[Zundfolge]

Is there any way you can legaly put a pre ban upper on a post ban lower?

Will just removing the bayonette lug (and any other pre ban features) be enough to make it legal?

[Hkmp5sd]

Yep, perfectly legal. Remove the bayonet lug, flash suppressor and permanently install something over the threads and you're good to go. Semi-auto assault weapons do not follow the machinegun's theme of "once a machinegun, always a machinegun." AWs are based solely on the number of evil features. ATF has even stated that if you take a preban AR, completely strip it and sell the receiver alone to another person, that receiver is no longer a grandfathered pre-ban. It can only be assembled into a post-ban configuration.

[AR-10]

The BATF has stringent requirements for permanently attaching a muzzle brake or such to make the threads on the end of the barrel no longer accessable. The muzzle brake also has to avoid being the same diameter as a flash suppressor, so that a grenade launcher cannot be utilized.

[MiniZ]

A tip if you are converting a pre ban upper, but like the "look" of the bayo lug.

Just grind off the "ears" on the sides, leaving the lug intact. As long as a bayo cannot be afixed-and grinding the ears will accomplich this-you can have the look.

Of course, if you like it shaved, then take it all the way off like the factory does.