To add to what Tom said...
A flash hider can be pinned to a 14.5" upper to make it 16". That's a very common thing to do. That keeps you out of NFA territory.
Once you spend the money on a good FH or muzzle brake (why pin an A2 when you could have something better) and the pin/weld work, you'll probably be close to the cost of a $200 tax stamp. However there's no wait, no forms, and no worries about where you can take the rifle in the future.