No one is going to run 110s in the ARC either, that bullet is shoved way down in the ARC case too. On top of that all of the Grendel cases are very thin and bulge like a belted magnum when you push pressures. See the photo below from the Grendel forum. To make things worse they decided on a bolt with a .136" recess, that pulls the case .012 further out of the chamber leaving more unsupported by the chamber walls. Look at the sectioned cases and how far down the thin walls of the Grendel case goes compared to the thicker 6.8 case. That should be clear why we can push the 6.8 harder than they can the Grendel cases. I tried to get Starline to make the lower area in the cases thicker only up to the point that the case was supported by the chamber walls but they would not do it...they are in the business of selling brass. Norma brass will outlast all other brands Grendel brass.
Grendel/ARC brass can barely hold up to 52000psi. It will be interesting to see what that ammo does in 120* temps.
one on left is like all bolts on the market including JPs, mil spec square lugs. 2nd is superbolt, Titan, 750XD and 800 series. The 800 series was made first(2007) so I could chamber the 6BRX(.473). 750XD(2013) was second (.473, .442, .421 cases), Titan(2015) was the last version because the long ogives of the .224 and 6mm bullets were laying too low in the barrel extension lugs to enter the chamber correctly.
Nice! Did not know these existed. I assume you have to use a proprietary extension? Similar to the KAC E3 bolt?
And here I thought we were talking about this 6mm ARC in AR15s, who knew?
Have you ever noticed the chamfer at the breech of all AR15 barrels? Did this guy have a goof?
Yes, The 800 series and 750 XD used the same extension but the newer Titan extension is different. The 750xd and 800 series bolts are a lot larger than the KAC. I would guess the Titan bolt is .010 larger in major diameter than the KAC.
Thanks LedZep, that's where I got confused. I figured it was from 52k yet you mentioned 60k a while back but I see that was for a bolt gun. That clears up my questions and puts things back on track a bit. Hey H, your ARC barrels will be coming set up to a Titan bolt correct?
The Titan bolts have a .124 recess. I designed the extension so a normal mil spec bolt could be used with them if it was an emergency , so that means a 7.62x39 bolt. You can see how that may be a problem with all other ARC barrels using a .136 recess type 2 bolt. I'm sorry I have no idea what I'll do yet. I've been making 6mmARs/243LBCs/6mm Predators and Banshees since 2007 and now we have something just a little different.
No sweat, it came back to me a couple minutes ago of the recess difference between the two but you had already replied before I could edit my post. Multiple ways to skin a cat
Who do I call to order a barrel?
Does it appear the belts are being caused by the dies or by firing where that portion of the case is unsupported?
At the moment I am swamped with other orders, I don't have a production spot open for them. I do not know if I will produce barrels or not since I have stopped machining Grendel bolts and would need to buy them from someone.
It is caused by being unsupported, the chamfer at the breech. Since 2007 the 6mm stuffed in a Grendel case has always been a wildcat, there were no loads printed so we had to work up loads and in doing that some are going to be high pressure I mean you don't know what is high pressure until you get there. Most other cases, 5.56, 308 etc will show flat primers first or ejector swipes but with these thin cases you may get that belt or even a rupture before the primers flatten and flat primers is usually the big sign to back off. I've been a wildcatter since 1982 and in all of those years I have only had 4 case rupture when developing loads, 2 of those were from a 6mm in a Grendel case in an AR. The first was using H4895 with a 95gr SMK bullet. Now that this thing has been SAAMIed and there will be many more people working up loads there will be many more belted magnum photos popping up. I suggest measuring the diameter of the bases just above the groove as a pressure indicator instead of waiting for the primers to show signs.
I've had a lot of frustrations over the years with Grendels. Part of it seems that no Grendel rifle had been gone through and matched machine-wise to the loads, capabilities, and choosing one criteria over the other that limits performance. If it was set for one load, it would be easier. I've come to find, you get what you get, and it's generally not velocity, for the whole rifle as a system to work. But, I have really liked my Grendels. Although, looking back, I wished I had gone with 6mm.
you know the rim on a Grendel case is around .050 thick right? And you drew the chamfer how big? Some barrels makers use a flat angle like you have drawn but better companies use a radius chamfer. I think if you ever get you hands on an AR barrel and measure the chamfer you will see that most are not what you think they are. Bolt guns don't need much of a chamfer.
If i could physically through my money from here at Frank i would. You'll have to go find those 4 threads and tell those 4 guys why their cases are swelled, they sounded pretty worried.
