@thehombre Grapeshot, for sure. Round shot is better for fortifications rather than massed enemies, whereas grapeshot is the reverse; a decent cannon filled with grapeshot has an effective killing range of about 500 yards, if I remember correctly. Let the enemy close to about 30-50 yards and a few canisters of grapeshot would hit them like the hammer of god.
That said, while the technology behind a musket or a cannon are pretty simple, and forging the barrels aren't exactly difficult, relatively speaking, that still doesn't make them easy to make, especially en masse. You need enough powder to propel the projectile at a lethal velocity while
also having a barrel wall thick enough to withstand the pressure generated by the powder igniting. And all muzzle loaders carry the risk of double charging or even under charging. Both tend to make things explode, the latter just in a more delayed fashion unless you realize that the projectile didn't exit the barrel, which can render it basically useless.
There's also the worry the fact that, as soon as you introduce firearms of any kind, you'll ignite an arms race; all it takes is a few captured examples and the enemy will eventually figure out how to reverse engineer it, so unless you kill them to the last man, it's better not to bring firearms to battle unless you want to be staring them down yourself before long.