• Watch Out for Scammers!

    We've now added a color code for all accounts. Orange accounts are new members, Blue are full members, and Green are Supporters. If you get a message about a sale from an orange account, make sure you pay attention before sending any money!

ar308 bolt hard to come out of battery/ weird wear marks

Jmccracken1214

Supporter
Supporter
Full Member
Minuteman
  • Dec 10, 2018
    2,296
    1,045
    Thomasville, NC
    My ar10 cycles back and forth smooth, but when put into battery, without a round, the bolt locks up tight in the chamber and is pretty tough to pull back.

    What could I be looking for to cause this? The rifle shoots and cycles fine also, i just pulled it out and was dry firing it and noticed how hard it was to cycle the bolt. Happenes with or without a magazine.

    Toolcraft BCG and a rainer arms ultra match barrerl.
     
    I just noticed wear marks in the buffer tube, in the threads where the bolt has made contact with them.
    F5A4AFF7-2676-4BBA-ADE9-F8A9990842D9.jpeg
    497D8DB7-9C3F-4C8D-B6F9-C50C46812285.jpeg
    4E8D58D6-D24C-4B53-80E5-12C852950062.jpeg
    EF8A8383-3A7D-4379-8119-3DBC288D97CB.jpeg
     
    I had issues like that with a 308 build. The cause was the index pin on barrel extension was off center just enough to cause alignment issues with bolt. No idea if that's your problem but it's something to check.
     
    • Like
    Reactions: Jsp556
    I assume the gouges on your lower match up with features on the bolt carrier. If the bolt carrier is slamming into the receiver tower and gouging it, the BCG is traveling too far to the rear. You may be looking at two different issues.
     
    I assume the gouges on your lower match up with features on the bolt carrier. If the bolt carrier is slamming into the receiver tower and gouging it, the BCG is traveling too far to the rear. You may be looking at two different issues.
    I think the armaspec recoil spring is allowing the bolt to come back further than it should, even when I pull it back manually, it seems to go back into the tube all the way. I just went out and turned my gas down 2 clicks, and it wouldnt lock back, so its not overgassing.
     
    I'm going to wildly speculate that it shouldn't be doing that.
     
    • Like
    Reactions: SSBB
    What does the other end of the bolt carrier look like?
     
    factory ammo.

    I took the lower off, and pushed the bcg into the tube, against the spring and can easly push it all the back back into the receiver.
     
    How much slop is there with the BCG in the upper? Does it tilt a lot when you wiggle it around?
     
    How much slop is there with the BCG in the upper? Does it tilt a lot when you wiggle it around?
    It wiggles a little bit, side to side, and up and down, but I just checked 2 of my ar15's and they both have the same amount of play.
     
    Seems to me the only way it can hit your lower on the way back is if the tolerance fit is too large on the upper. I would take out the buffer/spring, remove the bolt, assemble BCG into upper, attach to lower and see if I can manually jam the BCG into the lower by tilting with my fingers.
     
    I dont think I have a dedicated ar10 buffer/spring here, but I have a spring that measures 10.75" with a carbine buffer, its tough to get it back enough to lock back, by hand, but i can do it.. I fired 3 rounds with it, adjusted the gas once to get it to lock back and it did and no more marks, (I colored the marks with a sharpie)

    Might have to ditch the armaspec and get a regular buffer/spring
     
    • Like
    Reactions: GParitsky
    I think you have two issues.

    1. Your BCG is traveling too far to the rear. This can either be because your buffer needs a little spacer (you can use a quarter dropped into the buffer tube if that's the case), or you have an AR-10 length buffer tube (~7.75" length) with a stubby AR-10 buffer. I don't recognize what buffer you have in there. You should measure the length of your buffer tube, and the length of your buffer. If your buffer is the same length as an AR-15 you may just need a little spacer. If your buffer is 3/4" shorter than an AR-15 buffer, you need to swap either your buffer tube to a regular AR-15 7" tube, or you need to swap buffers to a regular length AR-15 buffer.

    2. When you've loaded rounds into the gun, have you extracted one and inspected the bullet? Is the bullet engaging the rifling? That would make the cartridge difficult to remove when loaded but it would extract normally after firing. Or, if your BCG is used and has carbon buildup inside the gas key, the gas tube can lock up on the carbon in it. Or, it could be bad headspacing - did you use Go and No-Go gauges to check headspace? Or it could be a few other things but I'd start with checking the bullet for rifling engagement and checking headspace.
     
    I think you have two issues.

    1. Your BCG is traveling too far to the rear. This can either be because your buffer needs a little spacer (you can use a quarter dropped into the buffer tube if that's the case), or you have an AR-10 length buffer tube (~7.75" length) with a stubby AR-10 buffer. I don't recognize what buffer you have in there. You should measure the length of your buffer tube, and the length of your buffer. If your buffer is the same length as an AR-15 you may just need a little spacer. If your buffer is 3/4" shorter than an AR-15 buffer, you need to swap either your buffer tube to a regular AR-15 7" tube, or you need to swap buffers to a regular length AR-15 buffer.

    2. When you've loaded rounds into the gun, have you extracted one and inspected the bullet? Is the bullet engaging the rifling? That would make the cartridge difficult to remove when loaded but it would extract normally after firing. Or, if your BCG is used and has carbon buildup inside the gas key, the gas tube can lock up on the carbon in it. Or, it could be bad headspacing - did you use Go and No-Go gauges to check headspace? Or it could be a few other things but I'd start with checking the bullet for rifling engagement and checking headspace.
    The armaspec silent spring Im using measure 7 1/8"
    The buffer tube is a Aero enhanced tube
     
    Yup! That was the issue here.

    I bought that lower complete too, the tube was screwed in just far enough to catch the buffer pin, now that it's 2 more turns in, when I take the bcg out and push it down into the spring/tube, it doesnt come close to contacting the receiver.
     
    You bought it complete and I’m assuming the castle nut wasn’t staked if you just screwed the buffer in more? Easy fix. Well anyways, that’s the fix for the overtravel of the BCG then. For the lockup issues I’d confirm any rifling engagement and headspace as I noted above.
     
    I had something similar with a kak bcg. The tolerances were pretty tight where the gas rings were. After about 150 rounds it worked itself out. Ensure you clean out the carbon buildup on your bolt and inside the carrier.

    And I just saw you fixed it... so nevermind.
     
    You bought it complete and I’m assuming the castle nut wasn’t staked if you just screwed the buffer in more? Easy fix. Well anyways, that’s the fix for the overtravel of the BCG then. For the lockup issues I’d confirm any rifling engagement and headspace as I noted above.
    I’ll check a round, but it locks up tight without a round too.
    I had something similar with a kak bcg. The tolerances were pretty tight where the gas rings were. After about 150 rounds it worked itself out. Ensure you clean out the carbon buildup on your bolt and inside the carrier.

    And I just saw you fixed it... so nevermind.
    That may be the case here, I haven’t put 100 rounds through it yet.
     
    If these other checks don't solve it, the extension may be improperly aligned. causing the lugs of the bolt to hang, I've seen it many times and while it may function, it will wear at an accelerated rate.
     
    • Like
    Reactions: rpoL98
    I’ll check a round, but it locks up tight without a round too.

    That may be the case here, I haven’t put 100 rounds through it yet.
    If it's doing it with no round then I would borrow another BCG and test it. That is abnormal. First thing though, remove the bolt from your BCG, separate your upper receiver, make sure the lugs and area the bolt goes into the barrel is all clean, and try to insert the bolt by hand. If you can put the bolt into the bolt extension and rotate it into its locked position, then it's a bolt carrier problem. If the bolt will not easily go into the locked position with a little fore/aft wiggle room, then you have either a bolt or barrel extension issue.
     
    yours:
    ef8a8383-3a7d-4379-8119-3dbc288d97cb-jpeg.7905733


    mine:
    XekVxn7.jpg


    had a similar situation, Rainier 6.5CM Ultramatch. Bolt lugs seriously impacting, dragging on barrel extension lugs. tried 2 other bolts, same behavior. Tried barrel in 2 other Aero M5 uppers (M5E1 enhanced and regular), same bad. The barrel extension feed ramps lined up good enough with the upper feed ramps, as good as my other M5 uppers, visual optics were good, used a small shim on one side of the index pin to bias the barrel extension over a little so the alignment of the feed ramps looked really good. but still some drag on the lugs.

    when I hand tightened the barrel nut to the upper/barrel, the BCG seemed to go into battery ok, some drag on the lugs, but as soon as I torqued it to 65 ft lbs, the BCG, regardless of which BCG or upper receiver, always clanked into the barrel extension lugs. I could force it in, but then when I went to pull the charging handle back, I couldn't get the bolt to come out-of-battery.

    I replaced the Rainier barrel extension with a Lilja barrel extension. I did have to address headspace, and gas port clocking, though.
     
    Last edited:
    I have a bolt/barrel combo from AR15Performance which came with an notice that the bolt may feel like it sticks in the barrel extension due to the tight tolerances. I don’t think he sells that combo anymore, but it wore in fine in my gun after a couple of mags. Probably not the OP’s issue, but a relevant observation none the less.
     
    Is it an Armalite bolt, DPMS barrel extension?

    Or is it the other way with a DPMS bolt in an Armalite extension

    Which bolt lugs are thicker?
     
    Is it an Armalite bolt, DPMS barrel extension?

    Or is it the other way with a DPMS bolt in an Armalite extension

    Which bolt lugs are thicker?
    not sure who you're addressing the question to, but in my case it's a Rainier DPMS-pattern AR-308 barrel in 6.5CM, and an Aero 308 DPMS-pattern BCG. FWIW.