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Discussion Starter · #1 ·
Hi all,

I am new to the forums, but have been the owner of a 556 since I purchased a 1st-gen rifle back in 2007. To be clear, this means the original version of the rifle with fish gill grips and adjustable (NOT folding) stock. So far, I've kept all of the factory equipment on the gun (I know, heresy for some of you but I don't mind the spartan setup). Lately, though, the frame attachment for the stock has started to loosen up and I'm looking for some options.

There's an image below. Apologies for the crappy pic and MS Paint, but the ring under the arrow rotates (not talking about the buffer tube threads, which have always been a bit loose and easy to spin a half-turn or so - my only complaint about the gun really). I think I can unthread the buffer tube, but I have spun it about a full turn or so and it gets significantly harder to turn. I've been hesitant to turn it any further because I don't want to damage anything.
Revolver Trigger Air gun Shotgun Bumper


Anyone have any tips or info? Can I just wrench the buffer tube off of this thing? Is there a screw I can tighten (looks unlikely). I did a few searches of the forum but everything I found had the folding stock on the 556 Classic.

Thank you
 

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the castle nut (the ring under your yellow arrow) needs to be loosened far enough back that you can pull the receiver end plate away from the receiver. there is a tab in the bottom center that "locks" (or rather used to lock) into a groove in the bottom of the buffer tube treaded portion to prevent rotation of the tube. Pulling the plate back will free a hump that is inserted into the rear of the receiver that prevents the plate from turning.
 

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Discussion Starter · #3 ·
Thanks for the response. I honestly feel like kind of an idiot - of course it's a castle nut! Looking at buffer tube kits for ARs, I see it now.

Even without the right spanner, the nut spun off with some elbow grease. Several threads on the tube itself are absolutely wrecked and the staked grooves between plate and nut aren't engaging properly anymore. Thankfully, the threads on the receiver look OK. After looking around at some of the builds on the forum, I splurged on a Magpul UBR. Here's hoping it fits!

Thanks again. Will update when the new stock arrives.
 

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The UBR will fit your Sig 556 just fine. I've put 3 on my Sig 556s over the years. Looks great too. Especially with a cheek riser. Which, sadly were phased out of adaptability very early-on with the original UBR design. Probably because too many thought you could run a riser with an AR build. No dice. Works perfectly with our rifles, though. My only complaint with the UBR is; it's a heavy stock on an already heavy rifle. However, if you ordered a gen 2 UBR-- it will be significantly lighter than the gen 1. Good luck!
 

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No problem @ChefJeff789 , glad the info helped. If the UBR requires an AR armorers tool make sure and buy/borrow one and install it to torque spec so you don’t have the same issues you were experiencing with the previous stock.

Good luck with the instal!
 
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