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Posted: 10/21/2011 8:44:35 AM EDT
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I have built a 6.8 SPC. I used all new parts. The bolt will not turn and lock.
I have taken a 5.56 bolt and carrier from one of my other guns and it will close and lock on the 6.8 SPC. The 6.8 SPC bolt/carrier will also close and lock on my 5.56mm gun. I also took the 6.8 SPC bolt and put it in a different carrier and it would close and lock on the 5.56, but not on the 6.8 gun. It looks like the 6.8 bolt and 6.8 gun are not compatible. Any thoughts? |
| This is due to the face of the 6.8 bolt being to tight to the face of the barrel which is probably on the larger end of the spec, "or did you use an upper receiver lapping tool? This could have caused this as well, if you removed too much material when lapping the upper " Your 5.56 bolt is probably a little on the smaller end of the spec. Also your 5.56 barrel probably is as well, which is why your 6.8 bolt will close in the 5.56 extension. If you didn't lap the upper, I would send the bolt back and let them know the issue and maybe they can send you a different one to try. Hope this helps. |
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Quoted:
This is due to the face of the 6.8 bolt being to tight to the face of the barrel which is probably on the larger end of the spec, "or did you use an upper receiver lapping tool? This could have caused this as well, if you removed too much material when lapping the upper " Your 5.56 bolt is probably a little on the smaller end of the spec. Also your 5.56 barrel probably is as well, which is why your 6.8 bolt will close in the 5.56 extension. If you didn't lap the upper, I would send the bolt back and let them know the issue and maybe they can send you a different one to try. Hope this helps. Just out of curiosity, are you saying the locking lugs are too thick to fully seat in the barrel extension? |
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Quoted:
This is due to the face of the 6.8 bolt being to tight to the face of the barrel which is probably on the larger end of the spec, "or did you use an upper receiver lapping tool? This could have caused this as well, if you removed too much material when lapping the upper " Your 5.56 bolt is probably a little on the smaller end of the spec. Also your 5.56 barrel probably is as well, which is why your 6.8 bolt will close in the 5.56 extension. If you didn't lap the upper, I would send the bolt back and let them know the issue and maybe they can send you a different one to try. Hope this helps. The .001-.003 material removed when lapping well have no effect on the bolt fitting the receiver extension. This is one reason I like to buy barrel and bolt as a matched set. Look for any burrs on the bolt lugs and the extension. Put some machinist blue on the bolt and see if you can determine what is interfering. |
| You are correct. .001-.003" will not affect it that is why I said if "too much" material was removed. I know of uppers that have had too much material removed and ended up too close to the bolt face. It is very easy to remove too much material. And to answer the question before it could be the barrel face or extension surfaces with a burr like was also said. Not necessarily the bolt itself, but it could also be the bolt or a combination of both. |
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UPDATE:
Thanks for all of your responses. I decided that I was going to bring it to a local gun smith since I was driving right by there last weekend. The smith said that he had never seen a bolt with little dimples on the back side of the locking lugs and that it most likely was machined wrong. We tested a DPMS 6.8 bolt that they had in my gun and carrier and it worked as it should. I won't tell what company that I bought this from, since I have not contacted them yet, but it is a bolt and barrel combo. I have been out of town and am going to call them and send the bolt back tomorrow morning. I will let you know what the outcome is and get some pics of the bolt and gun. |
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Bolt lugs should be .275" long, the dimension from the front of the lugs of the extension to the breech of the barrel should be .285 or .010 longer than the bolt lugs.
That is why the bolt worked with the 5.56 barrel but not the 6.8 barrel, it wasn't the bolt but the barrel extension. Some of the barrel extensions being made by AO are short so when assembled on barrels the space between the breech and extension lugs is not large enough to get some bolts in. CMT/Stag bolts are usually .277 or .002 longer than most other bolts. The condition should be adjusted by the company making the barrels which is usually ER Shaw or Wilson Arms but usually isn't. |
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