The "Big bolt"

[noone]

On another forum, I noted that there is a fellow making and using what he calls a "big bolt". This bolt has longer and thicker lugs for Grendel type cartridges. Of course, it also requires a special barrel extension.

The bolt lug design looks very well thought out, and if made with the right kind of steel and properly heat treated it could be very useful. The maker claims that he is getting what I consider extraordinarily high velocities because with this bolt/barrel extension, he is not tied to a pressure level of around 50-54K CUP. There is no way I would attempt any of the loads he is using with a "normal" Grendel.

Has anyone heard about these bolts, and if so, what additional information can you provide? I'm presuming that the person who makes and uses these bolt/barrel extension units does so because he likes them, and has a vested interest in claiming success. As a result, it is possible that the information he provides needs to be independently verified.

I doubt that the person would be deliberately dishonest. I just like independent verification. Like President Regan said, "Trust, but verify."

[LRS_Ranger]

If this is the case, it might make heavier bullets like the 140's a viable option, as one would be able to launch them fast enough...

[stibuzz]

They would be an option, but there is still the OAL that ultimately will be your limiting factor in the AR. Subscribed

[ss355]

Are you referring to the AR Performance bolt that looks a lot like the KAC improved bolt, with lugs that have a more rounded profile?

[StoneTower]

There is a big bolt concept that is used by Olympic Arms. They use it in their WSSM uppers. They run a modified carrier and use an AR-10 bolt and barrel extension. It is super strong and with a setup like that, you could probably use Grendel loads that were designed exclusively for the bolt guns.

http://www.olyarms.com/index.php?page=sh...&Itemid=37

This guy takes the Olympic Arm's parts and builds custom uppers with them.

http://www.dtechsuperstore.com/WSSM%20page.htm

Cheers,

David

[noone]

Nope, it is definitely a different bolt setup than the one Stonetower's dtechsuperstore link shows. The "big bolt" lugs have a longer bearing length, rounded lugs, much more metal between the cartridge and outside of the bolt's radius. Obviously, such a bolt also requires a matching barrel extension, but they are designed for use in an AR15 size rifle.

The maker claims to be able to get safe working pressures of around 60,000 (CUP I presume) rather than 52,00-54,000. I'm just not sure that the higher pressures would get me that much. Brass life would be dramatically shortened. And the Grendel works just fine for me as it is.

The big bolt concept seems like a neat idea if it works. Although I'm not interested in the super high pressures, it would be nice to be able to load a little hotter and have a wide margin of safety to do it with.

Using a 140 grain bullet with it's super high BC looks very attractive. Even if the 140 is only going 2200-2400 FPS, that would be a huge increase in downrange energy. I'm just not sure if the dramatically shortened brass life would be worth it.

Plus, all my Grendels work great now. Why fix them if they work? I'm just looking for more information on this idea.

[StoneTower]

It may be a different bolt, but remember the one in the picture is cut out for a WSSM case. The rim diameter of a WSSM is (.535). The Grendel rim diameter is (.44). If you cut that bolt for a Grendel, there would be more metal between the cartridge and outside of the bolt's radius.

Nope, it is definitely a different bolt setup than the one Stonetower's dtechsuperstore link shows. The "big bolt" lugs have a longer bearing length, rounded lugs, much more metal between the cartridge and outside of the bolt's radius. Obviously, such a bolt also requires a matching barrel extension, but they are designed for use in an AR15 size rifle.

The maker claims to be able to get safe working pressures of around 60,000 (CUP I presume) rather than 52,00-54,000. I'm just not sure that the higher pressures would get me that much. Brass life would be dramatically shortened. And the Grendel works just fine for me as it is.

The big bolt concept seems like a neat idea if it works. Although I'm not interested in the super high pressures, it would be nice to be able to load a little hotter and have a wide margin of safety to do it with.

Using a 140 grain bullet with it's super high BC looks very attractive. Even if the 140 is only going 2200-2400 FPS, that would be a huge increase in downrange energy. I'm just not sure if the dramatically shortened brass life would be worth it.

Plus, all my Grendels work great now. Why fix them if they work? I'm just looking for more information on this idea.

[tony549]

AR15performance has the big bolt as referenced above. Check out Harrison Beene's blog note on 68forums:
[URL="http://68forums.com/forums/showthread.php?13626-New-bolts-for-the-264-LBC"]
These are not cheap and I don't know if he's taking custom orders right now.
By the way he's taken his big bolts to 60,000 psi, not CUP, according to his note.

[JASmith]

Being able to go to 60 ksi with the Grendel would make it much more attractive as a military cartridge. This environment stresses reliability and safety but doesn't require significant reloadability.

The real question: "Is this real and can it deliver the pressure safely and reliably?"

[noone]

I didn't know it was AR15 performance making them. I have had wonderful dealings with them in the past. Guess I'll have to call them for more info.

Yes, it is 60,000 PSI, not CUP...my bad. Still lots more working pressure available than the standard for the Grendel. And I'm still not sure the shortened case life is worth it, but it would be nice to have the option.