Brother Hiriam,
Fed law is followed here, so if you are legal where you are coming from AND where you are going to you transport locked/cased in trunk.
No out-of-stater needs a LTC or FID if coming into MA for participation in a match or going to a gun show (bring evidence of same). If you are just coming to MA and hope to do some shooting informally, bringing guns/ammo into MA would be illegal without a LTC. There are NO FIDs issued to out-of-staters, only LTC (which is a good thing).
Non-Res LTC takes many months (last I knew) to obtain, required a MA Certified course certificate (for the first issuance), special fingerprint card, application and $50/1 year.
Non-Res (even with LTC) are legally prohibited from purchasing any firearms, ammo or ammo components (exception is ammo at a range and it must all be fired there, non can be carried away with you). That said, I've never been asked to show my LTC (I am a MA Resident) at a gun show when buying ammo or magazines/clips (all require a LTC or FID). Most gun shops don't ask for any ID to sell ammo or mags either (although this is illegal for them to do).
Mag capacity is NOT restricted in MA, although residents must have the proper category LTC to possess or purchase them.
Best bet would be checking
www.packing.org and
www.goal.org and calling GOAL at 508-393-5333 to make sure that whatever you do is legal.
The law is uniform across the state, but the knowledge of the law by any particular LEO will vary all across the board.