Dimple in Brass Cases

[196kirka]

Had two strange occurrences while shooting some reloads at a range last week.

I fired 10 reloads each of a 7.62x54R load using Varget, IMR 4895 and BL-C(2) and ten reloads of a .303 British using BL-C(2).

The last fired case for the 7.62x54R using 41.0 gr. BL-C(2) (starting load as listed in the LEE "Modern Load" manual). and a 150 gr (.312) Hornady SP exhibited an elongated dimple in the brass starting at the shoulder and running aprox. 3/8" length, 1/8" deep and 1/8" wide. The brass used was twice fired PRIVI PARTIZAN brass. The brass was neck sized only. The firearm used was a Mosin-Nagant M91/30 rifle. I had fired this rifle many times using mil surplus and reloads. I have never seen a dimple appear in any brass using mil or the reloads using IMR 4895 or VARGET.

I then fired 10 reloads of .303 using 42.0 gr. BL-C(2), 150 gr PRIVI PARTIZAN FMJ, The ninth fired case exhibited a similar elongated dimple but slightly deeper. The brass used was unfired Remington (thin brass) brass that had been neck sized only. After the 10 reloads, I fired 20 rds of PRIVI PARTIZAN , .303, 150 gr. SP BT. There were no dimples in the brass after fired. The firearm used was a No. 4, Mk 1* (Savage) Enfield.

I have always used IMR 4895, IMR 4064, AA 2460 and Varget powders in the older mil rifles. This was the first time I have ever used BL-C(2) powder. I only purchased this powder because I have not been able to find IMR 4895 or H4895 powders.

I have never seen the elongated dimple appear in any rifle or pistol reloads, ever. I am wondering if anybody has had this experience before, why it would happen. I am wondering if the BL-C(2) powder has something to do with it. I was going to do some .223 and .308 loads with the powder but am holding off to see if anybody might know if the powder is the cause or what else might be the cause. Hopefully someone has some info.

Information

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[MarkB]

Were the cases sooty? I have not experienced this problem, but I've heard that it can be caused by using a light load of a slow powder. Apparently unburned powder can be blown between the outside of the case and the chamber wall, where it subsequently burns and creates a situation where the pressure outside the case is higher than inside it.

I see that Hodgdon lists no loads for 7.62x54R. For .303 Brit, they recommend a starting load of 53 grs of BL-C2 for a 150 grain bullet.

You might want to mike your bullets. I suppose .308 diameter bullet could cause similar issues.

[Litt'le Lee]

sounds like your bolt is doing this while chambering new rounds

[Patrick Chadwick]

I knew I had it somewhere, but it took me nearly an hour to find this info!

This effect is described in detail in a case study on PP. 341-2 of DWJ (Deutsches Waffen Journal) 3/1983. The photos show cases dimpled just as described by 196kirka.

The elements that combine to produce this effect are some or all of the following:
1. A powder charge that is insufficient to press the brass against the chamber and make a gas-tight seal.
2. An oversized chamber, so that a recommended minimum charge which ought to be sufficient to achieve sealing is no longer adequate.
3. Excessive freebore, so that the bullet flies free of the neck, which can then spring back as the gas leaks around the neck.
4. Increasing pressure as the bullet is thrust into the forcing cone (a.k.a. throat).
5. When the bullet is fully into the rifling, rapid acceleration of the bullet cause a drop in pressure in the barrel and the cartridge case. BUT the extra-high pressure gas between the case and the chamber wall cannot escape so quickly through the thin gap between the neck and the chamber.
6. The result is a very brief time where the pressure within the case is lower than the pressure between the case wall and the chamber - so the case collapses. As described by 196kirka and shown in the photos already referred to.

So the question from MarkB is spot on - the none-dimpled cases will probably also have sooty marks on the neck and shoulders.

Considering the above factors, it would seem advisable to A) check bullet seating depth and B) check freebore, then C) use a larger charge of a slower powder.
And, if you're really cautious - make a chamber cast and measure it very, very carefully and compare with published maximum/minimum dimensions.

Patrick

[196kirka]

A huge amount of thanks for the information MarkB and Patrick. You all are right on but I have never experienced this dimple thing before with any light loads. I believe it must be the powder (BL-C(2)) and the light loads. I did not notice any sooting on any cases except the 7.62x54R case. The case was shiny brass and there was small amount of residue where the dimple was.

The 7.62x54R load came for "LEE Modern Reloading". The start load was 41.0 gr. so I used that load. Hodgdon Powder Co. load data showed 52.0 gr. start which I thought was a rather stout starting load since it was 11 grains more than the LEE data. Also the Hodgdon data refers to the use of a .308" bullet. None of my Mosins use such a small bullet. The Mosins I have measured so far are .313-.314.

The .303 load data of 42.0 gr came from an internet source. The "LEE Modern Reloading" manual lists 43.1 gr. as a starting load so I figured that 42.0 could not hurt. The "Hodgdon Data Manual" lists 45.0 gr. as a starting load. This load should be more like a proper load to ensure a seal upon firing.

I will increase the amount of BL-C(2) powder. It may be a few weeks until I get to do the reloads and try them out. Maybe in the meantime I can locate some IMR 4895 or H4895. I have used the IMR 4895 for years and have never had any such situations using light or starting loads. Thanks again for the information. I have copied it for future reference.

[Patrick Chadwick]

No surprise that you never experienced dimpling before - the effect is so rare that it is hardly common knowledge. As you can see, I had to go back 26 years to find a documented case, as opposed to hearsay.

I would keept the dimpled case as a curio - you may well never see it again!

BTW, did you check the chamber dimensions?

Patrick

[196kirka]

Thanks Patrick. I need to get some of that stuff called Cerrosafe, I think, to check out the chamber dimensions on the Enfield. I did check the neck size of fired Remington (starting load) and the Privi (full power commercial load) and compared to the specs. as listed in the Lyman #48 manual.

Rem. low power load: Neck .338 (spec. size .338), shoulder .409 (spec. size .4010)

Privi full power load: Neck .3415 (spec. size .338), shoulder . .4085 (spec. .4010)

The low power BL-C(2) load in the Remington brass was not sufficient to properly expand the neck to seal. I will increase the charge for both the 7.62x54R and the .303 British.

[MEHavey]

QuickLoad shows the following data for the two 7.62x54R starting loads you mentioned above (using the .312/150gr Hornady SP and 24" barrel):

41gr - 66% fill - 2,200fps - 23,700psi
52gr - 83% fill - 2,800fps - 44,500psi

Quite a difference -- no wonder you were getting underpressure signs w/ the first load. I'm also surprised you didn't experince hangfires w/ that load because of the low case fill ratio.

That (similar) 42gr of BL-C2/150gr FMJ in the .303 showed 31,700psi -- borderline low pressure as well -- but functional as a (very) mild load at 2,400fps

[John Kepler]

Thanks Patrick. I need to get some of that stuff called Cerrosafe, I think, to check out the chamber dimensions on the Enfield. .

It probably won't help you any, at least not with the Enfield. Enfield chamber dimensions bear absolutely NO relationship to the dimensions of the cartridge case! The chambers are cut 0.010" longer than the case dimensions...could be more...and are in spec! The Enfield obturates at a datum point just ahead of the case rim....anything that happened ahead of that datum-point didn't concern them that much (easy extraction did), so the dimensions forward of that point are, shall we say, generous and variable-signifying nothing....distinctions without much of a difference!

Also, Lee must have a fairly large number of lawyers on staff writing their load manual! I've noticed that ALL of their "starting loads" tend to be VERY anemic compared to other loading manuals.