Question about uneven wear on bolt's lug faces

[SDW]

Hi all. I'm wondering about the cause of the uneven wear on my Grendel's bolt. Quick stats:

• Bolt is from Ballistic Advantage. Purchased with barrel.
• Barrel is BA 20" Premium DMR model.
• Bolt carrier is a PSA Premium NiB coated.
• Upper receiver, can't recall. Not anything "top tier".
• Rounds through this upper, less than 200 I'd say.

Tools at my disposal for dealing with this are a bore scope, which I can use to more closely exam barrel extension up close if need be, and on the way (by sheer coincidence) is an upper receiver lapping tool from MidwayUSA. Been wanting to check and ensure the "trueness" of my uppers.

Not the most focused pic, but it shows pretty well the finish worn off mainly on the two lower lugs. But also worn of partly on the two lugs adjacent to them. Seems to scream off-axis contact between the bolt and barrel/extension. But what say you good folks?

IMG_2012.JPG

[A5BLASTER]

Lapp the upper and it should fix the issue.

You for sure aren't getting a square bolt lock up.

[SDW]

Thanks. I'll do that. Just pulled apart my upper in prep. The lapping tool should arrive on Friday, and I already have some 220 grit lapping compound. I will report the results once I've got some lapping done. FWIW, I anticipate that there will be some material coming off the top.

[SDW]

BTW, here is a bedding question. I think the convention is to use the green 620 retaining compound for bedding, right? If so, have you ever had to remove a barrel that's been bedded this way? I'm curious how much heat it would take and how it would be applied.

[Klem]

SDW,

The bolt wear is the finish coming off a new phosphated bolt, so there's probably nothing wrong with it.

The worn area of your bolt is the part of the bolt that first impacts the top of the rounds as they sit in the magazine. They then push the rounds forward clearing the mag lips and up on an angle into the chamber. If you load a loaded mag into the gun and pull back the charge handle you can see where the bottom lugs touch the top of the round. 200rds is just enough to show the wear on that new bolt. In time the whole finish on that bolt will wear, especially in places where it rubs.


If you are worried about the bolt not being square to the barrel extension look at the back of the lugs, not front. It's the back of the lugs that are hard up against the teeth of the extension when the round goes off. The front of the lugs are not touching anything on shot release.

On a side note, I was always taught to oil the backs of bolt lugs before firing while keeping the front of the bolt relatively dry.

[tdbru]

what Klem said!! it's the back of the lugs that you worry about full engagement on, not the front of them. Klem is spot on.
-tdbru

[SDW]

Thanks guys. That is a relief. I was honestly puzzled about what part of the extension might be contacting, since obviously it's the back of the lugs that bear the force. I thought the fronts only touched air! Hah. I'd just never noticed that kind of wear form on any of my 5.56 bolts.

That's what I get for watching so many SOTAR videos lately! That guy has me wondering about every little thing. No regrets on pulling my rifle apart. Been wanting to do that with a few of them anyway.

On to the bedding question I slipped in, if you don't mind. Anyone ever learn to regret bedding their AR barrel using Loctite 620? I've used their 680 before (still have some) and know how solid you can lock parts together. Taking it apart at some point later might be a challenge.

[tdbru]

Criterion uses Loctite 609 for their assemblies. it takes the least amount of heat (300F if i recall correctly, but if i were you i'd double check) of their sleeve assembly compounds to disassemble. i think 620 takes like 450F to loosen. others measure OD of the barrel extension and ID of the upper tunnel and wrap in a shim in of the correct size to take up the gap. if i recall the BCM upper has a slighty undersize extension tunnel on their upper and you have to heat the upper extension tube area to get a barrel slid in. it's an interference fit. check for previous reply links in the forum on upper barrel bedding. there are a lot of them.
-tdbru

[Klem]

SDW,

If you can't see the wear in your other bolts they are either differently finished or evenly worn by the time you inspected them closely.

To be clear, I am not suggestion you should skip the squaring of the receiver when that lapping tool turns up. It could improve things and it certainly won't hurt it. It's also peace of mind. If I had one I'd use it on every build.

I tried gluing once to see for myself but it made no improvement to group sizes. I used Loktite 641 and it came apart easily (which it is designed to do). The join was tight to start with and the receiver a billet so I don't think it was fair to expect much better. Again, to be fair I can see how gluing might help in a sloppy fit and don't doubt the other guys who say their group sizes shrunk. If this were me however and the join a sloppy fit I would trade the upper for a better one. Uppers don't cost much relative to the rest of the gun, especially when you think you might halve your group size.

Here's a thread on the subject

http://www.65grendel.com/forum/showthread.php?18407-How-do-you-bed-a-barrel-to-the-upper-receiver

[SDW]

Thanks again guys for the info on bedding. My barrel extension actually fits pretty snug in the upper. Takes a little persuasion to get it out and push it back in again. So think the clearance is tight enough that if the two aren't darned perfectly aligned with one another, it binds. I think I can forget the gluing idea, which I'm relieved about. One less thing to fool with.

For the lapping, I'll just give it some quick, minor polishing to start with to see if it reveals bright metal all the way around. If so, there won't be much of a need to go further.

Back to the other thing. Just for fun, I dragged out my little Teslong camera because I noticed something strange looking inside the extension. And odd series of scuff marks in the lower half. Mostly lower half. The sections/segments of scuffed surface run from about 2 o'clock to about 8 o'clock. Coincidence you think... something done while the barrel was being built? The marks do roughly match up with the scuffed lug faces. I don't even know if it's possible for the bolt face to reach that far into the extension. Maybe if the two planes weren't in parallel and the bolt had been dropped on an empty chamber? But I never do that. Always believed that was a big no-no.

Scuffs_none.JPG

Scuffs_on_extension.JPG

[Klem]

I would clean the marks off that chamber with a dedicated AR15 wire or nylon chamber brush. Or some flannelette on the end of a screwdriver.

[SDW]

Didn't quite use those same tools, just a dry piece of paper towel wrapped around some tool. I went around that surface a bunch of times. What you see is what was left behind after that process.

I have a ton of wood-shaft gun cleaner Q-tip type swabs that I can use next. Plus some solvent I guess. I can even run that in a drill I suppose. But what will cleaning it further prove?

[SDW]

Sorry, that was an incomplete thought. I can buff a little of that black stuff off. FWIW, the camera shows higher contrast and makes it look blacker than it really appears to the bare eye.

I'll buff it a little, then try to take a pic without the blinding light shining on the surface.

[SDW]

Took another pic, more high-level, outside in indirect sunlight. Give a better view of the big-picture. I'd cleaned off the surface a little better here using one of my super-duty swabs and a drop of Kroil. Upper half is devoid of these blemishes.

It's the best I can do with focus I'm afraid. I miss the days of actual cameras with macro lenses and manual focus. (Obviously I could buy one I suppose.)

Scuffs.JPG

[SDW]

I'm thinking there is not too much I can do at this point. I will go ahead and ensure that the upper receiver is trued, and put everything back together. I don't have another bolt to swap in as a test, unfortunately. Been meaning to buy one to have as a spare. Probably get one from BFT/Six5. At the same time I do that, I'd also want to get a 6.5G a headspace gauge. A NO-GO gauge, at the very least. So the "spare" idea is a little expensive.