Quoted: So does anyone know the deal with the "permanently attached" FH thing? Specifically on a colt MT 6400C (M4). Can I just remove the pinned muzzle brake and thread the barrel to attach the phantom (5c2)... Or do I need to do this "permanently attached" thingy. (how? pin it?) Thanks
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Here's the deal. A barrel needs to be at least 16" long according to federal law. If not than you need to do one of two things...
Register the lower as a SBR (Short Barrel Rifle) which involves a $200 tax stamp and a lot of time & paper work. You would also have to check to see if that's legal in your state.
The other option is to permanently attach some form of muzzle device. A welded on FH or comp becomes part of the barrel. Those that have a 14.5" barrel have either a Phantom or Vortex (both are 1.5" long) FH permanently attached bringing the total barrel length to that magic 16" length.
In you case, I believe your barrel is already 16" long. I do not know if the muzzle device that come with it can be removed or if it will need to be cut of? If it can be removed and the barrel threaded without cutting it down, you will be able to install any muzzle device that you want without worrying about it being permanently attached. If it has to be cut off then your best bet will be to have it cut down to 14.5 and have either a Phantom or Vortex permanently installed.
PS - the best method of permanently attaching a FH would be to pin it and then weld over it. Some people suggest soldering it on but I feel there are too many risks involved with that.