Yes, there is. I have seen it. MANY MOONS AGO. It was a scan of the letter to the tech branch. I believe it was on Gunsnet, before Gunco even existed.
It was a letter to the tech branch and it said the muzzle nut is not a "muzzle device" since it serves no function. The letter stated a thread protector (aka muzzle nut) is not countable as a part.
I say that, however I have looked everywhere for a copy of that scan, and I'll be darned if I can't find it. I know what I read because at the time, my NHM-91 was being converted from thumbstock to pistol grip configuration and I was counting and re-counting to make sure it was done right. I have a muzzle nut on it BTW.
Suddenly everyone thinks it is a countable US part. My concern is that people will be fooling themselves on their parts count in case anyone ever got called on the carpet about it. The definition of a muzzle device is that - a device. Thread protection is not a function of the muzzle, nor does it "interact" with the muzzle as would something that alters the visual element (flash hider), or the inertial element (muzzle brake). I hope that makes sense.
I know what I read, however my searches simply have been fruitless. So I can't prove I'm not crazy.
I suppose one of us could write the tech branch for a copy of the statement.