The problem is not likely a field check by LEO, but rather when you find yourself in an "they are out to get you" mode.
If you are involved in a 100% legit defensive shooting in MA as a trigger puller, all your guns will immediately be confiscated while the system sorts things out. In a best case scenario, you will eventually get them back and only pay a few tens of $K for an attorney to look out for your interests while the investigation plays out. That's best case.
You can count on the police and prosecution going through every one of the confiscated items looking for some charges they can pile on - even if the shooting itself does not result in charges. DAs and ADAs like wins, and loathe letting someone go without getting at least a few ounces of flesh.
A removeable attachment on a barrel that can put you into unregistered SBR territory means the state can force a plea bargain on you. Sure, you'll talk tough about defeding it until they threaten a federal referral and your attorney points out that (a) you are guilty, and (b) the feds almost always sentence NFA violators to prison time, even absent any previous record (but it moves you into a lower box on the federal sentencing matrix).
At the minimum, you should get a NFA registration to cover the federal end of things.
