Some history,a lifelong friend had bought an early Bushmaster 6.8 before everyone realized the barrel specs were all wrong.

I talked him into replacing the barrel with a SS 18" SPEC II chamber 1/11 twist, lands, etc from a respectable company, may have been Rainier so he could shoot the good ammo, like SSA.

At the same time, he upgraded a lot of the rest with DD rail, Aimpoint M4, ya da, ya da. Having built many ARs, I did the work for him. Bolt cycled great after the rebuild.

He eventually had range time with it, not much, but claimed to have sighted it in, pretty low round count using SSA ammo mostly.   Couple of years pass, he needed money, sold me three nice guns at great prices, like new.

He said he never had a problem with the Bush in the little time he shot it. After I got it, I was simply function checking it, EMPTY, and the bolt locked so hard I had to get a mallet to pop the charging handle back. I cleanded it, chamber brush and all. Lubed.

Put it back, and it did the same thing. Some other forum suggested the bolt face may be too deep and need to be sanded down a tiny amount. I'm surprised this couldn't have happened at the range at least once. Any other thoughts or ideas? I don't have a spare 68 bolt yet.

Shoot some ideas, black hearts.....

What 6.8 bolt is it? Would one of those go / no-go gauges help determine if your barrel and bolt are in spec?

I'm thinking its the original Bush bolt. He did have another SBR 6.8.

Another forum suggested sanding a bit off the face of the bolt in case it was pounding the forward recess of the extension.

I have a go , no go somewhere packed up. But I've never really had to use it in building about a dozen ARs without problems.

Thought about putting a std 556 bolt in it to see if it would lock up. Didn't know if the lugs were any bit different than the 68's.

I'd guess the bbl extensions are probably the same cut for about any caliber, IDK......

What is the barrel stamped with?  Might help identify it.   Other than that I might be getting a set of 6.8 head space gauges and checking.

I too bought an early Bushmaster 6.8 but I got one with a SPC II / 1:11 twist chamber.   Not a problem with that and it's a good shooter too.

Get some Dykem blue from any machine shop supply and paint the lug area of the bolt before doing any sanding.  This will help you to understand what is causing the lockup.

Thanks, I'd thought about a little white paint to do the same. Don't want to lighten the thing, for sure.

Oh you really should be headspacing that barrel/bolt combo, anything else is a wild ass guess.

I agree, you are right. I've just done so many without a single issue. And, I also realized my Go, No go gauge for typical 5.56 wouldn't have worked anyway.

I thought, Bushmaster, quality bbl, ( I talked him into either White Oak, or Rainier ) they should have it down. The thing is, it worked fine for my friend for a least two range sessions without a problem, or he would have told me about it.

I've known the guy since we were babysat together, like 48 years. We are still best friends. When he has overspent at times, he's always given me a nice price break on gun deals. He also gave me a hellofadeal on about 300 rds of SSA ammo, when you could not find any, anywhere.

I'll have a nice LWT Commander he sold me in my will, that my dad and others did some custom work on, that he will get back someday, if he outlasts me.  He's been like a brother, or better.

Well, it is not a bolt issue at all I learned today. I tried another complete 5.56 BCG, and it fit just fine. Then, I grew a smart cell in my brain, and swapped out the 6.8 bolt into the other carrier.

Slid in with no problems. I took the problem carrier, minus bolt, and tried to slip it down the upper, and there is an apparent misalignment with the key, gas tube. Although, to the eye, with a bright light, the gas tube looks very straight.

I'm thinking it is the key, although it looks normal just looking at it. I may try a new key on it and see if it is salvagable. I'm thinking the carrier got swapped out sometime after it was last shot. I don't know.

Has anyone had to replace just the key on their carrier for a total fix? I may post this as a new question.

I had that exact issue with an upper several years ago. Put it together using all Bushmaster parts, and found the BCG wouldn't go in all the way. After trying to figure out what I did wrong, finally found that the gas tube would not fit into the key. Frustrating to say the least ! Bushmaster replaced the key and I was good to go.

With the upper detached, stick your pinky into the rear hole of the bolt. Can you easily move it in and out of the chamber? If not, chances are the gas rod is out of alignment. If you messed with it or if the front sight is not the pinned kind then it could be off or canted.