I’m suggesting that those with pistol braces on guns with under 16" barrels to pin/weld a solvent trap on the muzzle to increase the barrel length . 
Just kidding, folks.
LMAO 
I just added another barrel, no law against a double barrel AR pistol🤣
Just don’t attach a pointy thingy to the end of it, that would be illegal. Depending onto which end you attach the pointy thingy, you might be the one that gets hurt…
Your muzzle device may double as a mace or war-hammer, but not as a pike.
Wonder if we could get by with a falx attachment? What is the legal definition of a bayonet? Just any pointy item, or an actual knife, or what?
Now, mind you, I think a falx would not work well on a rifle. This was simply a joke, now a theoretical thought exercise.