Case Rupture

[parshal]

Hey folks, I hesitate to post this since I'm sure I'm going to get all sorts of responses of what I did wrong, should have done, didn't do, how stupid, etc., etc., etc. but it's worth it to me to post as a cautionary tale for other reloaders.

I was shooting my 6 Creed yesterday. Load is 44.5 grains R23, CCI 200, 105 hybrid and Hornady cases. The cases have been reloaded less than five times and never been over-pressured. 44.5 grains is halfway up the shoulder in the case. I've run it to 46 grains which is a completely full case with zero pressure. The action is made by Defiance for Brownells and called Stryker Ridge. Powder came from an 8 lbs. container which I bought sealed and I've used over five pounds so far with no issues.

I loaded 50 rounds with the FX-120. Each charge is then dropped in the case with a funnel and I move on to the next case. There's no way to overcharge since the case is so full and I've filled the case completely with no pressure. Low charges are unlikely since I'd have to spill much of the powder and it would be obvious it wasn't fully charged. Missing a charge completely seems unlikely given resulting overpressure.

I shot 30 shots at 600 yards and on the 31st, BOOM! Extractor broken in half, extractor spring gone and the bolt stop somewhere on the range never to be found. I jump back, pull off the ears and glasses and stand dumbfounded. Then, I feel the blood coming down my face. I had powder/gas burns on my cheek and nose and couple little spots under my eyes. I was wearing reading glasses.

The bolt easily opens. The case is still in the chamber. After collecting myself I set the rifle aside and pick up my other rifle to finish group testing with a few loads. Gotta get back on the horse so to speak.

I get home and take the rifle apart. There's no obstruction in the suppressor or barrel. A cleaning rod pushed the case head out with no resistance. I chucked the barrel in the lathe expecting to have to work at getting the rest of the case from the chamber. To my surprise I was able to pull it out with my fingers. A bore scope shows nothing abnormal in the chamber or throat. The action body looks fine with the exception of the missing bolt stop. I remove the firing pin assembly and nothing odd there except the overpowering smell of burnt powder/gas. The firing pin itself shows no marks from gas and the firing ping hole looks the same, normal.

I still need to pull the rest of the bullets and weigh the charges to verify. After reviewing my loading procedure I can only think of two items as a cause barring finding undercharged cases.

1) Case head rupture - I've had case head failures in other hotrod rifles but those usually come apart after firing and retracting the case. Those cases are also pushed hard which these weren't. I don't think this is it but I'll be checking the remaining cases for incipient case head failures just to be sure.

2) Stainless steel media - I don't use it often but I do use it. After running cleaned cases in a media separator I blow out each case with an air compressor from the neck and primer pocket. It's possible one was still in the previous case and lodged in the barrel. The 'boom' shot could have dislodged the pin but still been way overpressure.

The case clearly came apart violently at the unsupported portion.

 

[spife7980]

Scary but good thing you were wearing your glasses. A nice PSA for safety equipment if nothing else. Not sure what else I could contribute to this, you seem to have a pretty tight line on it. I sort of surprised that the bolt stop blew out way in the rear of the action and nothing else on it shifted beside the extractor which would be the first thing to go. No cracked lugs or bulged receiver threads? If you dont feel comfortable checking that stuff Id be scared myself and Id probably have defiance check it out or send it back to brownells for that badass warranty to check on it.

Whats your sizing regimen? Could be over worked but the rest should be showing signs if this one was that bad.

 

[Carlos Danger]

Could it be at all possible you got a random 6.5 bullet mixed in with the 6's?

 

[Huskydriver]

Could it be at all possible you got a random 6.5 bullet mixed in with the 6's?

Exactly what I was wondering

 

[parshal]

Every case is annealed with an AMP before sizing. Each case is individually measured for shoulder bump.

I wouldn't rule out a 6.5 bullet in the box. It's a 500 count Berger box. I'd like to think I'd notice the difference in length because the only other Berger 6.5's I have are the 140 Elite Hunters that I sold a while back. It would have to have been mixed in from the factory.

I've not measure the threads in the action. It was still very tight when I pulled it from the barrel but didn't feel any tighter than others i remove that were torqued to 80-100 ft. lbs. I did visually inspect the lugs and see no bulges or galling.

The action survived and so did I. The damage was pretty impressive to not have a stuck bolt with severe lug damage. The marks on my face indicate the gas came through the bolt shroud directly back to where my cheek was welded to the stock. Straight back! It's a testament to strength of the action.

My next call is to Defiance to pick their brain. The next call is to Brownells to check their lifetime warranty for what boils down to user error.

 

[Threadcutter308]

Exactly what I was wondering

Or, a Magnum primer mixed in with non-Magnum ? I don't shoot 6 Creed and you did list CCI 200's, so odds are slim that's what happened.

 

[parshal]

I thought of that, too. Those were pulled directly from the 100 pack. No other primers on the reloading bench. Even if that were the case I don't 'believe' that would cause the overpressure given I was 1.5 grains below a full case which never showed pressure either. That's one of the reasons I like this load being as I was unable to ever reach pressure and it's very accurate.

I like the feedback. It's making me think about my process.

 

[Threadcutter308]

I thought of that, too. Those were pulled directly from the 100 pack. No other primers on the reloading bench. Even if that were the case I don't 'believe' that would cause the overpressure given I was a grain below a full case which never showed pressure either. That's one of the reasons I like this load being as I was unable to ever reach pressure and it's very accurate.

I like the feedback. It's making me think about my process.

Makes sense, agreed.

 

[ShtrRdy]

Oh man, glad you're okay. Thanks for sharing this experience.

 

[Fig]

I wouldn't think a 6.5 would chamber at all in a 6 though...

I would think if it was a steel pin and an obstruction the failure would have been a lot more catastrophic and not just blown the case head and bolt, but maybe not... I'm kind of dumbfounded too. Seems like the obvious culprits have ruled themselves or been ruled out.

I would say a bad piece of brass, but would that cause the actual damage? I wouldn't think so. The chamber contains the explosion, not the brass. Mechanical failure of the bolt itself? That would seem to be very rare, but I'm sure it happens. Defiance makes some really good shit, but it's possible some sort of flaw escaped QC. I would type up a detailed letter and send it in for analysis.

I don't want to disappoint you though.
You did it wrong! You should have done it differently! You didn't do what you should have done! You're stupid, etc., etc.,

 

[parshal]

LOL

The action has perhaps a thousand rounds or less on it. I doubt the action was any cause. I'm leaning toward a case issue but it was darned catastrophic to be a case issue. I'm dumbfounded.

I'll report back what I find when I weigh the remaining charges, cases and bullets.

 

[VP47PPC]

I saw a Johnson Automatic Rifle blown to shreds because round was fired off before bolt locked into battery. Escaping gasses can cause a lot of damage. I had a 22 Hornet that would separate case heads after a few reloading with normal powder loads, all I could figure was the chamber or sizing die was out of spec and the brass was worked too much. You said you only shoulder bumped to size so its hard to imagine the brass would get over worked. I would pull the bullets on a few loads and inspect the cases and see if you can see signs of stretching just ahead of where the case head thins into the case body wall. Could the rifles chamber be a little oversized? All signs point to a case failure and remember that the Creedmoor's run at +60'000 psi which sounds like plenty of energy to cause some damage.

 

[parshal]

Brownells' lifetime returns doesn't work the same way for firearms. You've got to work with the manufacturer and Brownells will step in if the manufacturer give you the runaround.

My first call was to Defiance and they asked if the bolt lugs were galled or set back. They said they've never seen the action body stretch in any way and it's always the lugs being set back. They said they'd send me a new bolt stop kit and extractor. I've sent them the picture to see if the bolt stop could be installed properly given the stretching around the pin holes. All in all a great response from them.

I have 200 more prepped cases I was going to load after this initial 50. I'll be checking them all for case head separation. If I find more than a couple I'll toss all of them and start over with my new box of 500 cases. What a waste of cases!

In regard to an oversized chamber, I don't think it is. The go-gauge drops perfectly but tight with .001" tape so it's good there. It's possible the diameter near the base is larger given it was chambered between centers but nothing different than all the other rifles I've chambered. I'll measure the fired cases from yesterday and compare to the sized cases that weren't fired to comparison.

 

[Fig]

The paperclip thing works if it's really bad, but you may want to slice one in half with thin a cut-off wheel to really see if there's an issue.

 

[Doom]

I agree with VP47PPC.

You state you are not overpressure but what you really mean is that you are not seeing signs of over pressure but that doesn't mean you are not above the design pressure. Not saying that this is the cause just that without actual pressure measurement that's really all we know. Not being familiar with the 6 CM and RL23 I will assume you are shooting a safe load and assuming that's the case I would consider that the failure is due to failure of the case. It seems you full length size and anneal at each loading. How many times has the case been trimmed? Have you done the paper clip check for the ridge on the case internal.

Sounds like you have a handle on headspace so I wouldn't suspect that.

Another possibility is overheating of the case during annealing. You can check your annealing with a pair of pliers. Properly annealed brass should crush easily at the mouth and shoulder and be much harder to crush at the base and lower portion of the case. Do your cases show a defined area of annealing (dull) while the lower portions remains polished?

Is it possible that the bullet was seated long and into the lands?

AS for magnum primer I doubt this is an issue unless you are well over pressure anyway.

Finally, if you have a Dremal tool you can cut a case lengthwise and inspect it internally to see if you have thinning.

Good luck.

 

[Milo 2.5]

Could it be at all possible you got a random 6.5 bullet mixed in with the 6's?

Try seating a bullet that is .020" over in size in any case, you will make yourself a nice chess piece. A bullet does not act like an expander ball.

 

[parshal]

I'm using a Wilson hand seating die and an oversize bullet won't even fit into the die to be seated so I've eliminated that as a possibility.

Regarding pressure, you're right, I'm seeing signs but it's always possible it's too high.

To get my AMP setting, I tested five cases in Aztec mode so I know it's pretty set there. Yes, I barely see annealing marks and it does end just below the shoulder about where you see it on new Lapua brass, maybe even a little less.

Seating it into the lands is a possibility. I'm about .005" off at my seating depth so it wouldn't take much to be into the lands. I would expect to have felt bolt resistance when closing. I certainly won't rule this out as a possibility, though. It's on the list of culprits now.

You can see the picture of the case how it expanded about as far as it could in the unsupported case head. That's what broke the tip off the extractor.

 

[mcfred]

This looks very similar to another thread about the same type of failure. How much unsupported case is there between the bolt and the chamber?

Suspiciously similar case failures from too much exposed/unsupported case web:

You are being redirected...

You are being redirected...

You say the action is a brownell's brand. Is it a Rem 700 style push feed? Who did the barrel/chamber job?

Edit: yeah, It's a Defiance re-badge. Sorry for poor reading comprehension. This post describes what the potential problem may be. If you can't see the pics I'll rehost them for the membership here. It's a similar old glock Kaboom! issue from a couple decades back in G22's and similar shooting high-er pressure 40 S&W rounds in a barrel with too little case support in the 6-O'clock/feedramp area. It does not require an over-pressure round to disassemble the gun.

 

[Hondo64d]

Hey folks, I hesitate to post this since I'm sure I'm going to get all sorts of responses of what I did wrong, should have done, didn't do, how stupid, etc., etc., etc. but it's worth it to me to post as a cautionary tale for other reloaders.

I was shooting my 6 Creed yesterday. Load is 44.5 grains R23, CCI 200, 105 hybrid and Hornady cases. The cases have been reloaded less than five times and never been over-pressured. 44.5 grains is halfway up the shoulder in the case. I've run it to 46 grains which is a completely full case with zero pressure. The action is made by Defiance for Brownells and called Stryker Ridge. Powder came from an 8 lbs. container which I bought sealed and I've used over five pounds so far with no issues.

I loaded 50 rounds with the FX-120. Each charge is then dropped in the case with a funnel and I move on to the next case. There's no way to overcharge since the case is so full and I've filled the case completely with no pressure. Low charges are unlikely since I'd have to spill much of the powder and it would be obvious it wasn't fully charged. Missing a charge completely seems unlikely given resulting overpressure.

I shot 30 shots at 600 yards and on the 31st, BOOM! Extractor broken in half, extractor spring gone and the bolt stop somewhere on the range never to be found. I jump back, pull off the ears and glasses and stand dumbfounded. Then, I feel the blood coming down my face. I had powder/gas burns on my cheek and nose and couple little spots under my eyes. I was wearing reading glasses.

The bolt easily opens. The case is still in the chamber. After collecting myself I set the rifle aside and pick up my other rifle to finish group testing with a few loads. Gotta get back on the horse so to speak.

I get home and take the rifle apart. There's no obstruction in the suppressor or barrel. A cleaning rod pushed the case head out with no resistance. I chucked the barrel in the lathe expecting to have to work at getting the rest of the case from the chamber. To my surprise I was able to pull it out with my fingers. A bore scope shows nothing abnormal in the chamber or throat. The action body looks fine with the exception of the missing bolt stop. I remove the firing pin assembly and nothing odd there except the overpowering smell of burnt powder/gas. The firing pin itself shows no marks from gas and the firing ping hole looks the same, normal.

I still need to pull the rest of the bullets and weigh the charges to verify. After reviewing my loading procedure I can only think of two items as a cause barring finding undercharged cases.

1) Case head rupture - I've had case head failures in other hotrod rifles but those usually come apart after firing and retracting the case. Those cases are also pushed hard which these weren't. I don't think this is it but I'll be checking the remaining cases for incipient case head failures just to be sure.

2) Stainless steel media - I don't use it often but I do use it. After running cleaned cases in a media separator I blow out each case with an air compressor from the neck and primer pocket. It's possible one was still in the previous case and lodged in the barrel. The 'boom' shot could have dislodged the pin but still been way overpressure.

The case clearly came apart violently at the unsupported portion.

How much have you been bumping the shoulder when resizing?

John

 

[parshal]

I can't see the pics any larger than they appear in that thread. They do look suspiciously similar.

I did the chamber. If I remember correctly, the bolt nose clearance was .012". But, I'll reassemble it and do an actual measurement with a shim stock that I use just for this purpose. I used Manson's bolt body counter bore cutter.

.001 - .002" on the shoulder bump.

 

[Sheldon N]

I lean towards it just being a bad piece of Hornady brass....

Out of curiosity, how fast was the velocity with 105's and your load, and what length barrel? The net speed is a good indicator of how much pressure, regardless of whether you see pressure signs.

Other questions that pop into my head.... how much champfer on the chamber entry? That would also be a part of the unsupported case head in addition to bolt nose clearance. I'd also be curious how much brass working was going on at the base diameter. What's the base diameter of a fired case vs the base diameter of a sized case, using a micrometer. Is it more than 0.001 difference?

 

[parshal]

I'm going to measure fired vs. sized case base diameter.

The first six cases of the 50 round batch averaged 3191 with an SD of 6.2 yesterday.

There is no chamfer on the chamber itself. There's a chamfer on the outside of the bolt nose recess.

 

[Sheldon N]

The first six cases of the 50 round batch averaged 3191 with an SD of 6.2 yesterday.

Barrel length?

 

[parshal]

25"

 

[Sheldon N]

Not scientific but Quickload says 3191 fps from a 25" barrel with RL23 takes almost 70k PSI. I'd put it up in the category of a hot load.

 

[Milo 2.5]

Not scientific but Quickload says 3191 fps from a 25" barrel with RL23 takes almost 70k PSI. I'd put it up in the category of a hot load.

No doubt, 3191 is scooting in a 25" barrel.
That said, I've fired a few cases with WAY more pressure than that and never had a problem. There is something else going on, even case head sep brass does not leave a distinct ledge when it comes apart, gap somewhere, he just accelerated the issue.

 

[Doom]

This looks very similar to another thread about the same type of failure. How much unsupported case is there between the bolt and the chamber? ...

I doubt thats the case here and it definately wasn’t the case in the accurateshooter post. The OP claimed the bolt was blown back. For that to happen the lugs must fail, the bolt rotates open, or the round fires out of battery. That incident sounds like a trigger issue that allowed the firing pin to release on bolt closing.

 

[Threadcutter308]

I can't see the pics any larger than they appear in that thread. They do look suspiciously similar.

I did the chamber. If I remember correctly, the bolt nose clearance was .012". But, I'll reassemble it and do an actual measurement with a shim stock that I use just for this purpose. I used Manson's bolt body counter bore cutter.

.001 - .002" on the shoulder bump.

I usually chamber mine for about .005" nose clearance, but .012" doesn't sound unreasonable at all...

 

[parshal]

I seem to remember one manufacturer recommending .015" but I can't find that anywhere in my saved paperwork so maybe I dreamed it.

I reassembled and tossed in .015" shim and it closed easily so it's definitely too much. On the first few I left them at .012" and now keep them .005-.008". I don't know how I could have left this one that much but I apparently did.

I cut open a couple of cases and none are showing signs of separation. Comparing the cut ones to the failed case I can see it separated above the web, right about where it was sticking out of the chamber. While I'd say could be a contributing factor there has to be more to this. I will certainly be setting the barrel back a couple turns and get the clearance more reasonable. I may go ahead and trash this brass just to make myself feel better. It won't feel better in the pocket but mentally it will.

 

[Threadcutter308]

I seem to remember one manufacturer recommending .015" but I can't find that anywhere in my saved paperwork so maybe I dreamed it.

I reassembled and tossed in .015" shim and it closed easily so it's definitely too much. On the first few I left them at .012" and now keep them .005-.008". I don't know how I could have left this one that much but I apparently did.

I cut open a couple of cases and none are showing signs of separation. Comparing the cut ones to the failed case I can see it separated above the web, right about where it was sticking out of the chamber. While I'd say could be a contributing factor there has to be more to this. I will certainly be setting the barrel back a couple turns and get the clearance more reasonable. I may go ahead and trash this brass just to make myself feel better. It won't feel better in the pocket but mentally it will.

Buy a box of Lapua.......

 

[mcfred]

Doom, Read the linked thread again. In that thread, the bolt did not leave battery per his description post-mortem later in the thread.

Anyway, here are some of the full-res pictures from that thread. The OP (first attached pic) did not really provide a lot of empirical infomration other than "[Nosler tech support guys] think it is the 'unsupported case' issue." You can see a LOT of soot going well up the side of the case web.

The second anecdotal picture was captioned: "Too much unsupported case as stated above. This one was the 10th round and was an X. Shooter almost lost an eye."

The third case picture is reportedly a factory Rem 700 with a severe overpressure factory Winchester .243Win round. The brass flowed into all the nooks and crannies but did not have a head separation implying an in-spec factory bolt face/chamber relationship.

The last two annotated pictures are the OP's depiction of "out of spec" and "correct" chamber support.

I'll let the rest of you guys sort out what happened to Parshal's rifle. Like so many different kinds of catastrophic failure, there are probably a few contributing facotrs.

Parshal, Glad you're alright. Stay safe out there.

 

[parshal]

Those are much easier to compare and my failed case looks fairly similar. I don't have nearly that much case sticking out of the chamber as the casefail4 pic.

Logic would say that particular case was either a bad case or was in horrible shape compared to the others I cut open which showed no signs of separation. If the case was in sad shape, add more unsupported case than is necessary and BOOM!

 

[pitdog85]

OP all I can say is wholy shit, thanks for sharing that is scary man. In regards to bullet going into the lands causing a catastrophic failure like this would be unlikely unless you were way way over pressure already. I get only a slight velocity increase from bullets jammed hard into the lands vs jumped certainly nothing that would even come close to causing what you have here.

 

[DannC]

Proof loads are nominally 133% of maximum charge weights. I have never seen any gun damaged with an in spec charge. It takes an enormous amount of over pressure to cause the damage shown. The case is always blamed but it is only there to hold the powder and bullet and to contain the pressure from a standard charge.

The proof of the over charge is in the fact that the case was still in the chamber when the gun fired and the high pressure forced the bolt back ripping the head of the case off before chamber pressure could fall to normal.

To the post about the Glock 22 and its reputation of Kbooms generally blamed on unsupported chambers or firing out of battery is pure bunk. Yes I have destroyed a 40 S&W Glock but it was the wrong powder and a charge weight one and half times maximum and probably compressed.

Sorry to be such a downer.

To the OP I am glad that you were waring some eye protection and were not more seriously injured. I have stopped using any eye protection when shooting my rifles because they interfferred with my cheek weld but not any longer.

 

[clcustom1911]

The only question I have is:

Trimming?? It hasn't been mentioned.

Maybe you missed trimming one case and when you chambered it, the uncircumcised case mouth crimped down on the bullet and caused massive pressure spike....???

 

[parshal]

Honestly, I don't think the bolt was forced any further back than the lugs. There's no gouging or setback in the lugs. Headspace is less than .001". If I pushed the shoulder back .003" that's a total of .004" the case can move back. If the lugs don't give that's not much movement. Based on the primer still being in the case head and the fact that 80% of the case head was separated I think all the gas and force simply went around the case and bolt. If that makes sense.

All cases were trimmed with a Giraud. Actually, they were cut to .905 by accident so shorter than necessary.

 

[918v]

Looks like detonation. Did you maybe half charge it?

 

[Supersubes]

Maybe I missed it but can you show us a pic of a fired case in the chamber? How much chamfer do you put on the chamber? The damage to everything else is expected given the separation was too far south to keep a seal of the chamber. Any more pics of the case? Like a side view of the case head?

Load specs seem super hot to me.


Edit: I think it was Sheldon that mentioned a possible bad case. What does the profile of the inside of the case head look like? Does it differ from any of your cross-sectioned cases?

 

[Doom]

Doom, Read the linked thread again. In that thread, the bolt did not leave battery per his description post-mortem later in the thread.

I did and its a shame the OP didn’t post what he ultimately found.

You are correct that he did say the bolt was closed and in the same post implied he was not the shooter so I question which is the correct story. Obviously, it is critical to the determining the root cause.

Note also that the last two pictures you show are not from the barrel in question but are an example from Nosler showing an example of what this issue would look like. The screwdriver is elevating the round.

I wonder if they bothered to check the bolt serial number?

Let’s let the discussion of that thread die.

 

[RTH1800]

Bad case or damaged by setting shoulder back too far too many times.

 

[whatsupdoc]

If you look at the primer it does not look overly flattened from excessive pressure.
If the case was sound and properly supported in the chamber and the failure was from over pressure the primer should look worse.
To me it looks like bad case or unsupported case head.

 

[Fig]

I thought of that too, but I discounted it because I think that with total separation like that the primer probably didn't get subjected to much pressure. All the pressure went out the sides.

 

[whatsupdoc]

If the pressure was high enough to rupture the case the same pressure would have been applied to the primer.
Now if the case web was thinner than the primer cup that would be another story. I cant tell from the picture but it looks like the case head separation is lower on the case than I have seen from stretch due to pushing the shoulder back too far.


@parshal I would cut some of the other cases you fired and check the web for any thin spots.

 

[Wormydog1724]

A gremlin came into my reloading space and screwed my sizing die down an 1/8th turn after about 4 or 5 reload cycles. I realized this when I had a case separation on about my 7th reload cycle. Out of 150 pieces of brass I had to throw 39 away, using the paperclip method for checking for the thinning ring inside the case. When the case separated I didn't even realize it until I tried to chamber the next round so I don't think this was his issue, or at least not the only issue. I didn't blow anything apart on my rifle/bolt/face and once the separated case was removed and compromised brass chunked in the trash, everything is back to normal.

 

[Jake the dog]

Please excuse what may be a stupid question, but if the bolt was engaged and didn't move back, what caused the bolt release to take a flyer? If the bolt lugs were engaged, what force acted on the release? Am I seeing some distortion on the action at the top of the release cutout?
I'm fairly new to hand loading at this level, so I would realy like to avoid this particular part of the learning curve.

 

[parshal]

The case separated right where chamber support stopped. I measure the shoulder bump on each case as it's sized to verify shoulder bump.

I reported above that I cut numerous cases open and found no evidence of thin spots. Maybe this was just a case that was thinner than others.

 

[RTH1800]

I’m leaning toward overpressure event.
This do to the brass flow on the case logo at the OR of the Creedmoor. That brass was moving.

 

[Sheldon N]

Did you figure out how much bolt nose clearance you had? I know you said it was greater than 0.015.

Any base diameter measurements? Just thinking along the lines of ruling out brass working at the base being a contributing factor.

 

[TheGerman]

If its already a hot load; maybe it was made hotter and put over the 'limit'?

- What was the temperature outside while shooting as opposed to when you normally shoot and/or when you tested that original load?

- You said this happened after shooting 30 rounds. How many rounds did you shoot in the string #31 was in and over what time?

- The string you shot previous to the string #31 was in; how long between the end of that string and the #31 shot?

With what you've reverse engineered that it couldn't be, I'm also thinking it wouldn't be a sizing issue because why is only 1 case so out of spec that it would do this? You can rule out the shoulder bump question with a simple shoulder guage on the rounds you loaded but didnt shoot. If nothing there is crazy I wouldn't think #31 was sized that out of spec in regards to the others that didn't explode.

Assuming the shoulder 'test' comes back as fine, the inability of being able to double charge the case, the primers coming from the same box, same powder, same gun, etc. I can only think of 2 things:

Your load is on the verge of dangerous and was put over the line with a combination of higher than normal enviromental temperature which in conjunction with a hot chamber put the case into a overpressure scenario -or- you had stainless media clogged up in the flashhole when seating the primer

 

[parshal]

If its already a hot load; maybe it was made hotter and put over the 'limit'?

- What was the temperature outside while shooting as opposed to when you normally shoot and/or when you tested that original load?

- You said this happened after shooting 30 rounds. How many rounds did you shoot in the string #31 was in and over what time?

- The string you shot previous to the string #31 was in; how long between the end of that string and the #31 shot?

70 degrees. The last time I shot this load was 50 and 52. So, definitely warmer.

The chamber was definitely hot although case didn't sit there for more than five seconds. I shot those 30 cases in groups of 6-8 and then the rifle sat for 5-10 mins. I think this shot was about shot 6.

Your two thoughts are exactly what I suspected originally. The load was warmer than expected due to the temperature, the chamber too hot or steel media in the barrel. Combine either of those with extra bolt nose clearance and I see the results. However, I'd shot others when it was just as hot with no pressure signs.

Sized, unshot cases are how I sized them, about .002 shoulder bump.