Sure, that's arguable, but there's a big caveat. The OP's rifles existed as such on some FLL's books, be it manufacturer or reseller. As a free state AR, it would have needed to have been transferred by an FFL to a MA LTC holder to meet the federal requirements for intra-state transfer, even though it was not a firearm under MA law. If that FFL trail doesn't exist, you can't say it was legally possessed by MA LTC holder on 8/1, and it's therefore a no-go regardless of pinning, welding, fixed magazine, and so on.