Didn't you read what he wrote? There is no damned FA10 because he bought his rifles out of state. The FA10 would only apply if he bought them in MA. The problem is that we in MA sometimes think that what we have to do applies everywhere because of our conditioning, but that is not the case outside of this and a handful of commie states.
Also, it appears that you watch too many CSI type shows. The cops don't come looking for a firearm that was used to kill someone. They would already have to suspect YOU for that crime based on a connection or possible motive, THEN after querying their "registration" database would see that you owned a gun in the particular caliber as the murder weapon. They can't just do ballistics testing on a slug and suddenly have the serial number of a particular gun to track down it's owner! All they can do is match the bullet to the gun if they ever find it.