I recently purchased a Post-Ban DPMS 16" CAR that came with a additional complete upper & SS Bull Barrel. My problem is that the bull barrel / upper has a bayonet lug - but no flash hider.
If I read the ATF rules correctly, it is not legal to install this per-ban upper on my post ban lower....
Can I remove the bayonet lug (mill it off / cut it off)without hitting any gas ports or weaking the front sight assembly? Trust me - I am not a gun smith "and do not know".
There are no numbers / date of production on the upper showing it as pre-ban. All I want to do is use the heavy barrel... and not directly break the law.
I would appreaiate any input.  

Check closely, only the actual "locking tabs" of the bayonet lug have to be removed. If they are still there, they can be easily removed and the finish touched-up.

I was told by a vendor (CMMG inc) that the bayonet lug can be ground off, and the upper will be legal as long as you have no more than one feature per BATF regs(usually most people have only the pistol grip on post ban rifles). I would protect the surrounding parts/metal with duct tape or something, but you should be able to accomplish this without affecting functionality if you are very careful. You will need a good vise with padded barrel jaws (I use wood blocks) and a good high speed dremel, with diamond or carbide cutting attachment and fine finishing stone. I would not try it with a conventional power drill, you would risk damaging the surrounding area if it slipped and it's too hard to get fine detail. A good excuse to get a dremel tool if you don't have one. You can blue the metal where the lug used to be if it is carbon steel, or polish it if is stainless.
I plan on doing the same with a DPMS gov't profile upper, since I want the 1 in 7 twist that is not available in a postban upper. after grinding the bayo lug I will remove the flashhider, put on a twist-on muzzle brake, then permanently pin it to the barrel, weld over the pin and hole, then grind and polish that area of the brake,  to be legal.

I just received two CAR kits from Model 1.  They are both post ban kits.  

It looks to me like the barrels were made with the bayonet lug on them and then ground off.  Are all post ban kits or barrels like this?  I'm sure it depends on the maker of the barrel.

I just finished removing a bayo lug from an AR barrel. First I just removed the "ears", and was going to leave it like that.  The more I thought about it, I decided to take the rest of the lug off.  A bayonet will still slide on a lug with no ears, and it can be locked on with safety wire.  I did not want some DA somewhere trying to screw me over a technacality, so off with the lug.

I kept breaking dremmel cutoff wheels, so I cut it off with a carbide hacksaw blade.  Very controllable, no danger of colataral damage (barrel was well taped).  

I already had the brake welded, so I just refinished from the handguard ring foreard to the Brake with Norrell's Molykote.  Beautiful job, excellent results.  But my wife wants to know why there is now a 3" hole in the back side of the toaster oven....