Dave G., you are welcome to believe what you wish. I base my decisions ONLY on written content of ATF letters and other relevant data as written in law, and other relevant technical data.
A NATO standard grenade launcher attachment has a diameter of 22 millimeters. This is a 'magic size' that any reasonable man would AVOID in the making of any muzzle device. I would not want such a device on my rifle if it was within 1 mm of that spec. I see no rational reason to tweak the dragon's tail.
I further refer any interested parties to the ATF letters, and the letter of the law, regarding threaded muzzles. If anyone chooses to think that 'other threads' are OK, they're willing to take a chance that I will not. The writing of the law and the ATF letters on the subject are EXCRUCIATINGLY clear on the issue, and they say that a THREADED MUZZLE on a rifle with one other 'assault feature' makes the rifle an assault rifle.
Whether or not muzzle-launched rifle grenades are readily available or not does NOT change the meaning and intent of the law.
It would be highly inadvisable to pursue a variation of this scheme unless you intend to become a test case, and if you were to do so, be advised that the law as written, and the opinion of the relevant authority, is against you.
I take this seriously. So seriously that both of my post-ban AR's (both Frankenguns, one has a homebuilt receiver I milled from a forging) have barrels that have had the threads turned completely off and had plain fillers installed in their place, giving plain muzzles. I'm not comfortable with the idea of having a heat treated steel barrel welded or soldered with silver solder at at least 1100 degrees Fahrenheit. That's bad for the temper.
No one could even QUESTION the configuration of my post-ban rifles, and they shoot just fine without bayonet lugs or flash hiders, though I admit I'd like to have them there if it were allowable.
CJ