8So

Well documented.

-Continued-

“Unlike its contemporary, muzzle-loading muskets, which required the rifleman to stand up to reload with powder and ball, the shooter could reload a ball from the magazine by pulling a transverse chamber bar out of the breech which allowed a ball to be supplied to it and which then rebounded back to its original position with the aid of a spring, all while lying down.”

“The air reservoir was in the club-shaped stock. With a full air reservoir, the Girardoni air rifle had the capacity to shoot 30 shots at useful pressure. These balls were effective to approximately 125 yd (114 m) on a full air reservoir. The power declined as the air reservoir was emptied.[12] The air reservoir was similar to the disposable carbon dioxide cartridges used on some modern air guns.”

Sure sounds like ‘assault to me’…

The ‘Lewis and Clark’ air rifle (Girandoni) had an internal magazine capacity of 20 rounds…

https://en.wikipedia.org/wiki/Girardoni_air_rifle