Questions about my first AR build???

[customcutter]

LRRPF2,

Thanks for the link. Yes I'm familiar with "blue printing" a bolt action even though I haven't done it. I probably won't do it with the Grendel, but I will put a little dykem on the lugs just to see how much contact I have. The main reason I probably won't do it is I don't want to mess with the headspacing on the bolt/barrel combo. I've read several threads on the subject now, I'm not sure how much it does for accuracy. However, I am convinced that it can be a contributing factor to broken bolts.

Nugginfutz,

Thanks for the diagram. As they say a picture is worth a thousand words. I'm not sure why, but I was visualizing using the spring and the back half of the case.

thanks,
CC

[LRRPF52]

AR15 head spacing is sloppy by design. There's no way the mass and inertia of the bolt carrier group can generate enough force to deal with variance in shoulder location like the camming action of a turn-bolt Mauser. So precise shoulder locations aren't an issue, and have to be under-sized to feed in a gas gun. There's no way you're going to affect headspace with lapping, unless you use heavy grit media, and go to town on it for hours after ingesting some serious amphetamines.

You don't need to blue print as far as receiver face is concerned, unless it's obviously out of spec, which will break bolts all day long. Some smiths start from scratch, and will true the barrel extension, all the faces, and go super-anal on it, and end up with a gun that shoots sub-1/2 minute consistently. There is a lot of work involved in getting that extra 1/4" that most people won't bother with.

[NugginFutz]

My understanding of the reason behind lapping the locking lugs is as follows:
The primary purpose is to increase the bearing surfaces of the lugs. This ensures the load encountered during firing is evenly distributed. The secondary reason, but the one most desired, is the potential for improved accuracy. There seems to be a perception that the Grendel is a bolt breaker, but I think that the reputation is more anecdotal than factual. Regardless, I can see the practical application of lapping the 6.5G bolt's locking lugs, if for nothing else, than to evenly distribute the force loads across all the lugs, thus reducing the possibility of any one fracturing.

What I am uncertain of, however, is the accumulated wear of the the barrel adaptor. If, for example, you have more than one bolt, do you lap them all? And, if so, does that translate into unwarranted wear of the barrel adaptor (manifesting as additional headspace)? It seems to me that there is a distinct possibility of overthinking this.

[customcutter]

I think the main areas I'm going to concentrate on are the squareness of the upper and the fit of the barrel extension to the upper. I won't be punching paper for that last 1/4". It will be a hunting rifle that I can hopefully get to shoot sub MOA. To start with I'll assemble without bedding the barrel extension and I'll use anti-sieze on the barrel nut. If accuracy isn't there I'll back up and start over.

[NugginFutz]

I think the main areas I'm going to concentrate on are the squareness of the upper and the fit of the barrel extension to the upper. I won't be punching paper for that last 1/4". It will be a hunting rifle that I can hopefully get to shoot sub MOA. To start with I'll assemble without bedding the barrel extension and I'll use anti-sieze on the barrel nut. If accuracy isn't there I'll back up and start over.

A very sensible, organized approach. If you know anyone with an upper receiver lapping tool, it may be worth offering a 6-pack for the use of it. Half of my uppers, when checked, were not square (some a little, one a bit more). By lapping them, I at least had the peace of mind that squared receivers were not going to be an issue. Mind you, now, some of mine are bench riders, and others are mud guns. I treat then all the same, though, in case I ever interchange parts.

[Tedward]

All this talk about squaring up the extension to the upper is really deceptive. If the extension is installed on the barrel by the barrel maker that should be square. Then you put the barrel into the upper and when the barrel nut is snugged then torqued, it presses the extension flat against the upper receiver. There is no adjustment. If you find it off by the method LRRPF52 (Midway) said with the dykem to see the contact points, then using the compound is just to smooth and make it contact evenly.

Is that all your talking about doing? Mate the surfaces of the back side of the bolt lugs to the barrel extension with the lapping compound?

[customcutter]

All this talk about squaring up the extension to the upper is really deceptive. If the extension is installed on the barrel by the barrel maker that should be square. Then you put the barrel into the upper and when the barrel nut is snugged then torqued, it presses the extension flat against the upper receiver. There is no adjustment. If you find it off by the method LRRPF52 (Midway) said with the dykem to see the contact points, then using the compound is just to smooth and make it contact evenly.

Is that all your talking about doing? Mate the surfaces of the back side of the bolt lugs to the barrel extension with the lapping compound?

The procedure that LRRPF52 and Midway were talking about is to lap the bolt to the lugs inside the barrel extension.

Squaring the upper receiver to the barrel extension is another matter. The uppers are not always square to the bore. Brownells sells a lapping tool that can be used to square it up and make sure you have 100% contact around the upper receiver to the barrel extension. Then you get into, is the barrel nut machined square, are the threads on the upper machined square, etc, etc. It would be similar to "blue printing" a bolt action receiver. An awful lot of work to chase that last 1/4" of accuracy. Maybe worth it in National Competition, but not a hunting rifle.

I don't have a jig, or know of anyone that has one. I do have a large surface plate and machinist squares, so I'll check it that way. If necessary I can turn a jig on my lathe and lap the upper, but from what I've read the VLTOR's are machined pretty well.

[customcutter]

Any thoughts on this BCG and Supplier? Maybe I'm looking in the wrong places but looks like any decent BCG is $130-150??? Seems like it would be worth the extra $50?

https://www.primaryarms.com/WMD-Guns-NiB-X-NiBH-X-Complete-Bolt-Carrier-Group-p/nibxbcg-0002.htm

[NugginFutz]

The procedure that LRRPF52 and Midway were talking about is to lap the bolt to the lugs inside the barrel extension.

Squaring the upper receiver to the barrel extension is another matter. The uppers are not always square to the bore. Brownells sells a lapping tool that can be used to square it up and make sure you have 100% contact around the upper receiver to the barrel extension. Then you get into, is the barrel nut machined square, are the threads on the upper machined square, etc, etc. It would be similar to "blue printing" a bolt action receiver. An awful lot of work to chase that last 1/4" of accuracy. Maybe worth it in National Competition, but not a hunting rifle.

I don't have a jig, or know of anyone that has one. I do have a large surface plate and machinist squares, so I'll check it that way. If necessary I can turn a jig on my lathe and lap the upper, but from what I've read the VLTOR's are machined pretty well.

What he said.

Because some of my rifles do shoot for that last 1/4", I find the effort worth while. Just not explicitly for a hunting rifle, as CC says.

[NugginFutz]

Any thoughts on this BCG and Supplier? Maybe I'm looking in the wrong places but looks like any decent BCG is $130-150??? Seems like it would be worth the extra $50?

https://www.primaryarms.com/WMD-Guns...bxbcg-0002.htm

I've only heard about PA (most of it good), but have never used any of their stuff.

The first thing about your selection that jumped out at me was that this seems to be a 5.56 bolt, not a Grendel bolt.

[LRRPF52]

Most production rifles don't have evenly bearing bolt lugs. This is true of hunting rifles, as well as AR15's. You can check any of your rifles with Dykem and confirm this for yourself. It's simply a product of slapping production guns together on an assembly line where cost benefit analysis doesn't warrant ensuring 100% contact between the bolt lugs and their recesses or barrel extension.

My leaning is towards the durability of the system, then accuracy. Since we're dealing with a bolt geometry in the AR15 that was designed around the .222 Remington in as light of a gun as possible, there isn't a lot of bolt lug mass to begin with.

Yes, the barrel manufacturers "should" ensure that the barrel extensions are true to the barrel, and they "should" ensure that the bore is concentric to the OD, and they "should" ensure that the barrel breech end threads are true and concentric to the bore. Unfortunately, these are all very rare to find on a production rifle.

Same with receivers. Notice that NugginFutz stated that half of his AR15 receivers were not true on the face? Everything should be true off of the center of the bore. The more parts that are true and concentric to the bore's center, the more balanced and accurate the system will be from both a longevity and accuracy standpoint.

If the receiver face is not true, it will cant the barrel off to one direction, so that the bolt lugs only engage on one side. This configuration will in fact break bolts, shearing lugs off in very few rounds. On the billet receivers made by well-renowned companies like LAR, Seekins, and Mega Machine, you have a much higher standard of QC when it comes to the receiver face being trued to the bolt carrier raceway.

In the bolt gun world, this is why precision billet actions are so popular with accuracy nazi's. Why spend all the time trying to un-suck a Remington 700 action, when you can just get a top-end CNC billet receiver from GAP, Surgeon, Templar, etc.

[customcutter]

The first thing about your selection that jumped out at me was that this seems to be a 5.56 bolt, not a Grendel bolt.

Yes, I was thinking I could use the bolt carrier for the Grendel build, and have a spare bolt for my Rem R-15. But, that kinda defeats the purpose of the NiB treatment???

Newbie question, would the spare bolt (NiB) have to be head spaced to the R-15? Or a new bolt headspaced for any AR, or are the machining tolerances tight enough that it doesn't matter which bolt is used?

I know bolts should be properly headspaced on Grendels, but didn't know if that was due to different bolt configurations, specs, type 1, type 2, etc, before the Grendel went SAAMI.