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Discussion Starter · #1 ·
Robinson Arms confirmed that with a CTR stock, you can use either use the military spec version or the commerical version so long as you get the corresponding stock tube. My question is what else other than the M4 adapter from Robinson, the stock tube and CTR stock, do you need to attach this to the XCR? I have heard of castle nuts, various types of screws mentioned. Does anyone know for sure what misc. parts you need and do they typically come with the purchase of a stock tube. Also, to my knowledge Robinson does not sell stock tubes.
 

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The adaptor from RA will have a set screw in the bottom of the buffer tube. I guess you could run the stock with the set screw only, but I think there is enough leverage from the stock to override the set screw. I would use a castle nut alongwith the stock tube.

Here is a complete list of parts:

CTR stock
Compatible buffer tube
Castle nut
Stock adaptor from RA

That should have you covered.
 

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The castle nut tightens against the back of the receiver. I would liken it to a jam nut. I can tell you that any torque on my CTR will rotate the buffer tube without the castle nut. For light use, especially if you drilled an index hole (not all the way through) in the buffer tube to give more purchase, then you might be okay.
 

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Correct,the purpose of the castle nut is to be able to tighten the stock while in it's vertical position,because if you just screw on the buffer tube to the adapter,you'll be damn lucky if the threads just happen to stop where the buttplate is completely plumb.The castle nut tightens against the M4 adapter not the receiver.Screw the castle nut onto the buffer tube then screw on the tube,then while holding the buttplate just about 1/2 to 3/4 of an inch from being plumb,tighten down the castle nut.
Sometimes it's hard to hold it while torquing down the nut and you wind up with it crooked,that's why you should start with the stock being off just a little so when you crank down on the stock wrench it winds up being plumb.You might have to try this several times to get the offset right.By the way you need a castle nut wrench.They are about 3 or 4 bucks,but they are the only really good way of doing it without scratching hell out of everything.



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So, if the castle nut keeps the tube from rotating in the adapter, what keeps the stock from rotating on the tube? My PRS has a bolt that goes into the end of the tube, which I had to tighten the crap out of to keep the stock from rotating on the tube. It appears to be kept from rotating by friction only. I would like some sort of positive lock or catch. The front of the stock looks as if it was designed to engage some sort of feature on the rifle to prevent rotation, but no such feature exists on the XCR adapter or the tube.
 

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An AR buffer tube has a square ridge on the bottom with the notches for the different stock positions. The stock slides over this and this ridge keeps the stock in position front to back and from rotating. I would liken it to a crank key and a pulley.

My bad about the castle nut locking against the receiver--I had to inspect too many AR's yesterday, I guess.
 

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Discussion Starter · #8 ·
Anyone who has a Magpul CTR stock, can you confirm whether your XCR has a Mil Spec stock and buffer tube or if it has the Commerical stock and commercial buffer tube? Rob. Arm's tech department orginally told me the XCR would take either tube and corresponding stock, however, on further confirmation from them, they said that only the Mil Spec tube and stock would fit property. They said the commerical version will fit, but it would be lose. Can anyone confirm this who has a CTR stock???
 

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Discussion Starter · #10 ·
Thanks Terra for that info..lol..but I was needing to know if for certain if the mil. spec buffer tube or the commerical version is the perfect match for the XCR. A loose buffer tube is not a good thing. Can you ask him maybe? :(
 

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I got a commercial tube on mine and haven't noticed a problem. I do have a commercial CTR to go with it. From everything I have read, there is not a difference in the threads between a milspec or commercial tube or if there is, it doesn't matter. A commercial CTR will be loose on a milspec tube.

Hope this helps.
 

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It has already been said but the Threads are the same from commercial to milspec. Yes, a commercial stock will fit on a milspec tube, that is probably what they are talking about when they said there would be a little play. Keep tubes and stocks the same and you will be fine.
 

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Discussion Starter · #14 ·
Hey thanks for the forum response. I do not have the XCR yet, should ship soon, but do have the CRT commercial stock and buffer tube. The stock and buffer tube are rock solid, not play what so ever. My big concern was were the buffer tube screws into the XCR M4 adapter. Is there any play, even a little, there in your experience? If you look at the deminsions provided by Magpul of the comm vs. mil spec buffer tube, there is an every so slight differene in diameter and that is what has me concerned. Thanks again for your y'alls advice.
 

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Mine has zero play,just use the castle nut to lock it down and you'll be good to go.
 

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Hey all - I am just about to order a Magpul CTR for my XCR, but I had a few quick questions.

Mine came with a folding M4 stock - do I have a Milspec tube or commercial? (IIRC, it was Milspec, but I want to be sure).

Second, looking at the pictures above, it looks like the stock is threaded almost all the way into the M4 adapter - there are barely any threads visible behind the castle nut.

On my rifle, there are more than 1/2" of threads behind the castle nut - should mine be screwed in farther for stability's sake? I also think it might look a little nicer and it would shorten the length of pull a tad.
 

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I also have a question.

Am I right in thinking Magpul's ASAP sling mount plate will work with the XCR in conjunction with the M4 stock adapter?

-Vic
 

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Ok, I have read this thing about 20 times over and maybe I am just stupid and can't figure it out. If I want to put a CTR on my XCR what should i order? I have the Robinson Arms folder on my order right now but am thinking of changing my order. Could someone make a Do-it-yourself...LOL
 

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If you already have the M4 adapter from Robinson:
1)stock of your choice http://www.aimsurplus.com/acatalog/Magpul_CTR_Commercial_Collapsible_Carbine_Stock.html
2)Castle nut http://www.bravocompanyusa.com/Receiver-Extention-Nut-Castle-Nut-p/nut receiver extention.htm
3)buffer tube http://www.aimsurplus.com/acatalog/DSA_Inc._6-Position_Buffer_Tube.html
4)castle nut wrench http://www.aimsurplus.com/acatalog/ProMag_AR15_type_Rifle_Stock_Wrench.html
You need to thread the castle nut onto the buffer tube before screwing the tube into the M4 adapter.When it bottoms out back it out just back it out enough to make the squared adjustment row of holes under the tube perfectly vertical.Now tighten the castle nut up against the M4 adapter to hold the tube solidly in place.You also of course have to put the stock onto the tube.
 
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