Odd dimpling on once fired casings

[BULLBLASTER]

Is there usually black soot in the dimple?
It looks like the neck didnt seal in the chamber and gas blew back and made it.
Ive seen it from light underpressure loads.

Sounds like a possible cause.
Also, I think even with standard pressure if the gun starts to loosen up, similar things can happen. The lock up can feel tight when not under pressure. But the various components can wear or stretch, especially in a 130 year old gun. Then when firing, either bolt shifts back or barrel forward. With that much taper and being headspaced off the rim, seems quite possible.

Thats a good point. Or possibly some irregularity in the neck of the chamber prohibiting a seal?
I think that is a pretty low pressure round already.

[Rob]

I will chat with the smith about the low pressure dimpling that was mentioned on this thread and the castboolits thread (thanks for cross posting.)  that seems the most likely cause.

Not sure if there is excessive soot - I can check when I get it back.

I took a photo of six shells with the dimple - they all look identical to me.  The far right cartridge has been fired 2x out of the gun and you can see the remnant of the first dent which has fireformed about 50% out after the follow up shot.

[Rob]

Would "failure to seal due to a low pressure charge" put a consistent dimple of the same size, location and depth on the brass?

[JimmyHoffa]

Would "failure to seal due to a low pressure charge" put a consistent dimple of the same size, location and depth on the brass?

If the brass is the same structurally and all the other conditions close, I'd imagine so. Kind of like how sausages all seem to split in the same place when grilled.

[mountainman]

Is there usually black soot in the dimple?
It looks like the neck didnt seal in the chamber and gas blew back and made it.
Ive seen it from light underpressure loads.

This👍

[JDHasty]

I would be inclined to look further into post #17 on this thread.  It impresses me as a possibility.   

https://castboolits.gunloads.com/showthread.php?463175-What-do-you-figure-caused-this-case-neck-crease

[chukardogs]

Have you made a cast of the chamber? Just because the chamber looks clean doesn't mean there isn't an imperceptible high spot or depression in the surface. It doesn't have to be obvious to cause an issue when you're dealing with material movement of the cases. This may seem or actually be silly but unless it would be unsafe, I would try putting a filler of some kind (possibly a cotton wad) in the case. If the powder charge is small enough to leave a significant amount of empty space in the case, as the pressure builds on the material, it could be uneven enough to cause a concentrated load to be unevenly applied around the case. Have you measured the thickness of the neck around the case opening. If the case neck material thickness is consistently thinner 180 degrees from the dimples, I think you will have found your issue. Another way to know if this is your issue would be the case length. If the case is consistently longer (even a couple thousands) inline with the dimple, this means the case material growth when fired is uneven because of an uneven pressure build in the case. Just an idea....

[chukardogs]

What powder are you using for the loads causing the dimples?

[chukardogs]

Looking at the reload data, there's a huge difference in volume cc's between titegroup (.42) and  IMR trail boss (.98)
Throw in the fact that titegroup is a faster burning powder than the IMR trail boss. Just a change in powder may fix the issue you're experiencing. Good luck

[Rob]

the rifle is still with the smith and he is making a new sear for it as it is quite worn.  Once I get it back (probably Feb - I am not in a huge hurry), I can test stuff out a bit more.

I would be inclined to look further into post #17 on this thread.  It impresses me as a possibility.   

https://castboolits.gunloads.com/showthread.php?463175-What-do-you-figure-caused-this-case-neck-crease

I can say it chambers easy.  Only creates the dimple after firing, and the dimple goes into the case, not outward for what it is worth.

[chukardogs]

The dimple has to go inward. When material grows, it takes the path easiest traveled. Just a FWIW, If you look at your cases total volume cc capacity, it's 2.33. The volume of .42 cc's of titegroup is less than 1/5th of your capacity. This means when the gun is level, there's a very good chance that you have no powder in direct contact with the primer. Compare that with the IMR trail boss, being just a little under one half of the cases total volume capacity in cc's, I can't believe that just based on simple physics, that fact alone has wouldn't cause some pressure issues within the case at the point of ignition.

[Rob]

What powder are you using for the loads causing the dimples?

I reload with 15 grains of I-4227 with 180 grain LRN.  I would need to go back and find my data source but this was on the very low end of the scale - a starting load.  I want to say the range of charges was 14ish to 18ish.

The Black Hills factory ammo were cowboy action loads, but of course I have no idea what powder or charge they were shooting. 

A cast of the chamber might be interesting

[Rob]

The dimple has to go inward. When material grows, it takes the path easiest traveled. Just a FWIW, If you look at your cases total volume cc capacity, it's 2.33. The volume of .42 cc's of titegroup is less than 1/5th of your capacity. This means when the gun is level, there's a very good chance that you have no powder in direct contact with the primer. Compare that with the IMR trail boss, being just a little under one half of the cases total volume capacity in cc's, I can't believe that just based on simple physics, that fact alone has wouldn't cause some pressure issues within the case at the point of ignition.

I suppose I was thinking if there was a chamber issue where there was a crater in the chamber somewhere, that might create a dimple going outwards.  But these dimples look like the hydrolic dimples you see when there is too much lube on the case - the shape seems to "flow" which I think might give credence theory of the brass not sealing off the chamber which was allowing gas to flow past and create the dimples (as was mentioned earlier by a couple folks)

[chukardogs]

They make products specifically for this issue. Plastic shot buffer, dacron fiber pad, etc... If you don't find a distinct residue pattern on the outside of the case from this escaping gas, I'd have to look at other possibilities. I had a buddy that reloaded a bunch of 25-06 shells without trimming length. This caused each case to run into the end of the chamber during expansion from the pressure to crack down the full length of the neck area. Every one of these cracked cases had a distinct black residue along the entire length of the crack. If the case doesn't have enough room to grow in length during the firing, something gotta give. Good luck

[wadu1]

Any update on this @Rob? It's been a interesting follow along.