who said it was not allowed in MA? A threaded barrel is not an issue unless it has an additional evil feature which it does not. The gun is perfectly legal to own in MA both the regular and tactical models.
No dealer can legally SELL the gun as a complete gun because it is not on the approved roster. That has nothing to do with your ability to purchase or own.
Just find the right dealer. Right now they are hard to find as they were just recently released. They are only slowly making it out into the wild and the demand is higher than supply. It will take some time to get one but there are no serious obstacles.