Why would i reread the statue?
Instead you should do some more reading. Read the definition of Large Capacity Weapon. The reason a 9rd tube on a pump action shotgun is not considered an issue is because of this definition. A pump action shotgun can never be a Large Capacity Weapon. So while the tube, if removed from the shotgun, would be a LCFD and a felony, while attached to the weapon, the weapon is not a LCW. This sets up a bit of a challenge to prosecution and as noted by
@drgrant no one is aware of any actual prosecution.
The issue of shotgun shell size (length) only matters for tubes. Tubes on pump don't matter per above. Tubes on semi-auto matter. But a semi-auto designed for 3" or 2.75" wont cycle with 1.75" mini shells. The generally accepted way to determine capacity is what the gun/tube is designed for. The fact that one can create and stuff in arbitrarily short shells that the gun does not work with again makes a hard case to prosecute. If you had the gun and it was loaded with shorties, I think they have a case. Otherwise I think they do not.
Your question of 6 rounds of 410 in a revolver is an interesting one. The S&W Governor (on the approved roster) is such a gun. The Taurus judge is 5rd capacity. They also make shot shells for 22LR, 9mm, etc as "snake rounds". I think prosecution of the unloaded gun would be difficult. I think if it was loaded with 6 410 rounds they would have a case.
Go read 501CMR7.02 and the convoluted language for "Capable of accepting a large capacity feeding device". A Ruger 10/22 is capable of accepting a LCFD so by reading of plain english and the statue, it is a LCW and not permissible to be possessed by a FID holder. But the CMR defines "Capable of accepting a large capacity feeding device" in such a way that since the 10/22 is not on the large capacity roster, it is not a LCW if possessed by someone who has a LCFD under their control etc.
It does not get more ugly or mind-numbing. You have to go into this believing that if you own a gun in MA you are committing a felony every day and just dont know it. Gun control laws are not about controlling guns but controlling the populace. Making them confusing and self contradicting is key to scaring people off exercising their rights and owning guns.