I think that there are some things that we can all agree on, one of which is that the MGL covering this issue is poorly written (not unusual for MA gun laws). The next thing that we can agree on is that a pump shotgun can never, ever, be classified as a large capacity shotgun, as the paragraph that discusses this matter specifically excludes it from same criteria that describes a semi-auto gun as a large capacity gun.
Now we're left with the issue/description of a large capacity feeding device (LCFD). In that paragraph, it mentions a fixed or detachable magazine, box, drum, feed strip or similar device...that holds >5 rounds. Under these definitions, a tube cannot be a fixed or detachable magazine, a box, a drum, a feed strip or a similar device. However, it does make reference to a tube-fed .22 as being exempt. If it did list a tube in the aforementioned description, then I wouldn't be writing this response.
Under this same MGL C. 140, Section 121, in the description of an "assault weapon" it states that it has the same meaning as semiautomatics as defined in 18 U.S.C. section 921(a)(30). In addition, this same paragraph explicitly states and lists the following exemptions: "(ii) any weapon that is operated by manual bolt, pump, lever or slide action;..."
So now we're stuck with a conundrum, i.e., a non-large capacity weapon, a pump shotgun, that if it has a >5 round tube capacity, it has a "large capacity feeding device" that is or isn't legal. However, since the tube is integral to the shotgun and cannot be removed to reload like a mag, box, drum, etc., the definition of a large capacity weapon should take precedence, which, again, means the pump shotgun is exempt. Otherwise, the non-semiautomatic phrasing exempting pump shotguns would be meaningless.
Furthermore, I don't believe that MGL defines the size of a standard shotgun shell as being 2.75" or any other size shell, for that matter. Therefore, if we all follow "what appears to be the letter of the law" then any pump shotgun with <a 5 round capacity tube could also, in fact, be a large capacity feeding device if it can fit >5 rounds of mini shells, and thus, be illegal (for example, a 4-round Remington 870 Wingmaster).
For all intents and purposes, MA has been under the 1994 AWB for almost 25 years, so I would think that there would be reams of case law where people and gun stores were hung out to dry if they possessed or sold pump shotguns that were illegal. I could be wrong, but to my knowledge, no properly-licensed person or gun store has been nailed for possessing or selling these shotguns. If I'm wrong about this, someone will correct me.