I agree but the odds of someone getting rapped for this in MA (a tube fed shotgun capacity violation) are between slim and powerball level.There is nothing in the law that says "it is OK to possess something that is otherwise a felony to possess because a gun shop sold it to you." When it comes to possessing something that is a felony to possess, "it's not my fault" isn't going to save you from a felony conviction and a lifetime prohibition.
Part of this is it's not difficult for an attorney to show how poorly written the law is. It doesn't even dictate cartridge size(s) for shotguns.
Potentially BTW with Aguila minishells or other short shells (ive seen 2.5in ones too from federal) every 5 round shotgun is illegal. The law is silent on the constitution of the ammunition.