Severely Dented Cases….Secondary Explosion Effect?

[307WYO]

Any help, advice, or ideas would be greatly appreciated... So here's the details, I'm shooting a 7mm ultra mag. Two days ago, I shot a 0.5" group with 88 gr RL-25 in once fired brass, federal 215M primer, and 140 gr nolser accubond. Once back at home, I loaded another 5 rounds with the identical load but the only differences were that I used new Remington brass and had to open a new sleeve of primers. The following morning, I headed back to the range. It resulted in big dents being left right in front of the case shoulder. There was burned powder on the outside of the case like gas entered between the chamber and the case.
What I have read on this subject and the secondary explosion effect leans towards the idea that: 1.) the load is too light, 2.) the powder is burning to slow causing the case not to expand completely to the chamber wall. I then shot even a lighter load (87 gr) in a once fired case and everything was fine. Also re-tested it with the same load from the day before......
Long story short, the big dented cases were replicated 3 times, all with new brass and primers from a different sleeve. Anytime once fired brass was used, everything worked great. I am going to try a load with the new brass and some retumbo powder to see if the slightly faster burn rate will help….

I have shot this particular load (88gr RL-25) in multiple ultra mags prior to this one and they all shot amazing.

Could it be a bad batch of brass?
Or the new brass possibly stiffer, and therefore not expanding enough?
The primers are slightly defected causing the powder not to burn fast enough?

Any help would be great. Thanks everyone

 

[g0rd0]

GOOD question!
I have never seen anything that severe. The only thing that comes close in my memory was many years ago I was at a NSRA range where they where handing out 7.62 nato (military issue stuff), during a gentlemans competition. We where of course using bolt actions in 308win. In almost all of the brass shot there that day the same dents where there BUT, although the dents were there in almost the same place they were very small.
So it may just be the brass, reload it and see if it does not correct itself

 

[307WYO]

Thanks for the input. I am leaning towards the idea the the powder charge is not burning hot and quick enough to fully expand the new case. I am going to try 92+ grains of retumbo tonight and see if that will work. It makes me a little nervous to shoot the rifle with light loads now, but i am not knowledgable enough on the subject to determine how dangerous this is...Thanks again

 

[dig]

You may try cleaning the chamber real well. I know you said only one load causes tjis but something is not sealing.

 

[284nut]

Have seen this once before years ago (1985). Pre '64 264 Winchester Magnum with 140 grain Nosler solid base bullets, cci 250 primers and IMR 4350.

The only remedy we found at the time was to switch to a slower burn rate powder, back in 1985 options were limited. We found that H870 was the only powder that fixed the collapsed shoulders. It shot accurately so we never switched again.

 

[Sully2]

Thanks for the input. I am leaning towards the idea the the powder charge is not burning hot and quick enough to fully expand the new case. I am going to try 92+ grains of retumbo tonight and see if that will work. It makes me a little nervous to shoot the rifle with light loads now, but i am not knowledgable enough on the subject to determine how dangerous this is...Thanks again

90 grns is listed as the minimum load...and if you load too light and the burn efficiency is all out f whack...you can get cases like you show AND WORSE. My Nosler manual shows loads of 92; 94 and 96 grains and says the 92 grain load was their most accurate

 

[MagnumManiac]

Saw this the first few weeks after the 7RUM came out, worked in a gunshop testing newly rebarreled rifles, we had no load data and this occurred on our handloads and a batch of factory Rem ammo.
We determined it was from too little an amount of slow burning powder, anything below around 90% case capacity would produce this every time even with bullets around 160gr.

You need to bump up that powder charge, because SEE is a very real possibility, we had instances where the report didn't go 'bang' but 'kerwhoomp', and we had to hammer open the bolt! This was with light charges and light bullets.

Cheers.
gun)

 

[Mikecr]

Seen this before
http://www.longrangehunting.com/forums/f28/what-caused-my-case-do-help-79064/

I don't know the answer. It would seem to me any pressure outside the case would not be greater than the pressure inside the case, regardless of chamber sealing. So I don't buy the low pressure load cause. But again, I don't know the answer.

 

[307WYO]

According to the Nolser book, my load of 88.4 grains of RL-25 is in the middle of thd powder range though.....showing 96% capacity. The light load theory makes sense to me but just not with my load.

For an update:
I loaded the exact same setup in a new case from a separate batch. I didn't size the new case either. (I did run the previous cases that were dented through my sizer) The thought being the new brass is already smaller than once fired brass so bumping the shoulder helped create the issue.

Results:
1.) same load in new unsized brass from different batch= no dent
2.) same load with the original new unsized brass= no dent
3.) new brass ran through my sizer with 92gr retumbo= no dent.

All in all, I think sizing the new brass was a mistake on my behalf and maybe a bad batch of brass.

 

[mrultramag]

I've read that it is from too light of powder charge developing not enough pressure. I have a 7 rum. Keeping my loads towards the upper spectrum has resulted in this not ever being an issue for me. Right now I run a 140ab, rem case, 90.6 gr 7828, rem 9.5M primer and the load gives me 3400 fps @ 15ft as chrony'ed. While the charge wt of 7828 may be over max for the manuals, the velocity out of the 26" bbl is by no means out of line. I have run it up to over 3500 with rl25 and retumbo both. RL25 is slower than 7828 and your only at 88 gr so I would guess that its a fairly mild load. Now that doesn't explain all the variables you've mentioned but I would reason that running hotter loads will keep you from experiencing this phenomenon.

 

[mrultramag]

And my guess as to how dented cases happen in the 7 rum; a mild load that does not develop pressure fast enough to seal the neck to the chamber but does develop enough to seal the case body to the chamber. The gas that has sneaked back past the neck collapses the shoulder of the case. and as you have stated there are many contributing variables such as brass hardness, etc that can help contribute to this happening. When the conditions are right-it happens. JMHO but its just a wild assed guess mostly.

 

[500mag_guy]

Now see on my burn rate chart it shows RL-25 and imr 7828 being on the same level for burn rate and then Retumbo being one step slower then the two of them.

In both my 300 rums and in my 375 rum i run them on the hot side of the Scale. And in my semi custom 300 rum I run it over max for best performance. I also run them .020" off the lands and single feed in ever single rifle I have. But I have never had this problem.

I'm confused on what not sizing the case did for you when you used it as a variable??

I think alot of people have problems with the rum calibers because they are under loading them in hopes to have a longer barrel life... Rums were ment to push hard and fast, if a guy wants barrel life buy a standard magnum. 7mm rem mag, 300 win mag etc..

Just my two cents hope you get you problems worked out. Good luck sir.

 

[Mikecr]

I see it as a puzzle, and would love to learn the actual cause.

The cases seem not merely failing to fully expand at a local area. They're seem formed INWARD there.
And, there is no discoloration within the form. The sooting stops at the necks/shoulders.
I can think of nothing that would cause external to case pressure greater than internal case pressure.
If it were a pocket of powder late igniting there would be discoloration from it.
Any interference with full expansion would just cause that, and not inward formations.
The inward formation is what throws me.

Is it really inward, or was the brass sized so much under chamber that it's really a local failure to expand?
I don't see any SINGLE possibility as a cause, but some rare combination of factors.

 

[7 loader]

****! I would check other books also. this does look like a low pressure load.