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The norc has 3 lugs inside the flashider and grooves cut into the barrel , a castle nut and set screw hold the flashider in place .
If I remember correctly if the barrel is cut to around 18.5 or so in it gets threaded to 28TPI by 1/2 as the barrel is thinner than at the front .
Smith enterprise makes these and they are a good brake , they have the lugs inside so they are a direct replacement for the original flashider, someone had an ad recently on the EE with 3 of them for sale.
Springfield Armory makes this one ...I have a couple of these and they have the lugs inside as well , Brownells used to ship them but I read they wont anymore Spcamno has one for sale on the EE http://www.canadiangunnutz.com/forum/showthread.php?t=670744