It's a 16" CAR from J&T. It came with a muzzle brake. If I replace it with a vortex or A2, do I have to pin and weld them in place? At what point do you measure the actual length. It looks like mine would be 14.5" without anything on the end of the barrel, but I don't know how far out the threads stick, or where to measure from at each end. Anybody know?

Barrels are measured, with the chamber empty and bolt closed, by dropping a rod or dowel down the bore until it contacts the face of the bolt, and measuring even with the end of the barrel.  

Unless you become a "special taxpayer" and you obtain a "tax stamp" for it (REALITY--this means Registered and licensed) as a Short Barreled Rifle, and that is a NFA weapon, then the barrel must be 16" long.  

However you attach whatever to the end of the barrel, it must be "permently attached" and the result be over 16" long.  An A2 flash suppressor would be too short.

If it were mine, I would contact J&T, and see about having them change out the brake and properly attach another device for you.

Thanks for the response. Yes, I have already contacted them about doing that.  No response yet. We'll see.

Are you sure it's a 16"?  J&T's "tactical" kit has a 15" fluted barrel with (in post ban form) a Young's MFG muzzlebrake permanently attached.  If that is what you have (I do) you must permanently attach a flashhider to keep the length 16"+.  If you do in fact have a 16" barrel I'm sorry for wasting your time!

Scot