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Problem with my 1st .308 build...Help!


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I have been building my first .308 AR piece by piece to save on cost. I will describe the problem then I will list the possible culprits.

Drawing back the bolt with the charging handle all seems well, engage bolt stop, OK so far. NOPE, the bolt catch is catching on the bolt carrier and not on the face of the bolt. Disassemble and check, the bolt carrier will not travel far enough into the buffer tube for the bolt face to clear the bolt stop. The bolt face is about 1/2" forward of the bolt stop. My parts, with dimensions, are as follows:


REDX arms bolt carrier 7 7/8" long. With bolt installed and pulled forward 8 7/8'' long.

UTG Heavy buffer for .308 carbine 3 1/4" long.

JSE Surplus .308 carbine spring 11" long.

Mil spec Black Hawk buffer tube from Palmetto State Armory 71/4" outside length, 7" inside length. 


After all my reading and questions here and at other forums I thought I was proceeding correctly. I milled out an 80%Arms lower, bought a REDX arms upper, put in a Black Hole 308 barrel from Surplus Arms and Ammo, CMMG lower kit from 80% arms, added a CMC trigger and torqued and checked and fussed as best I could. Would someone check the length of their bolt carrier to see if mine is too long? Did I make a mistake with the mill spec buffer tube? The Heavy buffer? The spring? Was I supposed to sacrifice a goat on the alter of .308?


I would appreciate any help with this as there is some sort of simple noob error here. 



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I suspect your problem is your buffer tube/buffer/spring combo, with a AR15 type carbine buffer tube you have to use a really short buffer (around 2.5")

and a special spring with a higher spring rate that has a wider space between coils - the preferred combo is the CAR-10/Special AR10 spring from Slash:




If you don't have the dough for that, the standard LR-308 Carbine Buffer/Spring from DPMS seems to have the correct dimensions, I haven't fired mine yet but 

the measurements work out correct for proper cycling:





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I suspect your problem is your buffer tube/buffer/spring combo, with a AR15 type carbine buffer tube you have to use a really short buffer (around 2.5")

guruofhotrod got it right. Your buffer is too long. UTG doesn't make an LR308 pattern buffer (for use in AR-15 carbine receiver extensions). You need an LR308 pattern buffer for that tube.

Did you get the standard H-weight AR-15 buffer or H2 or H3?

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Thanks guruofhotrod and FaRKle. I suspected the buffer as I did not know of the length difference.  My mistake. Will replace with the correct buffer and spring. Thanks again gentlemen.

The reason I asked which weight heavy buffer you got is that it may be cheaper to convert it to AR-10 pattern and replace the receiver extension with an AR-10 one (or even a VLTOR A5 one since it's the same length) and get a AR-10 action spring. Those two parts will cost you ~$60, while a heavy buffer's heavy buffer will be $100 minimum. If the UTG buffer you got is H2 or H3, I'd definitely do the AR-10 pattern receiver extension route.

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