Odd dimpling on once fired casings

[Rob]

I have a Marlin 1889 Lever gun chambered in 38-40 Winchester.  This particular gun was manufactured in 1890.

When I shoot it, the brass comes out with a dimple near the neck.  The photo below shows it (I have reloaded the brass, but this case has been fired once already.)

The dimple does not show up if you cycle ammo thru the chamber without firing.  It only shows after firing.

In my mind, it looks a lot like the hydraulic dimples you get reloading when you have excess lube on the brass when runs thru the resizing die.

I have it at a gunsmith right now for another issue (looks like a worn sear).  He has inspected the chamber with a scope and is not seeing anything that would cause this.  He does not know what is causing it and is pinging some of his resources to get their take.  I am curious if anyone else has seen something like this.  Again, the dimple only appears when after a cartridge is fired and the chamber has been scoped and shows clear/clean.  I should note that it happens on every single round fired regardless if the brass is factory fresh previously fired.

[Henrydog]

I don't have an answer, but I am tagging a long for a lesson.  Does it do it if you just cycle shells through the action without firing or just after firing?

[elkslayer069]

I know it’s a marlin but you might ask the same question on the Winchester collectors forum. https://winchestercollector.org/forum/ Lots of knowledge on there. Good luck.

[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.

[Rob]

Only happens when firing.

These are light loads-given the age of the gun that is all I want to shoot.

The dimple is always the same shape.

At one time i slowly ejected to see what side of the chamber it was happening.  I cant recall and the gun is in the shop so I cannt retest, but i dont think it was on the extractor side.  That was my theory as to cause.

Not sealing is an interesting thoery...

[ghosthunter]

Can you try one factory cartridge and see if the same thing happens?

[Rob]

Can you try one factory cartridge and see if the same thing happens?

I have shot about 90 factory loads and every one dimples.

Every spent shell has the exact same shaped dimple in the same location. 

Shells I load and eject without firing come out without the dimple.

[Fidelk]

Seems like the shell case is impacting on something metal just after the moment of firing (and only after firing). Is the ammo loose in there and is bullet length correct? These are my guesses?   

[wadu1]

Bazar, tagging along with a positive outcome.

[jrebel]

If you don’t figure it out and you get the gun back….mark the case with a sharpie and index the mark 12oclock in the chamber.  Then see where the dimple is in relation to your indexed round.   Do this with a couple rounds and see if it occurred in the exact same spot.  Then you have a map and starting point that could help identify the problem. 

[Platensek-po]

Have you fired any of the dimpled cases again? Does it dimple twice? Cause issues with the round chambering or case splitting? This is very interesting.

[elkslayer069]

If you don’t figure it out and you get the gun back….mark the case with a sharpie and index the mark 12oclock in the chamber.  Then see where the dimple is in relation to your indexed round.   Do this with a couple rounds and see if it occurred in the exact same spot.  Then you have a map and starting point that could help identify the problem.

That’s what i was thinking as well.

[Rob]

I did a single shot with an index mark.  Even have the shell!  But i have forgotten where i indexed at!!  I think it was top of chamber and if so then it would be the bottom of the chamber leaving the mark.  I will fire it 2-3 times when I get it back.

It happens with every shot-i have reloads with multiple dimples.

One unique thing about this cartridge is that it has super thin walls.  It would dimple easier than other chamberings.

Might be interesting to reload a shell without powder and primer and see if a dry fire dimples it.  I somehow doubt it will but I think I will try that.

[JDHasty]

I put a link to this thread out for these guys to weigh in.  https://castboolits.gunloads.com/showthread.php?463175-What-do-you-figure-caused-this-case-neck-crease

It’s getting some action, but nothing definitive so far.  There is a tremendous amount of knowledge on that site.  If I were a betting man my money would be on either someone there will have an answer or nobody knows. 

The only time I’ve gotten anything resembling that without it being a lube dent is when sizing 30 carbine cases down to 17 caliber. 

[JimmyHoffa]

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.