What happened?

[acloco]

Bolt face is clean and undamaged. Case in front of extractor groove is .473 - .4735". Other fired cases average .467". Virgin cases measure .4655" on average. Blown case runs about .001 to .002" larger than other fired cases at various matching locations between the case head web and the shoulders. No primer found. Primer seating feel for the whole batch was normal.
Will post bolt face picture if I can get a good one.

Hold camera 16" away from boltface (with good lightly) and zoom slightly. Helps to capture the details.

 

[Bojo]

While I have never seen a case like the OP posted , I’m gonna guess that somehow that case head either cracked or had some sort of internal void. Make sure your chamber is ok and replace all the parts.
Bolt head ,
Extractor , ball, spring
Ejector , spring and hold pin
Maybe even firing pin and spring.

Take a real close look at the rest of your brass.

Was that case ever stuck up in a die where it took some serious extra muscle to pull it out ?
Something of the like could have put a stress spot in that case head.

 

[thatguyshm]

Had something similar happen with a known load for my 6x45 a few years ago. I couldn't figure it out for the longest time. I had put a detachable mag kit on the rifle and when chambering a round I noticed very light resistance closing the bolt so I pulled the round back out and inspected it.

The mag kit changed the angle of approach just enough that it somehow folded over a tiny portion of the neck, essentially doing a punch crimp on just one part of the neck, which must have caused my chamber pressure to spike extremely big.

Case looked very similar to yours. Who knows if that's what happened to you.

How old was the ammo? If you have any left, try seating a few a little deeper, looking for a pop as it goes deeper, on the offhand chance there's a way you're bullet cold fused to your case. Obviously a pressure issue, just need to find the cause.

 

[joshb]

Do you check the length of your cases before loading? I don’t read that in the posts.

 

[SSL]

Do you check the length of your cases before loading? I don’t read that in the posts.

Length checked at each loading and held to 2.035". Damaged case measures 2.033". All necks were skim turned as part of the initial prep.
Ammunition was loaded in September as a batch of 50. I now have twelve loaded rounds left from that batch. I had fired three five-shot groups prior to this cartridge with no issues. Barrel was cool since I was testing a couple other rifles as it cooled between groups (.17 Remington, .223, 22-250 so no ammo mix-up). In the interest of safety I think I will pull the rest of the loads and check for issues. I have a box of brand new brass so I might just replace all from that original batch and start over.

Again...thanks to everyone for their input and ideas. Sure a new one to me.

 

[SSL]

Hold camera 16" away from boltface (with good lightly) and zoom slightly. Helps to capture the details.

Best I can do. Done with my phone...my good digital camera battery is dead...of course!
I also borescoped the chamber and see no issues. Ejector plunger moves freely

 

[338 Mollett]

If every thing measures the same and weighs the same ,I can't imagine where to start looking. One thing is for sure the pressure to produce that result is off the chart ! I don't think the worst carbon build up can cause this. How full is the case with this load ? If you weighed by hand and poured in the powder I think you would have noticed a drastic change. It's almost like a over sized bullet kind of pressure but I doubt that's the case. Anyway I'm glad your ok !

 

[joshb]

.???...A conundrum for sure. You should pull the remaining shells apart and carefully weigh the charges in them.
Maybe your scale had a hick up on that one?
Wrong bullet slipped into the box?
Mud Dawber Wasp built a nest in your barrel when you weren’t looking?

 

[SSL]

Mud Dawber Wasp built a nest in your barrel when you weren’t looking?

About as good an answer as I can come up with!
I did pull the loads on a few of the rest of the batch. No variables in powder weight, but I was surprised when the bullet on one case popped out by merely slapping the inertia puller into the palm of my hand. The others took more effort. Wondering if maybe the case in question was loose enough to allow the bullet to work into the lands from recoil after the first rounds in the magazine were fired. Hard to imagine pressure would be that high, but not seeing anything else at this point.

 

[joshb]

About as good an answer as I can come up with!
I did pull the loads on a few of the rest of the batch. No variables in powder weight, but I was surprised when the bullet on one case popped out by merely slapping the inertia puller into the palm of my hand. The others took more effort. Wondering if maybe the case in question was loose enough to allow the bullet to work into the lands from recoil after the first rounds in the magazine were fired. Hard to imagine pressure would be that high, but not seeing anything else at this point.

That might be it. What if the bullet went into the case? That could do it.

 

[243winxb]

1.Your using a double based, ball powder, containing Nitroglycerin. A light load with below normal neck tension. If using a standard primer, makes it worse.
Bullet moves to soon, secondary pressure spike. The bullet sitting in the rifling, acts same as a plugged barrel. High pressure.
2. Trim length to long. Bullet crimped in by chamber?
3. Cleaning compound let in case, reducing volume?
4. Defective brass? Not if fired 3 times before.
5. Loaded neck diameter larger then .276" ? Not likely, fired before.

AA2700

44 grains of AA2700. Book lists 46.5 as max

To light of a powder charge. Best to use a magnum primer. CCI LINK. "We tested loads at both maximum normal pressures and at the starting loads (some labs calculate start loads — we shot them). Standard primers caused no ignition issues at the max load but posted higher extreme variations in pressure and velocity in the lower pressure regimes of the start loads. In extreme cases, the start loads produced short delayed firings — probably in the range of 20 to 40 milliseconds but detectible to an experienced ballistician. Switching that propellant to a Magnum primer smoothed out the performance across the useful range of charge weights and completely eliminated the delays."

Case in front of extractor groove is .473 - .4735". Other fired cases average .467"

The .473" a sign of very high pressure.

 

[SSL]

1.Your using a double based, ball powder, containing Nitroglycerin. A light load with below normal neck tension. If using a standard primer, makes it worse.
Bullet moves to soon, secondary pressure spike. The bullet sitting in the rifling, acts same as a plugged barrel. High pressure.
2. Trim length to long. Bullet crimped in by chamber?
3. Cleaning compound let in case, reducing volume?
4. Defective brass? Not if fired 3 times before.
5. Loaded neck diameter larger then .276" ? Not likely, fired before.


To light of a powder charge. Best to use a magnum primer. CCI LINK. "We tested loads at both maximum normal pressures and at the starting loads (some labs calculate start loads — we shot them). Standard primers caused no ignition issues at the max load but posted higher extreme variations in pressure and velocity in the lower pressure regimes of the start loads. In extreme cases, the start loads produced short delayed firings — probably in the range of 20 to 40 milliseconds but detectible to an experienced ballistician. Switching that propellant to a Magnum primer smoothed out the performance across the useful range of charge weights and completely eliminated the delays."

The .473" a sign of very high pressure.

All good points, and I thank you for your input.
My trim length is 2.035" as listed as minimum by Hornady.
Neck (with bullet seated) is .2755"
Starting load for the 75-grain VMax listed by Western Powder load manual is 41.8 grains with a max of 46.5. My 44-grain load is one grain below (45-grains) where extraction gets a bit sticky and primers start to flatten with my rifle. Accuracy is very good and I don't need to burn up a barrel to get more speed.
And yes, .473" is off the charts. Blown out to match the nominal rim diameter!
Primers are CCI 200, a chart match with the Remington 9 1/2s (not 9 1/2M) listed by Western for this cartridge.
I have used this same load/primer/bullet for at least 3 years in temperatures ranging from 20 to 95 degrees without a problem. The day it happened it was 72 degrees, overcast and I was shooting from my gun room.

I appreciate the info, but after finding a second loose bullet while pulling loads from the rest of the cartridges I'm leaning towards @joshb suggestion that this one case had slipped through with loose neck tension and the bullet migrated into the case as a combination of recoil from the first two cartridges and chambering.

Again, thanks all for your input and ideas.

 

[joshb]

Years ago, when I was first getting into this game, I was shooting a 308. I had found a good, although warm load, touching the lands at 2.96 COL. ( yeah, I was green and measured to COL) I wanted to see how it would shoot at mag length, so I loaded a bunch using the same powder charge but pushed the bullet deeper in the case to 2.8 COL. The first shot locked the bolt. I had to pound it open. It was an eye opener and a learning experience about how “small changes” could have dangerous results.

 

[dellet]

1.Your using a double based, ball powder, containing Nitroglycerin. A light load with below normal neck tension. If using a standard primer, makes it worse.
Bullet moves to soon, secondary pressure spike. The bullet sitting in the rifling, acts same as a plugged barrel. High pressure.

This actually has a lot of merit, knowing that there were other bullets loose in the neck.

The same type of thing happened to a friend with worse results. When the throat went in his barrel, it went quick. Best they could come up with was the bullet jumped with the primer and became a blockage.

There was a lot of finger pointing back and forth with the barrel manufacturer because the rifling was just about missing an inch or two down the barrel.

Something like this might make more sense than bullet set back alone.

Bullet sets back in the magazine, creates a longer low pressure jump that seats the bullet deeper down the barrel acting like an ubstruction.

 

[SSL]

Years ago, when I was first getting into this game, I was shooting a 308. I had found a good, although warm load, touching the lands at 2.96 COL. ( yeah, I was green and measured to COL) I wanted to see how it would shoot at mag length, so I loaded a bunch using the same powder charge but pushed the bullet deeper in the case to 2.8 COL. The first shot locked the bolt. I had to pound it open. It was an eye opener and a learning experience about how “small changes” could have dangerous results.

This actually has a lot of merit, knowing that there were other bullets loose in the neck.

The same type of thing happened to a friend with worse results. When the throat went in his barrel, it went quick. Best they could come up with was the bullet jumped with the primer and became a blockage.

There was a lot of finger pointing back and forth with the barrel manufacturer because the rifling was just about missing an inch or two down the barrel.

Something like this might make more sense than bullet set back alone.

Bullet sets back in the magazine, creates a longer low pressure jump that seats the bullet deeper down the barrel acting like an ubstruction.

OK, an update and possible answer....
I found the primer, totally by accident - it was laying on the floor under where I disassembled the rife. Must have dropped out then.



I assume the side closest to the extractor is the part where no 'skirt' is left of the cup.

In light of the theory of loose bullets, a little background: I had been using the Lee Collet neck die on these and other cartridges. Of course the time comes when the shoulders need to be set back. My practice was to size fired cases through the neck die, check for chambering issues and, if I felt resistance, follow by a trip through a Redding 'body' die set for bumping the shoulder. My theory at this point is that certain cases elasticity caused the trip through the body die and setting back of the shoulders to slightly 'flex' the case necks and cause a looser fit than normal. A better method, if this scenario is correct, would have been to run cases back through the neck-sizing die after the Redding die.
I abandoned the use of the Lee dies some time ago when I noticed that longitudinal case-neck cracks were more frequent and appeared much earlier on cases processed with it over time than with normal sizing dies even with annealing. In every instance, the cracks were evident under strong magnification as being along the stress marks left by the neck die collet jaws (adjusted for a firm but not crushing grip). I returned to standard sizing dies for all of my cartridges and noticed no change in accuracy, fewer neck splits and eliminated one more tedious task. By my records (thankfully I keep them up to date most of the time) this batch of brass is the last one processed through the Lee dies followed by the Redding body die.
If my theory is correct, this would have allowed the bullet to either move forward to create a lot more 'dead air' space (not too likely - fed from magazine) or as @joshb suggested, forced back into the case by recoil or even just chambering causing pressures to jump, even with a less than maximum load.

Or maybe something else...crap shoot at this point!