Even the pretty Grendel bolts can shear

[StoneHendge]

JP with about 3500 on it. Was headspaced to JP barrel. Never ran anything hot in front of it. Was running 123 gr Hornady FMJ with 25.5 gr XBR at 2450 (22") in 6.5x39 Starline cases with Fed 210 when it happened.

IMG_20210209_145852_copy_592x722.jpg

[VASCAR2]

Ouch, you might call JP and see what they say.

[StoneHendge]

I'm guessing at this point that it was the bullets. Barrel had been almost exclusively fed a diet of 123 grain Scenars. I measured a bunch and they are all .2625 diameter. As are all of the other 6.5 bullets I have on hand: 140 gr RDFs, 90 gr TNTs and the 85 gr Sierra Varminters. Barrel has always gotten max speeds (and pressures) 0.6-0.8 gr below what others report, so I'm guessing it's a little "tight".

Measured about 100 of the Hornady FMJs. All were .2630 except for 3. Which were .2635. My only thought at this point is that I got a pressure spike sending a fat one down the tube.....

Try to save a few bucks and buy cheap bullets, trash a $130 bolt.

[LR1955]

I'm guessing at this point that it was the bullets. Barrel had been almost exclusively fed a diet of 123 grain Scenars. I measured a bunch and they are all .2625 diameter. As are all of the other 6.5 bullets I have on hand: 140 gr RDFs, 90 gr TNTs and the 85 gr Sierra Varminters. Barrel has always gotten max speeds (and pressures) 0.6-0.8 gr below what others report, so I'm guessing it's a little "tight".

Measured about 100 of the Hornady FMJs. All were .2630 except for 3. Which were .2635. My only thought at this point is that I got a pressure spike sending a fat one down the tube.....

Try to save a few bucks and buy cheap bullets, trash a $130 bolt.

SH:

I don't think it was the bullets. I think it had something to do with using 7.62 X 39 brass. I too used Fed 210M primers in my fireformed brass and didn't notice anything weird for the most part but then I would get exceptions of blown primers and high pressures. One thing is that I always got donuts to various degrees when I fireformed 7.62 X 39 brass into Grendel. Some were pretty severe and others not so much but most of the brass had them. I started to internal ream the brass after fireforming and the first shot. What a PITA. No -- I don't think it was the bullets you are using. I got a feeling it has something to do with the brass and large primer.

LR55

[StoneHendge]

SH:

I don't think it was the bullets. I think it had something to do with using 7.62 X 39 brass. I too used Fed 210M primers in my fireformed brass and didn't notice anything weird for the most part but then I would get exceptions of blown primers and high pressures. One thing is that I always got donuts to various degrees when I fireformed 7.62 X 39 brass into Grendel. Some were pretty severe and others not so much but most of the brass had them. I started to internal ream the brass after fireforming and the first shot. What a PITA. No -- I don't think it was the bullets you are using. I got a feeling it has something to do with the brass and large primer.

LR55

I always use an expander ball. Got in the habit of doing so when necking Hornady 6.5G brass down to 6mm magnified the inconsistency of it's neck thickness.

Load had a 4 shot sd of 8 during development and a 10 shot sd of 12 during confirmation, so the powder / primer combo is consistent.

I think I'm going to load some 123 Scenars holding everything else the same when it's back up and running. I'm interested to see the velocity since 27.2 gr XBR with a 205M in a Lapua case gave me 2510.

[VASCAR2]

[A5BLASTER]

Was the rifle overgassed? Where would it eject the case's? Was the upper lapped?

[StoneHendge]

Bid early; bid often. https://www.gunbroker.com/item/892164540

Disclaimer: see above commentary on bullet sizing

[dammitman]

well i feel better now as i broke a bolt last weekend as well and looks just like yours. yes i was testing handloads and although not positive 100% sure the cause i am thinking XBR8208 was involved. good powder but be sure of where your at.

[StoneHendge]

One of the lugs is wedged in the barrel extension and it looks like I'm going to have to take the barrel off to get it out. Should I be concerned about the integrity of the barrel extension lug? If the shear was do to overpressure, it was subjected to the same force as the bolt lug.....

[Klem]

SH

What 123gn bullet are you talking about - I was not aware Hornady has a 123FMJ. Were you shooting the scenars at the time? What was your OAL?

As for the extensions lugs, they are more meaty than the bolt so would not be too concerned they have been hammered. It's not like you can just screw this one off and screw another on without all the gear and being concerned about headspace. I would check them out carefully to make sure there are no chips off the corners and carry on.

Early thoughts would be the bullet was not held with enough friction and slipped forward on being loaded into the lands. Or the case was too thick at the neck and did not have enough clearance to release the bullet. What is the OD at the neck with a loaded bullet and those cases? What is the diameter of a fired case?

[StoneHendge]

Klem, the Hornady 123 FMJ is a bullet that's sold exclusively through MidSouth Shooters Supply. I don't know if it's something they actually make themselves or if they pimp their name out. https://www.midsouthshooterssupply.c...-count?Tk=DFBU

[Klem]

SH,

Thanks for those measurements.

I don't know about the die coming loose theory. The die may have come loose and your OAL crept longer, but at some point you would not have been able to load them in a magazine. Unless the chamber is shorter than the AR magazine, which is unheard of, it won't reach the lands. In an AR the mag is always the limiter for OAL length and we all load to the mag limits without fear of jamming in the lands. That eliminates OAL as the cause, unless the round slipped forward from intertia.

Your neck measurements are all fine. Plenty of clearance to release a normal diameter bullet. Plenty of grip on the bullet to stop it sliding from inertia and the diameter of the chamber at the neck is well machined at just under 0.300". So that eliminates those potential reasons.

That's cool that Midwest has an exclusive on no-packaged Hornady's. But that also seems coincidental. Not unusual for manufacturers to test market a potential new product in a geographic location, or in this case use one retailer. No expensive fancy packaging until they see it selling. With the cannelure it was manufacturerd to be fired out of an auto-loader so maybe it's an overrun for a Government contract - perhaps an overseas contract. Midwest are marketing them as both hunting and match but the customers are saying they are cheap plinkers suitable for neither. Typical bulk military type bullets... Intriguing.

Getting back to your bolt because they are bulk, cheap, limited run bullets what if one was a bit out of spec? Wouldn't take much to spike the pressure if one slipped through and was a tiny bit wider than .264.

You could simulate the bullet on QL to be sure. For that, measure (1) the length of the bullet, (2) the diameter at the shank, (3) the diameter at the narrowest part at the tail and (4) the length between the narrowest part and where the shank starts (so you are measuring the shape of the boat tail). If you want to do that I can run a QL simulation. But, it doesn't sound hot so you'd just be ticking this possibility off the list.





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[StoneHendge]

Thanks Klem - I don't think it's a hot load - I can get up into the low 27s with XBR with 123 Scenars in a Lapua case, so 25.5 shouldn't be hot. I was quite surprised though at the velocity though when I did my ladder. It's where I started and where the accuracy and sd was. SDs and accuracy widened I've the next 3 steps so I didn't even bother to finish it. But maybe the LRP enhances XBRs wonkiness with 123s in Grendel and contributed in a confluence of events. I do know I'll be using the remainder of my XBR keg with 77s in .223 where I've had great success with it.

I've got a tight chamber. I run C Products mags which offer the 2.31 COAL. When I first got the barrel, I didn't have the Hornady comparator and I started loading dummies with the Scenars around 2.29. They stuck and they kept on sticking until I got to 2.25, which is where I run them. The comparator (which I distrust immensely) tells me they are 2.26 to the lands. If throat erosion is what wears out a barrel, she has got a ton of life left! Tight chamber rather than tight barrel could also explain why my max charges are 0.6-0.8 grains below everyone else's.

I'm resilient enough to move along. My spare bolt locks down on and ejects my sized brass. All that's left is to put one of the loaded Scenars I have downrange to make sure the case doesn't stretch too much. My memory tells me the brass from the barrel it came from was about 0.005" longer than the JP set up. Then it's on to 1k of the 85 gr Sierras and H322....

[Klem]

SH,

An AR chamber shorter than a magazine. Woah! that is rare and now quite possibly the answer.