I want to make one of my preban uppers a post ban, it has a birdcage style  whatever. Does anyone know if this is considered a flash hider or a muzzle brake?? I can post a link to a pic if it's needed,  i just don't wan't to weld it on , then find out i have to change it. It's total length is 1 5/8" and it has 6 slots-7/8"long, 3/16" wide. can someone help me?  thanks.

A pic link would help, but you most likely have a flash suppressor.

Unless the front is closed (except for a small hole only slightly larger than bore diameter), it's a flash suppressor.

Unless the outer diameter is larger or smaller than the 22mm NATO standard, it's considered a grenade launcher.

A post-ban legal 'lookalike' brake or compensator must have both the closed front of a brake, and the non-22mm outer diameter.  Ideally, you'd like for it to have been submitted to ATF for flash suppressor testing, and have a ruling that it is considered neither a flash suppressor or grenade launcher.  The only AR 'lookalike' brake that has actually been submitted to and approved by ATF is the Wilson Combat.  

The Cav-Comp, Gunsmoke and Kurt's Kustom 'lookalike' brakes do meet the closed front and outer diameter criteria, but have not been submitted or approved in writing to my knowledge.

Does anyone think i could fill in the front with JB weld, then drill a hole in it?? would it hold up??

JB-weld?  Not a chance.  JB-weld would blow out on your first magazine, if not your first round.

Filling in the front doesn't address the outer diameter issue, either.

Welding a steel washer to the front of the flash suppressor would probably work (and has been done before).  Use a washer that's wider than the front of the flash suppressor, and that would make it not a grenade launcher, as well.  Whether it would still reduce flash or not, you'd have to test for yourself.

And, of course, any post-ban device must be permanently attached (welding or silver solder) to cover the muzzle threads in order for the upper to be post-ban legal.

Don't forget to grind off the bayonet lug.