Should I Prepare to Retire my 6.8 SPC [ Spec. II ] ? Any Comments & or Input ?

[Stopsign32v]

I'm looking at more than just performance, weight of the rifle, weight of ammo and recoil. If I'm building a CQB-300 rifle I would rather have a smaller lighter rifle with a better mag capacity. If I'm going to shoot 600 yds I'm not going to chop a 308 barrel short.

Then just run a 5.56

[pinzgauer]

Carp 158 or 9310, Microbest is good, 6.8 bolts don't break like Grendel bolts.

Can't let this mistruth go without a comment, it's been a decade or more since people have had issues with grendel bolts breaking.

I'm sure you could break one, but it's not common. Bill Alexander told me personally that the early rash of bolt breakage was due to a bad heat treat on some of the ones when he was working with Lothar Walther early on.

I will admit that the 6.8 bolt is stronger due to less bolt thrust and more meat on the bolt face. So I'll concede that point, but it's realistically not an issue now.

[constructor]

Can't let this mistruth go without a comment, it's been a decade or more since people have had issues with grendel bolts breaking.

I'm sure you could break one, but it's not common. Bill Alexander told me personally that the early rash of bolt breakage was due to a bad heat treat on some of the ones when he was working with Lothar Walther early on.

I will admit that the 6.8 bolt is stronger due to less bolt thrust and more meat on the bolt face. So I'll concede that point, but it's realistically not an issue now.

Anyone can go to the Grendel forum and read, seems like someone broke a bolt just a few months ago.

https://www.65grendel.com/forum/show...olts-can-shear

https://www.65grendel.com/forum/show...ken-6mmAR-Bolt

https://www.65grendel.com/forum/show...AR-broken-bolt

LR1955 LR1955 is offline
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Quote Originally Posted by FLshooter View Post
I have never broke a bolt in an AR 15. And I don’t know anyone who has. I have heard of this happening. But it had to be due to high pressure ammo.Stay within specs with your hand loads and you should be fine.
Well, I have sheared lugs on three Colt 7.62 X 39 bolts in a Grendel, one AA Grendel bolt, and a machined out 5.56 bolt. I have also sheared a lug on a Grendel bolt in a 6 AR.

However -- these bolts were before the enhanced bolts that seem to be holding up well today. Before these enhanced bolts, there were periodic reports on the forum of guys shearing lugs. It was not uncommon and seemed to plague the Grendel until adjustable gas blocks and the enhanced bolts.

LR55

[pinzgauer]

LR55 is a very respected shooter who has been shooting Grendel almost since it first became available.

Note that he said three were 762x39 bolts, which are specifically not Grendel bolts and have much less meat in key areas.

Early on, many mfgs would just open up a 223 bolt face. You can get away with that with 7.62x39, but not Grendel due to higher pressure. But even for 7.62x39 it's considered weak and not what Colt did.

That's what a lot of people used early on with grendel, incorrectly. And led to the stupid type 1 or type 2 stuff.

Also note that he said that since the "enhanced" bolts came out it is very infrequent. He's referring to the 9310 bolts with proper heat treating.

As I acknowledged, you can break a Grendel bolt if you load it above saami pressure. Which unlike SPC, is the proper pressure for ARs due to bolt thrust limits.

Just not an issue with factory or good reloads.

Many of us have shot many thousands rounds of Grendel and never had an issue.

[constructor]

LR55 is a very respected shooter who has been shooting Grendel almost since it first became available.

Note that he said three were 762x39 bolts, which are specifically not Grendel bolts and have much less meat in key areas.

Early on, many mfgs would just open up a 223 bolt face. You can get away with that with 7.62x39, but not Grendel due to higher pressure. But even for 7.62x39 it's considered weak and not what Colt did.

That's what a lot of people used early on with grendel, incorrectly. And led to the stupid type 1 or type 2 stuff.

Also note that he said that since the "enhanced" bolts came out it is very infrequent. He's referring to the 9310 bolts with proper heat treating.

As I acknowledged, you can break a Grendel bolt if you load it above saami pressure. Which unlike SPC, is the proper pressure for ARs due to bolt thrust limits.

Just not an issue with factory or good reloads.

Many of us have shot many thousands rounds of Grendel and never had an issue.

Yes I started shooting one in 2005 when Arne was selling them before Bill A trademarked the name "Grendel" I broke the first bolt in the first rifle I owned. That is part of the reason I designed the "superbolt" of course it was 2009 before I purchased CNCs and started machining them. I also broke a few Y/M 5.56 bolts in 3 gun rifles with high round counts back in the 90s, they broke at the cam pin hole. 2007 rolled around and Lietner Wise original owner of LWRC designed and manufactured his lobstertail extractor 6.8 bolt out of 9310. To test, I fired 131 rounds at a 3 gun event in Durango Co, they were loaded to 70,000psi on purpose. The lugs did not shear but I had chips around the cam pin hole. I tested the bolts for hardness and they were a touch over 70C converted. That gave me the last piece of the puzzle, they should be hard enough to have strength but not so hard they are brittle.

BTW if a Grendel bolt and a 7.62x39 bolt are made from the same material with the same heat treat the 7.62x39 will be 7-8% stronger at the lugs due to the extra .012" attachment/support of the lugs.

[pinzgauer]

Yes I started shooting one in 2005 when Arne was selling them before Bill A trademarked the name "Grendel" I broke the first bolt in the first rifle I owned. That is part of the reason I designed the "superbolt" of course it was 2009 before I purchased CNCs and started machining them. I also broke a few Y/M 5.56 bolts in 3 gun rifles with high round counts back in the 90s, they broke at the cam pin hole. 2007 rolled around and Lietner Wise original owner of LWRC designed and manufactured his lobstertail extractor 6.8 bolt out of 9310. To test, I fired 131 rounds at a 3 gun event in Durango Co, they were loaded to 70,000psi on purpose. The lugs did not shear but I had chips around the cam pin hole. I tested the bolts for hardness and they were a touch over 70C converted. That gave me the last piece of the puzzle, they should be hard enough to have strength but not so hard they are brittle.

BTW if a Grendel bolt and a 7.62x39 bolt are made from the same material with the same heat treat the 7.62x39 will be 7-8% stronger at the lugs due to the extra .012" attachment/support of the lugs.

I got my first Grendel in 2008, and shot it enough that I've worn out brass reloading. A lot of that was hotter than it should have been.

By that time bolt failures were pretty much a thing of the past.

As your comment about 7.62x39 versus Grendel bolts, neither Colt nor Bill Alexander's stress analysis agrees with you.

While it seems counterintuitive that a deeper face would be stronger, it's not just about the lugs, it's also about the extractor cut and how it the stress interacts with the lugs.

This thread is about SPC so you can get back to arguing about a SPC. I know you have a lot of experience with barrels and 5.56 bolts. But I also know that you take every opportunity to bash Grendel and have a strong financial vested interest in SPC over grendel. So we'll just leave it at that.

Much of what you're write about midpower cartridges at range I agree with. Grendel and SPC are so close in performance and each has slight advantages in certain areas. It also depends on what bullets you're using and what you're trying to do with it. So to me it's not worth arguing about. But when flat-out mistruths are repeated as gospel I have to speak up.

[pinzgauer]

One last observation: 70k PSI is amazing even with the smaller SPC bolt face.

[constructor]

t.

As your comment about 7.62x39 versus Grendel bolts, neither Colt nor Bill Alexander's stress analysis agrees with you.

While it seems counterintuitive that a deeper face would be stronger, it's not just about the lugs, it's also about the extractor cut and how it the stress interacts with the lugs.

.

Any engineer can explain to you why the 7.62x39 bolt is stronger. The lugs are .277" long, if you bore out the center .136" deep you only have .141" of full strength attachment. A 7.62x39 bolt would have .153" full strength attachment.
The cut for extractor has zero effect on the strength of the lugs unless you make it wider and undercut the lugs further.

The reason for the .136 depth was because in the beginning AA was re-machining 5.56 bolts and because the Lapua brass had a thicker rim they had to recess the bolt face deeper so the extractor would grab the thicker rim.
Fast forward several years when Grendel bolts were designed and machined specifically for the Grendel they could have drilled the extractor hole .012" further forward and accomplished the same thing. That is how I machined our Grendel bolts, that way I manufactured a stronger bolt and was able to use a stronger extractor, the same ones Bill A calls "extreme duty" made and sold by Black Rifle Arms.
Yes I have seen all of those out of scale drawings LRRPFFTT has posted all over every forum of the internet. I'm not a democrat, I don't believe everything I see and hear, I worked as an engineer for 25 years.