I have a 6.5 Grendel barrel set up for .127" headspaced bolt.
I have a 7.62x39 left handed bolt.
I have a 264 LBC reamer and Go gauge

What can go wrong!! I know how to set headspace on a bolt gun, but not on an auto with a barrel extension. Can somebody walk me thru this? It should only be .010" or so that I have to remove, so I will do this by hand.

I'm not quite sure what you're hoping to do here. If your barrel is chambered for a .125" bolt face depth, your 7.62x39 bolt is correct and it should be ready to use. What are you doing with a reamer?

doesn't lbc have a different throat? Thats what I think he is after maybe?

I think lbc is just the non copyrighted name for 6.5 grendal. The dimensions should be identical.

Fairly sure the LBC has a different throat, there used to be a lot of talk about pressure issues on some ammo in the Grendel as I recall and the lbc might have been more friendly to whatever factory ammo created the issue.

Since I can't recall the last time I really shot factory ammo of any substantial amount I load for my guns, which negates most issues others can run into.

My mistake. It is a .135 headspaced barrel, and I am trying to use a 7.62x39 bolt.
I'm not too worried about the throat as I have a 6.5 throat reamer and have throated my AA 6.5 G so I could shoot 120 Nosler Bal Tips. I'm not interested in a competition gun, I hunt with mine.

Just buy the correct bolt instead of messing around with a reamer. If you don't already know how to do it, and have the tools to keep it straight, you won't be doing yourself any favors trying to ream that chamber deeper. I've done plenty of chamber work on AR barrels and would not do what you're trying to do, at least not for that reason.

The best way to do this job is with the barrel extension loose from the barrel, which is not what it sounds like you've got.

Honestly - $40-$80 for the correct bolt and you're ready to shoot with a better barrel than if you try to ream it by hand.

Correct left handed bolts for a side charging receiver aren't easy to find.

OK, I get that.

So what are your capabilities with this stuff and what kind of tools do you have? Lathe, etc? Do you have some experience chambering barrels, or is this your first time? Are you the sort of guy who's pretty handy with tools and does quality work, or the other kind? The answers on how to do this depend a lot on what you can do. For example, there's not much point in suggesting how to do it on a lathe if you don't have one. You can do this by hand but it takes some attention to detail and a gentle hand with the reamer.

This barrel isn't nitrided is it? If it is, forget it, you'll most likely ruin both the chamber and the reamer; the nitrided surface layer is too hard. If it's bare stainless or parkerized chromoly you have a much better chance at getting it done.

I have access to a lathe and have done Mausers before. I usually do the last .015" by hand as I do not have a digital readout for the lathe.
Barrels are S/S. (I have 2 to do). I should be able to loosen the barrel extensions, but haven't tried yet. I hope they are not "gorilla" tight.

That sounds reasonable.

The barrel extensions should be tightened to 100-130 ft-lb, but may have been assembled with red loctite as well. Either way you'd need a decent barrel vice and an extension tool to remove them; I use the Brownells "reaction rod" that is sorta like the Geissele unit but only $60 and works really well. If they have red loctite on them, you'll have to heat them up a bunch to break that bond; maybe more trouble than it's worth.

IF you want to loosen the barrel extensions, you can insert a go gauge and your stripped bolt and screw the extension back on just snug against the bolt. Then use feeler gauges to measure the gap between the extension and barrel shoulder; that tells you how much deeper to run the reamer in. That's my normal AR barrel chambering practice anyway.

For this though, especially since you've done Mauser barrels and are used to taking the last little bit by hand, I'd be inclined to do it by hand and leave the barrel extensions tight. It's not much different than doing a Mauser, except that you have a lot less camming surface with an AR bolt, so just go in small steps and check frequently and watch the bolt start to cam in when you get close.

Don't use a piece of brass as the go gauge of course - you'll usually end up with a short chamber if you do. 4D Reamers has the gauges for rent if you don't already have them.

My preference is to mount the barrel vertically when reaming by hand; it seems easier to keep everything straight that way.

One potential issue to check is clearance for the bolt lugs between the barrel face and extension lugs. If it's too tight, your bolt won't lock in at all (even with nothing in the chamber), but you don't want it too loose either. I like to cut that dimension for .010" clearance.

I think you should be OK on this, but best to check it and be sure before doing any chamber modifications. I think the two different bolt sizes have the same lug length, but am not 100% sure on that and am not in a position to measure mine right now.

That's hard to measure with the barrel extension in place though, depending what measurement tools you have. You may have to get pretty creative with an indicator, or just do it by eye and feel, making sure the bolt has a little clearance but not too much. One crude method you can use: cut some paper circles of about the same OD as the bolt or a little bigger. Set one in the extension against the barrel face (covering the chamber) and try to lock the bolt in. If it does, try two pieces, etc. Personally I'd want clearance for at least one piece of paper (.004-.006") and no more than 3.

Hope that helps.

Sounds like I can leave the extensions in place. I usually do the final reaming vertically too. I have the bolt out of the carrier group now. Sounds like I should have it in to check the camming clearance. Are the pins in the extractor and ejector reusable after I strip the bolt? If not, is there a good source for these?

Yeah, you can re-use everything. The extractor pin is solid and easy to remove, just press the extractor in by hand and push the pin out with a punch. The ejector is a little more trouble, it's a tiny roll pin, but not a big deal if you have the right punch. Definitely strip the bolt down for this job.

You don't need the bolt in the carrier to check clearance, just the stripped bolt by itself is all you need. You're checking the distance between the barrel extension lugs and barrel face, nothing to do with the carrier.