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Old December 22, 2013, 04:50 PM   #1
mnptriggerman
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M&P/Rock River question

I recently purchased an M&P ar-15 looking to get a quad rail hand guard, a guy has one for sale that came off of a rock river arms ar-15, hoping that anyone with info could tell me if they would be compatible at all...
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Old December 22, 2013, 06:00 PM   #2
CTS
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Is the Rock River a free float handguard or standard two piece?
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Old December 22, 2013, 08:04 PM   #3
9x19
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Both the S&W and Rock River use standard AR parts, so they should inter-change, but if your M&P came with a standard two piece hand-guard the RR may require a bit of work.

If the RR is a free-float style, it uses a different type of barrel nut and no delta ring. In this case you'd have to dis-assemble your upper to install the free-float QR.

If the RR is a two-piece quad rail, it will easily mount using your delta ring and a couple screws to hold it together and in place.

If you use a sling or bipod when shooting, the free-float may offer an advantage (isolates the barrel from any pull or push on the fore-end), if not the two-piece will probably serve you fine.

I have a two piece on my Rock River mid-length.

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Old December 22, 2013, 08:56 PM   #4
CTS
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That is the other question, are the both carbine length hand guards. Even if the RRA is a two piece it has to be the same length as yours (assuming carbine).
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Old December 23, 2013, 08:05 PM   #5
BillyJack3
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One thing that steered me away from Rock River was the weight of their quad rails. Really heavy on the factory RR ARs I held. Might also be why he's selling. Check the weight and think about the balance of your AR. It may not be what you are looking for if all you want is a place to mount a couple of gizmos.
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Old December 24, 2013, 02:16 AM   #6
Sweet Shooter
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The RRA weight comes from the awesome barrel profile, not the rails (and really, I mean it, it is awesome). The H-Bar is heavy and it generally shoots less than 1MOA if free floated–with factory BALL. The free float half-rail is a bit heavier than the quad rail, but take them apart, if you've the muscle, and you'll see where the weight is coming from. Of course the M4 carbine hand-guards are lighter and worth looking into if you're downs with 2MOA.
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Old December 25, 2013, 07:18 PM   #7
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^^^ I didn't even think about barrel profile. HBAR would definitely make a difference so the weight difference in the rail might not have been that much.
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