Cannons that were massive and while they could deal a great deal of damage in a single firing, everyone would see it coming. Cannon required a lot of support or else their crew would get wiped out by enemy foot soldiers. You couldn’t just pop out, surprise, fire a cannon before anyone notices.

The writers of the amendment wouldn’t have conceived of small arms that would allow a single person to rapidly take down dozens of people at significant distance without any warning or support. Melee weapons were the only viable weapons for a prolonged single person fight, with small arms requiring a long reload activity between shots where the wielder was vulnerable and big weapons being huge, slow, and similarly vulnerable before even the first shot. All of the firearms of the time had the projectile go vaguely in the direction of firing, so at range it was essentially useless for a person by themself without a bunch of others firing balls chaotically in the same direction in hopes of hitting their target.