You are correct. Note that in Article I, Section 8 (the enumerated powers of Congress) is the power to grant "Letters of Marque and Reprisal." Essentially, Congress can grant anyone the power to be a privateer (i.e. a pirate or a raider) against enemy shipping. This was actually done during the War of 1812. Raiding enemy ships required that you have cannon - MANY cannon - that you owned personally. The Founders were not stupid men, they understood this VERY well. Thus, logic and the language of the Constitution indicated that cannon are protected arms.
I would also make the argument that ALL arms capable of being carried/used by a single person or even a 2- or 3-man team are protected under the 2nd. Our side employed rifles in the Revolutionary War, and at the time rifles were fairly high-tech, certainly higher tech than the Brown Bess muskets of the British. So we have the right to keep and bear ANY firearm now, since that right obviously existed back then.
Our side carried cannons too, and you can still own one. Blackpowder at least. I think your assessment that individual and crew fired weapons are covered is correct. I can’t look for it now, but I believe the cannon issue was resolved in court, Texas I think, not long after they became a state. Privateers also needed cannon.