What the hell happened -- Case pressure???

[wyomike]

At our 3rd. long range match, anywhere from 600 to 1200 yards, you get 3 sighters, then 5 shots for record. I was on my last target. On my 37th. shot, I had to pound the bolt to get it open. The primer was blown and ejector marks on the case, next shot was fine. The following shot was again over pressured, couldn't open the bolt without pounding on it, blew the primer and ejector marks on the case. I stopped shooting. I have been using this load all summer. 6.5x47 Lapua ,38.8grn. VV550, Federal primers, Peterson brass.
I pulled the remaining bullets when I got home, all the cases had the correct amount of powder (38.8grns.) This is a stiff load, but it's been safe in my rifle. When this happened, the ambient temp had risen into the low 90's. If it was temp related, why not all the shots. I reloaded 15 cases this morning and went to the range, temp was again in the 90's. All 15 shots were fine, normal bolt lift.
Any ideas?
Thanks,
Wyomike

 

[gravelyctry]

How long did you leave a round that was over pressure ‘cook’ in the barrel? If the gun is hot when you chamber a round, that round can increase temp quickly.

 

[joshb]

Trim length, hot chamber? Hard to tell.

 

[Bc'z]

How long did you leave a round that was over pressure ‘cook’ in the barrel? If the gun is hot when you chamber a round, that round can increase temp quickly.

This and my other thought was the load tuned in what I call fight mode.
I don't stop shooting or allow for cool down when I'm tuning charge weight, I like to know how the load is gonna preform at the end of 5 sighters + 20 for score.

 

[DShortt]

You were able to identify over-pressure from various signs. That's the problem.
Now, what caused it?
Heat? Powder charges inconsistent?

Those would be my primary suspects.

 

[Tommie]

Or cooking in the sun

 

[JMayo]

I might think too much shoulder sizing (headspace) popping primer.

Sticky case not enough headspace.

Another thing i think about is...
clean of cleaning fluid in the chamber.

I had some bullets that had trash in the meplat, it would come loose when i loaded a round. Had to beat it open

 

[Hal]

You might check for a carbon ring.

Hal

 

[DShortt]

I might think too much shoulder sizing (headspace) popping primer.

Sticky case not enough headspace.

Another thing i think about is...
clean of cleaning fluid in the chamber.

I had some bullets that had trash in the meplat, it would come loose when i loaded a round. Had to beat it open

OP stated it was the 37th and 39th shot. Cleaning fluid would be long gone.

You might check for a carbon ring.

Hal

Weird that it was fine the next day though.

 

[Hal]

D S

Well scratch my suggestion off the list.

Hal

 

[K22]

I had a similar experience several years ago while hunting groundhog in 90+ degree weather with a 22 250 and an H 380 powder load that never showed signs of pressure at the range which is shielded from direct sunlight.

While my experience wasn't as severe as yours, there was no question that my load exhibited all the signs of a high-pressure surge. I believe this was due to the rifle and cartridge being exposed to direct sun in 90+ temperatures. Like you, not all the cartridges showed it. It was intermittent and unpredictable.

Three things I changed:
First and most importantly, if ever encountering a sign of high pressure, STOP, investigate and do not resume with that load until you sort things out.

Second, I changed to extruded powder and never had issue like that again. However, I do not load at maximum charges either or seat bullet near the lands.

Third, I carry my extra cartridges in an Uncle Mike waist belt holder which shields them somewhat from direct sunlight as opposed to a holder in the side of the stock that exposes cartridges to direct sunlight.

All high-pressure signs should be taken seriously and remedied before continuing.

 

[JMayo]

I was just naming what I'd think about.
I
just like you, do not know what the problem was

 

[Forum Boss]

I think K22 may be correct, although often a powder load over-charge IS the problem.

Shooting my 6mmBR after a steel target reset was delayed I saw the next round go way high. My ammo was sitting in direct sunlight for ~20 min in an unshielded ammo box with about 85deg ambient temp. I noticed the rounds in the box were very warm to the touch.

I immediately moved over to another firing zone at the range and chron’d my hot temp ammo. It was about 85 FPS faster than expected! Note this was with Varget, which is pretty temp stable. But the time in the hot direct sun definitely did raise the pressure.

When I got home I did a reverse calc to see how much more pressure was needed to gain 85 FPS with a 105gr bullet — it was a LOT!

After that experience I never let my ammo sit in the direct hot Sun. After that day I always protected my ammo with a towel and kept it in the shade between stages.

 

[FNSafari]

All this reminds me of watching shooters leave their rifles and scopes in direct sunlight between matches. Most scopes are painted/anodized black, and are great at absorbing infrared radiation. Now, ask youself what that heat is doing to the erector tube supports and springs?

 

[wyomike]

Thanks to all. I strongly suspect that it was caused by the temperature rising. We started shooting at 8:00am. It was pretty cool, by the time this happened, as I said it was in the low 90's This happened when we were finishing about 11:00am. I had 11 rounds left and as I said I pulled the bullets and weighted the powder charge, it was right on. Our range has a roof over the benches, so there is no direct sunlight. The only thing that makes any sense to me is ambient temp. I plan on dropping powder charge when shooting during the hot summer months.
As for trying the same load Sunday morning at about the same temperature, I only fired 15 rounds. 3 strings of 5 rounds each, letting the barrel cool between strings.

 

[Bc'z]

Thanks to all. I strongly suspect that it was caused by the temperature rising. We started shooting at 8:00am. It was pretty cool, by the time this happened, as I said it was in the low 90's This happened when we were finishing about 11:00am. I had 11 rounds left and as I said I pulled the bullets and weighted the powder charge, it was right on. Our range has a roof over the benches, so there is no direct sunlight. The only thing that makes any sense to me is ambient temp. I plan on dropping powder charge when shooting during the hot summer months.
As for trying the same load Sunday morning at about the same temperature, I only fired 15 rounds. 3 strings of 5 rounds each, letting the barrel cool between strings.

Train like you fight, if you shooters long strings of fire, tune that way.
This way you'll get a real good understanding of how your load will respond as temperature and barrel heat rises

 

[Ken]

We alternated rifles to keep the barrel temp down.
That may not be an alternative in your match setting.

 

[NorCalMikie]

I might think too much shoulder sizing (headspace) popping primer.
Sticky case not enough headspace.

My first thought also.
(not a barrel/chamber headspace issue, brass problem)
Ejector marks on the case head?? Case HAD to move to get that!!
Brass getting "hard" and not moving as much during resizing?

 

[wedgy]

One thing to remember is that the temp stability rating is not linear. While Varget shows 0.13 FPS/degree this may hold from 40F to 85F but above 85F it can increase much more which is probably what you experienced. Most powders seem to increase speed around the mid 80's.

 

[Clancy]

My guesses are the cases were exposed to heat or need a trimming.