Does Mass. have it's own assualt rifle ban that prevents me from shipping 100 round RPD belts into their state?

A Mass resident needs to have a Class A or Class B licenese to carry to possess them, and they do have an AWB, but can possess anything made before Sept 14th, 1994.

Handgun hicaps need a Class A license.

You can always ask them to fax their Class A or Class B license, or send a picture of it, for verification first.