Are all Grendel extractors interchangeable?

[BluntForceTrauma]

Yes, the bolt issue for 6mmARC is a conundrum. My personal opinion is that the industry should voluntarily standardize on one or the other, on Mk125 or Mk136. I'm leaning toward Mk125 for the slightly greater lug support and because I can engineer it to get the same extractor claw dimensions as a 5.56 extractor. To me, that's the best of both worlds.

Availability of either bolt is NOT the issue. Both Mk125 (think the community of AR15 7.62x39 users) and Mk136 (6.5 Grendel users) bolts are out there in quantity. You can get one as easily as the other and the price is the same.

What IS the issue?

Extractor claw strength. (The issue is NOT the 0.125 boltface counterbore depth in and of itself, cuz even AR10s use 0.125).

If we had the extractor claw strength, we'd be using 0.125 and wouldn't think twice.

What if we get the extractor claw strength back to the same dimensions as a 5.56 extractor claw?

In that case, what is the argument for a 0.136 bolt?

Unless I'm missing something, the best argument is for those who want to use the exact same bolt unit in multiple weapons. It's a valid argument, cuz I've got more uppers than I know what do with and I'll just transfer BCG "X" to upper A, B, or C, depending on what upper I want to play with. Same bolt, multiple weapons.

But that's neither ideal nor recommended. I do it to save money and cuz I know my MONSTER bolts have always been dead nuts on in terms of headspace and are interchangeable.

Another example: I have zero interest in 5.56 AR15s. But many, if not most, of you own AR15s in both 5.56 and 6.5 Grendel.

You have Mk125 bolts for 5.56

You have Mk136 bolts for 65G.

If any of you have a commie AR15, then you've got a Mk125 bolt for 7.62x39.

And that's cool.

Taking it even further, most, of course, have Bolt A for Rifle A, and Bolt B for Rifle B, and Bolt C for Rifle C. In that case, the argument for using one bolt in multiple weapons evaporates.

So, I'm willing to have Mk136 bolts for 65G and Mk125 bolts for 6mmARC — provided the technical issues of lug and extractor strength are resolved.

Having said ALL that, nothing is set in stone. I want to hear what you guys think about the subject and your arguments for or against.

[A5BLASTER]

Yes, the bolt issue for 6mmARC is a conundrum. My personal opinion is that the industry should voluntarily standardize on one or the other, on Mk125 or Mk136. I'm leaning toward Mk125 for the slightly greater lug support and because I can engineer it to get the same extractor claw dimensions as a 5.56 extractor. To me, that's the best of both worlds.

Availability of either bolt is NOT the issue. Both Mk125 (think the community of AR15 7.62x39 users) and Mk136 (6.5 Grendel users) bolts are out there in quantity. You can get one as easily as the other and the price is the same.

What IS the issue?

Extractor claw strength. (The issue is NOT the 0.125 boltface counterbore depth in and of itself, cuz even AR10s use 0.125).

If we had the extractor claw strength, we'd be using 0.125 and wouldn't think twice.

What if we get the extractor claw strength back to the same dimensions as a 5.56 extractor claw?

In that case, what is the argument for a 0.136 bolt?

Unless I'm missing something, the best argument is for those who want to use the exact same bolt unit in multiple weapons. It's a valid argument, cuz I've got more uppers than I know what do with and I'll just transfer BCG "X" to upper A, B, or C, depending on what upper I want to play with. Same bolt, multiple weapons.

But that's neither ideal nor recommended. I do it to save money and cuz I know my MONSTER bolts have always been dead nuts on in terms of headspace and are interchangeable.

Another example: I have zero interest in 5.56 AR15s. But many, if not most, of you own AR15s in both 5.56 and 6.5 Grendel.

You have Mk125 bolts for 5.56

You have Mk136 bolts for 65G.

If any of you have a commie AR15, then you've got a Mk125 bolt for 7.62x39.

And that's cool.

Taking it even further, most, of course, have Bolt A for Rifle A, and Bolt B for Rifle B, and Bolt C for Rifle C. In that case, the argument for using one bolt in multiple weapons evaporates.

So, I'm willing to have Mk136 bolts for 65G and Mk125 bolts for 6mmARC — provided the technical issues of lug and extractor strength are resolved.

Having said ALL that, nothing is set in stone. I want to hear what you guys think about the subject and your arguments for or against.

Well all my barrels have its own bolt and back up bolt. So useing the .125 bolt face depth for the 6mm arc barrels wouldn't bother me because they would be dedicated too those barrels.

Granted that's if the extractor can be designed to be stronger.

[Lemonaid]

+1 for strength IF there isn't two versions out there causing confusion. Perhaps a unique color, finish, markings?

[Growler]

Everyone I talk to is going all out just to keep up with existing demand, never mind the hassle of a prototype. Anybody that seems remotely interested is giving me "token" quotes: their price per part is ridiculous and their NRE (one-time engineering fees) are unreasonable.

But I'm gonna do it when they are ready to do it.

Right now am finding the same difficulty getting someone to do an improved Mk125 bolt for 6mmARC that solves the thinned extractor claw problem....

Oh yeah, NRE is a cast iron beast—the troll of innovation.

I missed the bolt face depth spec in the 6mmARC thread. Is that design standardized on 0.125” depth?

It’s volume that drives quality. Common parts grow volume, raising all ships so to speak. I’d much rather see common parts where possible.

[BluntForceTrauma]

Is that design standardized on 0.125” depth?

Boltface depths are never denoted in SAAMI specifications. So, it's arbitrary for the manufacturer.

It’s volume that drives quality. Common parts grow volume, raising all ships so to speak. I’d much rather see common parts where possible.

Great point. Unfortunately, doesn't make the issue any less difficult. In the larger world of ARs, because of 7.62x39 ARs there are probably far more Mk125 bolts than Mk136 bolts out there. Seems THAT argument would push everybody to standardize on Mk125 for 7.62x39 and 6.5 Grendel and 6mmARC if one is to chase volume.

[Growler]

Boltface depths are never denoted in SAAMI specifications. So, it's arbitrary for the manufacturer.

That is unfortunate. I was under the impression that chamber depths were standardized. That means SAAMI spec doesn’t independently establish Mk136. No wonder there was so much chaos earlier.

[BluntForceTrauma]

That is unfortunate. I was under the impression that chamber depths were standardized. That means SAAMI spec doesn’t independently establish Mk136. No wonder there was so much chaos earlier.

That's why we fought so hard in the early days to standardize on Mk136. It was best for the whole 6.5 Grendel project that we stay unified in the face of competition by standardizing. There is strength in numbers. And here we are today: 6.8 SPC is the F4F Wildcat of cartridges while 6.5 Grendel is the F6F Hellcat.

Plus, Bill Alexander truly DID do the rigorous engineering necessary to make the strongest bolt — overall and all things considered — at the time. Nobody else was willing to spend the time re-engineering a 7.62x39 bolt to be optimum. They just hogged out 5.56 bolts and shoved them out the door.

Now, when we're on the ground floor of a new cartridge, the decision has to be made for industry to voluntarily cooperate in a standard for that new cartridge to make things easier for everyone and to avoid chaos.

[grayfox]

For me the issue comes down to interchangeability and whether a third "spare" bolt requirement becomes necessary. If the 6Arc bolt can be same dimensions as the Grrr then if/when I need to swap one in because of breakage or malfunction, I can keep the one spare for both calibers. And if by mistake the 6Arc goes into a Grr barrel no harm no foul since headspace will be maintained. OTOH a 0.125 6Arc bolt means I have to keep it scrupulously separate and buy a separate (now third) spare, keeping it separate also from my 6.5 monster bolts.
Same thing with extractor claws, they will need to be kept separate and require the third spare issue.
Because no matter how well something is made (I have only had the one Grr extractor fail in my years), parts do fail and you need to have on-board spares. Even more so as we head into more 2A headwinds.

I remember the earlier issues with the older toolcraft Grendel "short-bolt", which again set up interchangeability issues. I ditched that one that I had in favor of the standard style Grrr (ie, Monster) bolts.

[VASCAR2]

I’m with Grayfox, industry has pretty much standardized on Mk 136 for the 6.5 Grendel. I have spare 6.5 Grendel bolts and extractors and spares for the 5.56. From my reading bolt life is better with Mk 136 in 6.5 Grendel than AR-15’s chambered in 7.62x39 with Mk 125. For consistency sake I prefer a .136 bolt face for the 6.5 and 6 MM ARC.

One thing that really aggravated me about the 6.5 Grendel was the 6.5 Grendel II chamber designation created by Satern. It was already bad enough with confusion from 264 LBC, 6.5 CSS, 6.5 Sporter and Type I and Type II bolt nonsense. I just hate to see the 6 MM ARC get into this type of mess as it is very confusing for the general public. The 6 MM ARC success will partly be because of the success of the 6.5 Grendel and efficient case design including the .136 bolt face depth which was well engineered.

[grayfox]

I’m with Grayfox, industry has pretty much standardized on Mk 136 for the 6.5 Grendel. I have spare 6.5 Grendel bolts and extractors and spares for the 5.56. From my reading bolt life is better with Mk 136 in 6.5 Grendel than AR-15’s chambered in 7.62x39 with Mk 125. For consistency sake I prefer a .136 bolt face for the 6.5 and 6 MM ARC.

One thing that really aggravated me about the 6.5 Grendel was the 6.5 Grendel II chamber designation created by Satern. It was already bad enough with confusion from 264 LBC, 6.5 CSS, 6.5 Sporter and Type I and Type II bolt nonsense. I just hate to see the 6 MM ARC get into this type of mess as it is very confusing for the general public. The 6 MM ARC success will partly be because of the success of the 6.5 Grendel and efficient case design including the .136 bolt face depth which was well engineered.

+1, that's what I'm talkin' about...

[LRRPF52]

That is unfortunate. I was under the impression that chamber depths were standardized. That means SAAMI spec doesn’t independently establish Mk136. No wonder there was so much chaos earlier.

6.5 Grendel bolts and extractors come from the .50 Beowulf, which came from the Colt 7.62x39 development.

If you track the history of the Colt 7.62x39 and how after-market companies basically ignored the R&D that went into it, thinking they could duplicate it with a simple enlarging of the bolt face, you see where the big boys get separated from the imitators really quick. Another major oversight that should have never happened was that a lot of the early and even later 7.62x39 after-market barrel makers used .308" bores instead of .310", which spikes the pressure during swaging of the bullet into the rifling.

Every after-market 7.62x39 manufacturer experienced unusually-high extractor and bolt lug failures as the norm for many years as a lot of them were using 8620 run through amateurish processes in the metallurgy.

The strongest AR15-sized bolt for 7.62x39 is made by LMT using an exotic aerospace AerMet alloy that costs twice as much per bolt as 9310 bolts cost. Even those break extractors because they use a .125" bolt face depth.



Since the 7.62x39 and 6.5 Grendel have larger diameter case rims and heads, that are thicker than 5.56 brass dimensions in the longitudinal axis (front-back), so the extractor geometry needs to be different to be able to correctly snap over during chambering, and not grind against the Inner Diameter of the barrel extension teeth.

With a .125" bolt face depth, there isn't enough back-to-front meat on the extractor lip.

The reason the AR10 can use a .125" face depth is that the extractors are much wider than AR15 extractors, so there is far more root strength for the extractor lip as part of the extractor body.

If you place an AR10 extractor next to an AR15 extractor, you'll immediately see what I'm talking about.

You would need to use an exotic alloy for the .125" bolt face depth extractors in order to gain the lip strength you need in the AR15, and tightly control the QC on them dimensionally with an unacceptably-high reject rate that would drive the costs even higher.

As to SAAMI, if you look at bolt guns, Contenders/Encores, and AR15s, the chamber depth is going to be based on the bolt face or breech design, balanced against case support at the web. The bolt gun extractors can have really thin lips because they don't have to extract during residual case obturation. It isn't arbitrary, but is based on several engineering considerations for the type of action and constraints of that particular action design.

.125" bolt face depth in the 6.5 Grendel and 6mm Grendel variants is just asking for broken extractors in the AR15. I've had a representative/dealer of BHW claim that they never had problems with .125" bolts breaking extractors, immediately followed by customers mentioning they experienced that exact problem with the .125" bolts in a ".264 LBC" chambered BHW barrel/bolt combo. You see it all the time with high volume 7.62x39 AR15s, where many shooters just accept that they need to carry spare bolts/extractors with them to the range.

The .125" bolt face depth is just a non-starter from a production standpoint for this reason.

[crusader18]

That's awesome BFT, when I need another bolt, I'll be buying it from you.