If you're building a semiauto rifle that accepts detachable magazines in MA, you're subject to the AWB features test. You get no more than one of the following:
Background: What is the MA AWB? The "Assault Weapons" ban in MA is almost a verbatim version of the Federal AWB that existed from 1994 - 2004. This is how it is currently encoded in MA law: http://www.malegislature.gov/Laws/GeneralLaws/PartI/TitleXX/Chapter140/Section121 The "Assault...
Remeber when measuring barel length it is supposed to be on a closed bolt. Usually a cleaning rod or wooden dowel can help with that, although you would like to think that manufacturers selling barels marked 16" take that into consideration.