Bolt Question

[bob4432]

I was wondering why the Grendel had to go the .136 bolt when other 6 & 6.5mm wildcats are good with the .125 bolt? I am talking about the ar15 platform firearms that www.6mmar.com offers? Some of the wildcats on 6mmar use the Grendel case as the parent case.

Is it most are 6mm and thus a bit lighter than the 6.5mm bullets the Grendel uses?

Thanks in advance,
Bob

[Sticks]

A short while back, you could not find an active thread that did not have a post from LRRPF52 explaining this.

This thread has a fair amount of info on the matter. http://www.65grendel.com/forum/showt...ight=extractor

Search "Bolt", and "Extractor" and you will find lots of reading on why you should go with a .136 bolt face, designed and built that way.

[rabiddawg]

http://www.65grendel.com/forum/showthread.php?7420-The-Story-of-the-Grendel-bolt

Here is a link to the sticky thread

[bob4432]

Thanks

[LRRPF52]

I was wondering why the Grendel had to go the .136 bolt when other 6 & 6.5mm wildcats are good with the .125 bolt? I am talking about the ar15 platform firearms that www.6mmar.com offers? Some of the wildcats on 6mmar use the Grendel case as the parent case.

Is it most are 6mm and thus a bit lighter than the 6.5mm bullets the Grendel uses?

Thanks in advance,
Bob

It's one thing to wildcat a cartridge.

It's another to test fleet samples with pallets of ammunition.

Thankfully, Colt did all this back in the 1980s on the 7.62x39 AR15, and realized there was a need for more lip thickness in the extractor, since 7.62x39 rims are much thicker than 5.56 tiny rims. If you go forward into the extractor, you weaken it to the point that production lots will see unacceptably high rates of failure of the lips. You even see this with the DPMS LR-308 extractors, which are much wider than AR15 extractors by far, and still bend or break regularly.

In fact, DPMS recognized this was enough of a problem to totally re-design the GII extractor, source an alloy for it they won't divulge, and make it back-compatible with the LR-308. I've seen at least one of them broken already.

My DPMS LR-308 (on my .260) bolt face depth is .1275". GAP may have squared the bolt face when they did their accuracy work.

I've measured ArmaLite's and confirmed with them that they are .128", in order to allow more lip material on that much wider extractor.

The LMT enhanced 7.62x39 bolt is probably the strongest AR15 7.62x39 bolt on the market, made from one of the AerMet alloys, which are much harder to machine.

Even those extractors break because there isn't enough lip material for the thick rim on x39.

[bob4432]

It's one thing to wildcat a cartridge.

It's another to test fleet samples with pallets of ammunition.

Thankfully, Colt did all this back in the 1980s on the 7.62x39 AR15, and realized there was a need for more lip thickness in the extractor, since 7.62x39 rims are much thicker than 5.56 tiny rims. If you go forward into the extractor, you weaken it to the point that production lots will see unacceptably high rates of failure of the lips. You even see this with the DPMS LR-308 extractors, which are much wider than AR15 extractors by far, and still bend or break regularly.

In fact, DPMS recognized this was enough of a problem to totally re-design the GII extractor, source an alloy for it they won't divulge, and make it back-compatible with the LR-308. I've seen at least one of them broken already.

My DPMS LR-308 (on my .260) bolt face depth is .1275". GAP may have squared the bolt face when they did their accuracy work.

I've measured ArmaLite's and confirmed with them that they are .128", in order to allow more lip material on that much wider extractor.

The LMT enhanced 7.62x39 bolt is probably the strongest AR15 7.62x39 bolt on the market, made from one of the AerMet alloys, which are much harder to machine.

Even those extractors break because there isn't enough lip material for the thick rim on x39.

Thanks for taking the time and the explanations. So for the Grendel the .136 bolt face depth is the way to go (,136 bolt face depth, SAAMI Chamber, this is what I have been taught on this site, thought I would have already had one going by now but sometimes life takes you in a different direction), but if I were to venture into any of those other wildcats, which a lot use the 6.5G case as the parent case, would it be better to use a LMT 7.62x39 bolt or would having the barrel chambered so that a .136 bolt face depth be the way to go for the AR15 platform, if that can be achieved?

Very interesting info re: DPMS LR308/GII

Are the failures due to the pressure or just trying to use off the shelf items where they were not intended to be used? The DPMS LR308 GenII, does it fail when using .308 or is it the wildcats where the failures are displayed?

[rabiddawg]

The chamber used doesn't determine the bolt face depth. That is set by the extension. It is my understanding that you could have a wildcat based on a Grendel case and use the .136 bolt if that is what you want.

Keep in mind, Bill Alexander developed the Grendel and specs a .136 bolt face depth. Anything other than that is Not a Grendel bolt. Yes, they work but the man that spent the money and time testing said the .136 is the way to go. I'm not arguing with him.

Back in the day bolts were harder to get so some builders opted for other non-Grendel parts and chambers that fire Grendel ammo.

If I am wrong I will edit my post so someone please correct me. I'm getting way out of my comfort zone on this

[bob4432]

The chamber used doesn't determine the bolt face depth. That is set by the extension. It is my understanding that you could have a wildcat based on a Grendel case and use the .136 bolt if that is what you want.

Keep in mind, Bill Alexander developed the Grendel and specs a .136 bolt face depth. Anything other than that is Not a Grendel bolt. Yes, they work but the man that spent the money and time testing said the .136 is the way to go. I'm not arguing with him.

Back in the day bolts were harder to get so some builders opted for other non-Grendel parts and chambers that fire Grendel ammo.

If I am wrong I will edit my post so someone please correct me. I'm getting way out of my comfort zone on this

I am not in any way thinking of going against Bill Alexander, nor would I as he has spent so much $$ getting the Grendel right. I was just surprised to see some wildcats using the 6.5G as the parent cartridge using the .125 bolt face depth, well actually it being suggested to use the .125 bolt face depth bolt as that is all I could find on the 6mmar.com website.

From what I have read, Robert Whitley runs the 6mmar.com page and was under the impression that he was at the very least respected if not highly respected in the community. What caused my question/s was you have 2 people that are at the very least respected in the community for the 6.5G and wildcats based off the 6.5G case to differ on bolts used.

I guess my main question is why does Robert Whitley suggest the .125 bolt face depth bolt for 6.5G parent case wildcats in an AR15 platform. Is there something I do not know as to a reason why Mr. Whitley chooses the .125 bolt face depth? Anything to do with the chambering? I know it is .011", but this .011" makes a huge difference but I would think a gunsmith could chamber a barrel to accept either bolt face depth. This is where my confusion lies.

[VASCAR2]

Availabilty of bolts at the time a cartridge/chamber is introduced can have a bearing on choosing a particular bolt face depth. Since many manufactures were already producing 7.62X39 bolts with .125 bolt they were more readily available than .136 bolt. I bought my first 6.5 Grendel in 2009 and there were fewer sources for bolts than there are now. Many of the bolts on the market in 2009 were made by Alexander Arms.

Using a commonly available bolt can help a company meet a certain price point. Colt did the research proving the worth of the deeper .136 bolt face in their 7.62X39 AR-15's. Alexander Arms used the .136 bolt in the 50 Beowulf and with Colt's research AA chose to use the .136 bolt for the 6.5 Grendel.

[rabiddawg]

Don't take this the the wrong way but perhaps you need to ask Mr Whitley.

Another thing that caused this (as well as the chamber stuff) is ego and money. The 6.5 Grendel was a property of Alexander Arms. To build and sell you had to go thru AA. That's why we have all the variants that will shoot 6.5 Grendel ammo. Every one of those other players says their version is the best.

[Commander184]

My .130" bolt would jam my brass into the throat so hard I about pulled my trigger finger off trying to eject an Unfired hornady factory round.i bought a .136 bolt and problem solved. So it has to do head space and strength. I've read grendel bolts are stronger because of engineering not just material. I wonder if I backed my barrel off .006" would I have needed a bolt change? What ever AA says I do

[bob4432]

Don't take this the the wrong way but perhaps you need to ask Mr Whitley.

Another thing that caused this (as well as the chamber stuff) is ego and money. The 6.5 Grendel was a property of Alexander Arms. To build and sell you had to go thru AA. That's why we have all the variants that will shoot 6.5 Grendel ammo. Every one of those other players says their version is the best.

I do not take it the wrong way at all, and you are probably correct. I forget that I am getting into this after Mr. Alexander released his so there could be a SAAMI spec for the general shooting community. I do not know the timeline that Mr. Whitley releases his different wildcats, but it seems like I have the luxury of looking in the rear view mirror after all the R&D has been done and not through the windshield on a road with fog not knowing what those guys were getting themselves into.

Thanks for the suggestion,
Bob

[LRRPF52]

Simple answer is probably the reason: easier to get untested, .125" bolts that were more readily available than 6.5 Grendel bolts with the correct face depth.

What we witnessed with the AR15 market after Colt introduced the 7.62x39 Sporter in the 1980s was a few guys wildcatting those uppers, or using parts from those uppers once they saw it could be done.

The target guys had no interest in 7.62x39, but they did have a bit of interest in 6mm PPC, and a few smiths/shooters built some 6mm PPC AR15 uppers using either Colt BCGs from the 7.62x39 Sporter, or machined out existing 5.56 bolts.

They chambered fresh barrel blanks, used some accuracy enhancing techniques, polished extractors, bolt lugs, magazine feed lips-anything to increase reliability they could do.

Since they were using RLGS and longer barrels, stress on the bolts wasn't what it would be on something like a 16" or especially 18" MLGS, but there were still plenty of issues encountered.

A workable extractor is the real challenge, which is where the Grendel extractor geometry, and consequently, the Grendel bolt geometry comes from.

To have thick enough extractor lip, something has to move, and you can't go forward past the bolt.

The thing that is different with Bill A. is that he was always thinking about production, durability of the parts, and how to produce durable parts consistently. You can make one-offs and call it good, but to reproduce success and reduce failure margins as best as possible, it's an entirely different exercise. This is true with any type of product, and is the nemesis of production managers.

Bill started by tracking down one of the engineers who did a lot of the design work on Colt's 7.62x39, but for the .50 Beowulf first.

There weren't many Colt 7.62x39 Sporters made, and the product line only ran for a short while. You could still get them new in the early-mid 1990s, but they stopped production some time shortly after to the best of my knowledge. If anyone has better info on this, please contact me with reliable sources, as I would like to know. I was already in the market and bitten by the AR15 bug well before the Colt x39 came along, so I remember it clearly, but from a consumer perspective only at the time.



After Colt production ceased, a few other companies started offering complete rifles, as well as parts. Magazines were really the main chokepoint back then, because nobody made a reliable higher capacity one, and then the Clinton AWB was passed, which made it illegal to make anything more than a 10rd for us lowly peasants, so there weren't a lot of 7.62x39 ARs being shot in volume.

MGI made a modular system so you could shoot AK mags in AR15s, and this is where higher volume could be shot. All sorts of issues arose.

M&A Parts started cranking out AR barrels and bolts in 7.62x39, especially after the Clinton AWB sunset in 2004, none of which were tested to any respectable standards for functionality or durability. One of the biggest problems was bore diameters being Western 7.62 (.308") versus Russian .310" ammunition. Then add to that that parts suppliers don't test complete guns in fleet, so they just figured .125" bolt face depth is good to go, never mind that 7.62x39 has a much thicker rim than 5.56's tiny little thing.

ArmaLite Inc. (Really Eagle Arms, not the real ArmaLite from Fairchild) introduced one and sold them for a while. I don't know what bolt they used initially. They still make them under the new ownership as well.

Then the gunshow grade parts distributors thought they could have a crack at it, and failed miserably at anything close to a quality product, but succeeded in sales because a lot of hunters and aspiring riflemen who thought .223 Rem was for poodles, wanted a seven point six two in their AR15s. Nobody ever cared to look at what Colt did with bolts and extensions, nor did they even catch the glaring dissimilarity between .308" and .310" bores, which is a gross display of colossal incompetence, and resulted in substantial increases in chamber pressure, as well as bolt thrust.

But nobody really cared, because everyone was making parts, let the customer figure it out for themselves. Nobody took magazines, bolts, extensions, extractors, and barrels under one roof and solved the problems. The market was happy to continue to shell out their money for parts, and .125" bolt faces slowly became the default for franked-ARx39, unfortunately.

It's a sad state of affairs to see that level of failure spread across the market, leaving manufacturers with very limited options when compatibility is considered. Cleatus wants to buy his barrel of the month from Jim Bo's Tactical, but he wants the uber-cool, flavor-of-the-tactical-season coated BCG from Barnam and Bailey's parts supplier start-ups, so it had better be .125" face depth compatible.

It's one of the reasons why I never got into 7.62x39. There especially is no reason to now with Grendel steel case ammo. Same basic case, same exact bullet weight, one has 3-4 times the effective range, and less wind drift than .308 Win, and was actually designed to work in the AR15 from the start, with magazines that actually work.

[BluntForceTrauma]

It's one of the reasons why I never got into 7.62x39. There especially is no reason to now with Grendel steel case ammo. Same basic case, same exact bullet weight, one has 3-4 times the effective range, and less wind drift than .308 Win, and was actually designed to work in the AR15 from the start, with magazines that actually work.

This. And this again. Yes.

[bob4432]

Very much appreciate the education.
Was not aware of this history.