and costs $125... if you can find one.
the lowest cost one that the guys has suggested is that #7 from Model 1. Some hacksaw time and a little flat black stove paint and you'd be good to go.
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I also had a "legal" issue, bought a 3 1/2" flash hider from Northern Gun Parts for $12.50 plus shipping. The service was quick, the black oxide finish was mediocre, not as nice as the rifle's, but it works and is very inexpensive.
http://www.northerngunparts.com/ar-m16-parts.aspx
What length is your barrel? Or have you not even purchased it yet?