Something that just occurred to me, a very interesting point. The law only specifies 5 shotgun shells, it doesn't specify five 2 3/4 inch 12 guage shells, etc.
So let's say you load up your shotgun with a bunch of those Aguila mini shotshells that are about half the size of a normal shell. Even if the tube only holds 5 rounds of 2 3/4 inch shells, it'll hold a lot more than that with those mini's.
But here's where it gets really interesting. Under Mass. law, a magazine is large capacity if it's "capable of accepting" more than 5 shotgun shells. A shotgun tube that will only hold 4 normal sized shells will hold 1.5 or 2 times as many of the mini's.
Tube mag capacity depends on the length of the tube and the length of the shell, whereas box or drum mag capacity depends on the shell's circumference and the size of the magazine. If someone invented mini .223's that were half the length of a standard .223, an AR mag would still only hold the normal number. But in a tube mag, space savings in the length of the shell increases capacity, which depends entirely on the cartidge used, not the mag itself.
So the question you have to ask is, how many shells is your shotgun "capable of accepting"?