lr308 bolt/extension kinda question

[AXEMAN]

ok, bear with me on this. new lr308. new bolt. new barrel. not assembled. while i was trying to dry fit it all, i had an issue. while looking everything over, this is what i found. lets say i am holding my barrel in one hand (thats what she said) and the BCG in the other. i put the bolt in, it bottoms and it starts to engage but will not rotate. both parts are new, the barrel was ordered as a DPMS spec barrel, not ARma. is it just because they are new? im not forcing it, so i just want to see what to do next.

is there a known distance to measure here, like face of barrel to face of extension? does this sound like the extension/barrel or bolt? i send an email and called the barrel maker with the same questions. honestly, i am new to the ar15 and ar 308 and dont want to force anything. but i think there isnt room between the back of the extension and the face of the barrel.

any thoughts?

 

[AXEMAN]

Re: lr308 bolt/extension kinda question

lonewolf, i tried to fit it together. when i dropped the BCG in, it wedged. it wont go all the way forward as the bolt stops rotating.

the BCG is operational. it rotates and extends and retracts like it should. its just that the lugs cant lock and its like there isnt enought space as just the ramped forward edges of a few lugs seem to engage behind the extension. if that makes sense. is that a part of headspace in the AR world? is it different for dpms vs armalite? i know the answers may be somewhere, but this is all dpms but the barrel

 

[pagana]

Re: lr308 bolt/extension kinda question

First check that there is nothing sticking up like a burr on either the barrel face, front of bolt, or lugs of bolt. All of the above have an effect on headspace and collaborate to give the correct "timing" to the parts and could be preventing the bolt from engaging. When a custom barrel is added these dimensions should combine to produce the following: .001"-.002" total clearance between extension and bolt/barrel with the extension torqued to 50-65 ft/lbs and the headspace between go and no go depending on the chosen cartridge. In the above scenario, the headspace would be the measurement between cartridge datum in the shoulder of the chamber to the bolt face with the lugs seated against the extension lug surfaces. Less clearance, as mentioned above, prevents the inertia of the BCG assembly from affecting the headspace of the rounds fired when fed into the chamber. You will need some tools to get an accurate measure of the parts. You would remove the extension and measure from extension face to lug surfaces and also the distance from bolt front to bolt lug surfaces and cartridge face of bolt. You will likely need to remove material from the face of the bolt, but you must first determine your headspace as, if insufficient now, will be worse. By that I mean that you can screw the extension on with gauge installed and bolt in place to see if it stops prior to the extension being fully threaded on. All this should have been taken into consideration when the barrel was chambered including the .001" or so crush when the extension is torqued. In fact, this may be all that is causing the problem. Point is not to remove much material from bolt face or it may pose problems with feeding.

 

[Hellbender]

Re: lr308 bolt/extension kinda question

AXEMAN- Everything Toolmakr said, if you had a custom barrel made.

Did the gunsmith have the bolt when he assembled the barrel and extension?

Are you doing this test with the bare bolt ONLY, as you are adding another variable with the carrier installed, as the face of the carrier could also be hitting the extension.

Another possibility (slight) is your bolt could be out of spec or on the high side of tolerance.

 

[AXEMAN]

Re: lr308 bolt/extension kinda question

i am still re-reading what toolmakr said, lol. the smith did not ask for the BCG. other places required the BCG, this place did not. i have emailed them and i am waiting for their reply.

what i am trying is holding the barrel in one hand and the BCG in the other and putting the bolt into the extension and turning the BCG and it will not rotate and lock the lugs of the BCG behind the lugs of the extension. mechanically this will not happen and this is not allowing assembly of the rifle. the upper will not close as the BCG wont go all the way forward due to the bolt not rotating and locking up. barrel face and extension all look clear and free of burrs and debris. i think i will have to send the barrel and BCG to the maker. i was going build it myself, then i was gonna send the whole thing to GAP but this issue made me want to figure it out before i sent it to GAP.

 

[Hellbender]

Re: lr308 bolt/extension kinda question

From your description, it sounds like the barrel is installed on the upper, if so, is the gas tube installed?

If so, remove it and try again. Armalite has about .2" shorter gas port location and I have heard of makers drilling for the shorter Armalite tube. If you have the DPMS tube, it will do what you are describing.

If that's the case, the gas tube will be sticking into the upper too far and stopping the carrier prematurely.

 

[AXEMAN]

Re: lr308 bolt/extension kinda question

nope. sorry i wasnt clear. its a new build from parts. nothing is locked down at the moment. no gas tube installed. i had the rifle all dry fitted and just hand tight to check the weight and balance. when the BCG came, i opened the upper and dropped it in. i couldnt close the upper. after eyeing everything and taking it back apart to get the BCG unstuck, i realized that after the bolt head entered the extension, the bolt head could not rotate in the space between the extensions face and barrels face like it was supposed to, and that was why the BCG would not go into the upper all of the way and allow me to close it up. the barrel maker is on the west coast so i am waiting for them to open. im not really sure of my options at this point

 

[sandwarrior]

Re: lr308 bolt/extension kinda question

Axeman,

As it sounds you have a barrel that does not have enough cut out in either the backside of the barrel extension lugs or the face of the barrel (that would be the face inside the extension) is too short (possible the chamber is not cut deep enough. One other possibility is that the cut around that area is not deep enough. You can shove the lugs in but they won't rotate. But, they will rotate when you work it by itself in the BCG. The explanations above are pretty much the reason.

Bolt lugs are a lot easier to cut than the barrel extension and I don't see those being out of spec from a standard bolt manufacturer.

 

[AXEMAN]

Re: lr308 bolt/extension kinda question

i sent it back with the bolt and they said they would make it right. its there by now so i will wait and see whats up but i expect it to be back next week. i think by then ill have the install solved. i need to look for a crow foot so i can get torqued right

 

[sandwarrior]

Re: lr308 bolt/extension kinda question

In the meantime....sitting around would be killing me.... (that's where I'm at with my 6.5G)

Hope this comes out all good for ya!

 

[AXEMAN]

Re: lr308 bolt/extension kinda question

man i had a receiver at SSS for a T&T and before i sent it i knew it would take as long as it took. im not in a big rush

i dont get upset