This is a repost of an earlier post of mine on this topic.....
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I sent mine to JPgrips on Straight Shooters recommendation. Got it back in a week. $49 includes return shipping. If you have to ship your entire upper like I did then it is another $24.
http://www.jpgrips.com.
I sent him this DPMS Flash Hider ($5.69) and crush washer ($2.19) which he installed for me. He did a really great job.
DPMS Flash Hider A2 1/2"-28 Thread Pre-Ban AR-15 Matte - MidwayUSA
DPMS Crush Washer AR-15 - MidwayUSA
Here are the results:
And since this is a S&W forum, here is a pic of "Sweet Lips", my beloved M&P15T: