Converting
commercial fixed mag lowers would require destroying the pieces that hold in the mag release, don't even bother with commercial fixed mags if you care about mag removal, they are more "fixed" than what is needed to be legal in mass. More on that in the third paragraph. If you move to a free state stripped lowers are extremely cheap and you can mount your fixed ar components (upper, stock, grip, lower parts, etc) to a $50 stripped lower.
If you have an LTC in MA you can make a normal AR15 style rifle from a 80% lower legally. There is nothing in the MGL (ACTUAL law) that says you can't and it's been touched on 1000 times that the enforcement notice has 0 legal standing. You could also get a .22lr registered ar15 if you do want to abide by the EN and change the caliber as changing caliber doesn't require re-registering or does the EN say that makes it an "assault weapon". Or you could break the bank and get a "grandfathered" ar lower that pop up in the NES classifieds daily.
If you really want a fixed mag ar for any reason (dont have LTC, pistol build, worry of federal ban, or want post-ban full features), and really want to be able to convert it back to normal ar after have moved to free state (especially if it's an NFA item) OR want to be able to put in a different caliber magazine while in mass (and stay fixed), there is hope! You can have a gun that is legally a low capacity gun but still have the mag technically removable without destruction of gun parts (EG using power tools to release the mag catch that you need to do on commercial fixed mags). A gun is only "readily modifiable" to accept a large-capacity feeding device (lcfd) if you can insert a lcfd with only
immediate alteration of the weapon (basically either pressing mag release or bullet button). However, a gun is still low capacity if you can't
immediately alter. The SKS you can be converted to be capable of accepting a lcfd if you simply take out the fixed mag, which is only a simple 3 step process (remove trigger group, remove fixed mag, insert trigger group, done), but the SKS is still considered low-capacity as that is not immediate alteration of the gun. If a fixed mag ar is put together correctly its also basically a 3-4 step process.
For a fixed mag AR to not be able to be immediately altered to accept a lcfd ("readily modifiable"), you'll need a stripped non-registered lower (not a gun by MA law, so legal to not register) or have one you built yourself, which you'll need to turn into a functioning lower, and then you'll need the Cross-Armory fixed mag kit, and a mag of your choice (any caliber). Simply follow the instructions except
DO NOT PUT THE PLASTIC PLUG IN! Just the mag, mag catch, nut and screw, and you're good to go! Really that's it, go register it! To alter the gun to accept a lcfd you would need to take out the screw, then nut, mag catch, then mag, meaning it can't be
immediately altered to accept a lcfd. If you move to a free state you can remove the kit and have a normal ar (of course you need a normal mag catch, spring, and button to replace the kit). If you are in MA and want a different mag because wanting a new caliber or simply new mag type, just temporarily remove the kit and mag (when it is removed the gun cannot function normally), and reinstall it with the new mag!
501 CMR 7.02:
"
Capable of Accepting a Large Capacity Feeding Device means any firearm, rifle or shotgun in which a large capacity feeding device, as defined by M.G.L. c. 140, § 121, is capable of being used without alteration of the weapon; provided, however, that said feeding device is fully or partially inserted into the weapon or attached thereto, or is under the direct control of a person who also has direct control of a weapon capable of accepting said feeding device. "
"
Readily Modifiable to Accept a Large Capacity Feeding Device means any firearm, rifle or shotgun immediately capable of being altered so as to accept a large capacity feeding device as defined in M.G.L. c. 140, § 121; provided, however, that said feeding device is fully or partially inserted into the weapon or attached thereto, or is under the direct control of a person who also has direct control of a weapon capable of accepting said feeding device. "
Of course it is illegal to convert your gun to a large-capacity gun while in MA so don't do that.
THIS SAME METHOD WORKS WITH AR10 TYPE RIFLES