Need Real-World 7.62x39 Case Rim Dims

[BluntForceTrauma]

Fellow Horde Members and Vile Unwashed, I have a favor to ask.

I'm working on an improved 6.5 Grendel bolt that restores the standard 0.125 boltface depth with its slightly improved strength AND restores extractor strength — sort of a 65G Holy Grail — but also want to give a nod to 7.62x39 shooters.

SAAMI spec says 7.62x39 case rim is 0.447, but manufacturers may not actually make brass to that dimension.

Can any of you guys who have a pile of 7.62x39 brass and/or steel cases lying around get out your calipers and give me actual average dimensions of your case rim sizes from various manufacturers?

Thanks much in advance!

[bj139]

7.62x39
PPU brass case .444"
Tula steel case .443"

6.5 Grendel
Hornady brass .438"

[Djgrendel]

Winchester brass. .443
Federal brass. .441

[JASmith]

How do these dimensions compare with SAAMI / CIP?

SAAMI specifies nominal and how much smaller the sae diameter can be?

[BluntForceTrauma]

Excellent info. Thanks, guys!

SAAMI for 7.62x39 is 0.447 -0.010.

[IceAxe]

great idea, considering radiusing aswell

[olde sarge]

Hornady sst steel case average 0.445

[olde sarge]

Like the sound of this. Could use a couple of extra 7.62x39 bolts.

[Kswhitetails]

Can this be done while keeping the .136 face depth while staying true to the OD? That would be holy grail land.

[BluntForceTrauma]

KSW, we fought many a long and bitter battle to try and preserve 0.136 as a standard, not because those are magical numerals but because it represented a stronger and improved bolt over what was currently available. Conceding that a 6.5 Grendel case in an AR15 bolt necessarily weakens the bolt compared to a standard 5.56 bolt, everyone's goal was to make 65G bolts as strong as possible. Bill A's engineering said the extractor claw or hook becomes too weak in a 0.125 Grendel bolt for long-term reliable extraction. It was counter-argued, however, that deepening the boltface to 0.136 created a new problem by weakening the lug support versus 0.125.

So, what to do?

Keep the lug support and weaken the extractor, or keep the extractor and weaken the lugs? Through some truly clever engineering, Bill A. believed he had found a good compromise, and it has served us well.

But innovation cannot be stagnant.

My goal is the same as the original goal: Make Grendel bolts as strong as possible.

Why not get more strength back if we can get it?

We can.

I've got the first round of CAD drawings for the bolt body back from my manufacturer and they are working on the extractor drawings.

We've been in these trenches a long time, and Grendel deserves the victory it is now achieving. But not gonna rest on our laurels. (As you probably know, I've also been working on bullets — for example, got "Valkyrie-killer" bullet designs needing manufacture: 104gr in solid copper has a calculated G1 BC of 0.550 at Mach 2.5 or 2,790 fps, and a jacketed lead version depends on the amount of lead fill but the 132gr version has a 0.670 G1 BC at Mach 2 or 2,232 fps.)

Anyway . . . bolts.

It can be reasonably argued: You fought so hard for a standard, why muddy the waters now?

We fought so hard for a standard because we didn't want the Grendel program saddled with weaker bolts and a reputation for breaking bolts and we didn't want this new cartridge saddled with the chaos of competing standards in a dimensional free-for-all. It was Bill A.'s fear that such a situation would inhibit SAAMI adoption. Thankfully, we are way past all those early concerns and they are moot at this point.

So, to me, that argues for continually improving the strength of Grendel bolts, which is what I intend to do.

Currently, there are plenty of both Mk136 and Mk125 bolts available, so barrels can be headspaced for whichever the customer prefers and corresponding bolts will always be available for both new and currently owned barrels.

My plan is to offer Mk125 bolts for members at the same great price, and I will also be moving my barrels to Mk125 pending successful testing of this new bolt. These wheels grind slowly, though, so that is many months out.

Having said all that, KSW, I'm interested in hearing your thoughts about Mk136 vs Mk125, as you mentioned above?

[Kswhitetails]

If innovation is the interest, and ignoring standards the practice; Then why not change both the barrel extension and bolt together, enabling a vastly enlarged dimensional scope in which to innovate?

First, I admit to a vast, blissful, ignorance in regards to much of the following as far as the engineering involved, but you asked so here I post.

I like innovation, it's what makes good things greater over time. I also like standards, as they set a benchmark with which to measure growth; and a drawing board to return to, when new discoveries and knowledge are gained.

I believe that any innovation regarding the bolt strength increases of the Grendel are more likely to come if we don't allow ourselves to be limited to the standard 5.56 Barrel extension dimensions.

Observations are:

1 - Barrel extensions are now almost exclusively sold already installed on barrels. (1a) It takes a competently experienced, trained, and equipped individual to fit an extension to a barrel; (1b) and the makers have elected (correctly, I think) to almost exclusively sell retail barrels with the extensions already installed.

2 - Grendel already requires you to change the bolt and barrel when changing an AR15 into that caliber.

3 - The biggest limiter to the lug size and extractor thickness claw isn't only chambering headspace, but also the interior spaces of the barrel extension into which the bolt must fit to engage battery.

4 - The use of Grendel specific barrel extensions adds no untoward end user or installer changes or difficulty, if all other variables are maintained. Additionally, it allows Grendel specific bolt lug design and number. (See 8)

5 - Adding material to the bolt lugs and extractor claw is a definite advantage for strength and potential longevity - in any platform.

6 - Existing metallurgy and test data paves the way for simple changes to dimensional variables in the system, already proving strength of material; life expectancy; function; and most of all acceptable change limits.

7 - If loaded correctly, and as designed and intended - I.E. within pressure limits - Grendel has proven to be utterly reliable and effective within reasonable safety margins as it currently exists. Failures at Grendel chamber pressures are more likely due to poor material choice, condition, and manufacturing processes than the bolt not being able to contain detonation properly. However, it is understood that reinforcing the bolt would add some insurance and ability to absorb some flaws in this system.

8 - A safety benefit is created by changing the bolt lug and extension together is permanent, effective removal of the possibility of incorrect bolt usage. This is the kind of innovation that could in the end prove the most beneficial.

9 - This forum, and community provide a unique opportunity to share in these design and testing costs as it allows the spreading of the initial investment over the participating members. The ability to share, open source, and add minds to the team being advantages not available to design and testing departments in normal business.

10 - The Grendel case is small. Many of the maximum allowable loads are already limited by case capacity. Pushing strength into the bolt above it's current design may or may not lead to additional performance, and this innovation should be taken weighted more for safety purposes than performance gains.


I am sure there are lots of incorrect observations in the above list, but this post is to expose and discuss ignorance and incorrect assumptions.

There's an age old adage that states "If it ain't broke, don't fix it." I agree. And the AR15 barrel extension is definitely proven, and pretty much bomb proof. Which brings up new points of contention for me, as I am sure it has for other, wiser men before me. Chief amongst which is that it's MUCH easier to change a broken extractor or bolt than a barrel extension.

There is a bolt strength benefit of removing material from the barrel extension in order to add material to the bolt, but it obviously comes at the expense of weakening the extension. There is a "right" medium to what can be added and subtracted, and this can only be proven after extensive, thorough, and expensive testing. I don't think that much material needs to be removed from the extension to give the bolt quite a bit of added strength.


CMMG HAS ALREADY PAVED THIS ROAD WITH THEIR ANVIL, and GRENDEL PowerBOLT, but it does not lend itself to the major benefit of the original platform - parts availability, and interchangeability of the AR15.

[BluntForceTrauma]

KSW, excellent points, all.

1. One has to balance innovation vs. parts commonality vs. installed base. LWRC knows this from selling their proprietary lower receiver with an enlarged magazine well to fit larger Magpul 6.8 SPC polymer magazines. An improvement, yes. Wildly popular with the masses? Debatable. CMMG, as you mentioned, is also an excellent example. An improvement? Yes. Perhaps straying a bit too far from interchangeability? Probably.

2. My Mk125 bolt design uses standard barrel extensions and thus is completely interchangeable for all barrels headspaced for it.

3. Excellent point about "Why not improve the barrel extension while you're at it?" Good point that AR barrels are always sold with an extension, and if the 65G bolts are already a unique animal, why not mate them to a unique extension? Working on it! We might call this "Phase 2" of this particular product improvement project.

4. As LRRPF52 and others have pointed out many times, there is no SAAMI standard for boltface depth. Getting Grendel standardized via SAAMI and the market somewhat stabilized in the infancy of the cartridge has been achieved. I'm not gonna deviate from SAAMI spec, meaning chambers. I guess I already deviate in rifling, cuz SAAMI spec is six-groove and I do 5R.

What is to be gained — at this point — by "standardizing" on 0.136? Bolts and barrels are readily available for either 0.136 or 0.125. Today, if one goes 0.125 one knowingly is gambling on extractor strength. Until now. My extractor hook, for example, has 37% more metal in it than even a standard 5.56 hook.

5. We would not advocate this bolt as allowing the handloader insane new velocities, but, rather — as you mentioned — more "for safety purposes than performance gains."

[LRRPF52]

How do you plan on dealing with extractor lip thickness on a .125" bolt face depth? That is the real challenge.

Where does the 37% more material come in and still maintain the correct extractor ramp angle?

On AR10 bolts, they can use .125" depth because the extractor is much wider at .312", whereas AR15 extractors are .240" wide.

My East German steel case 7.62x39 ammo ranges from .443" to .445".

Blank cartridge head diameter is .444".

[BluntForceTrauma]

[VASCAR2]

Here are the results from measuring my 7.62X39 brass.


CBC head .444 +or- .001. Rim thickness .055, one case .053 and one .054. (Recorded 7 cases checked many more very consistent)


FC. head. .444 +or- .0005. Rim thickness. .055, +or- .001 but had one .053 and one .058 (measured 9 cases)

IMI head .445 +or- .001 (measured 8 cases one .448 and one .443). Rim thickness .055 one case was -.001

I had one Winchester case. head .443 Rim thickness .051