Inexplicable bump in .375 CT case

Hi guys,

I tested a CheyTac Intervention M300 in .375 CT.
I made some hand loads with SMK 350 grs and Peterson cases.
When the muzzel velocity overrises 900 m/s the cases got a bump in the shoulder.
Below this velocity there is no bump in the case.
Does someone know what causes this bump?

Best regards


Does the chamber have a fluted neck (Gasentlastungsrillen)?
May it be gas pressure “flowing back” around the neck and when dirty or oily it happens only on one side and pressure builds up maybe?

I think EOD is on the right track. I have had this happen with a 300 Weatherby that the projectile was making a “leap” from the case mouth to the rifling that left a gap (lots of freebore in a Weatherby factory chambered barrel). The cases had only been neck sized and the slightly smaller diameter of the neck allowed gas to blow back and created similar dents. The cases having been fired in the same rifle allowed the shoulder to to seat and seal preventing gas from flowing further down the case. This sort of dent is more commonly seen on cases that are sized with to much lube. (Prior to firing) Treading into reloading topic.

Looks like there was something in the chambering of the
rounds that resulted in the folds (Bump as you decibed it). Juts a guess on m,y part!

According to William C. Davis, this can be caused by the reason Sportclay described but also by slow ignition of the propellant. The surprising fact in your situation is that it appears with “hot” loads, while most explanations are based on weak loads.
Davis concludes: “The phenomenon has not been systematically investigated…”
(Source: Handloading, NRA 1986)

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I’d suggest you contact Peterson directly - I’ve had GREAT experience with their customer service dept.

Peterson Cartridge
761 Commonwealth Drive, Suite 201
Warrendale, PA 15086 USA
(+1) 724.940.7552

Question #1: Did you use their .375 CT brass, or did you make the .375 cases from their .408 CT (10.36x77mm)? If the later, did you anneal the brass (neck & shoulder) after resizing down from .408 to .375"?

Question #2: Were these fired in an auto-loading rifle? I’ve seen similar dents (although not in the cases’ shoulder) from violent extraction/ejection when the case hits either the ejection port or the brass deflector (e.g., AR-10/SR-25 style rifle) - especially with hot loads and/or over-gassed loads. In other words - could this be happening during the ejection sequence, and not in the chamber?


Hi guys,
thanks for all responses!

@ Alex the rifle has definitly no fluted chamber.

@ Jochem thanks for the explination

@ Ranger Joe the Peterson cases are headstamped with .375 (9,5x77). So the are not necked down from a .408 CT.
The rifle is a bolt action no automatic action.

Best regards


Surely there must be some kind of pressure relieving flutes in the neck?
See picture:

Looks to me like a set of flutes around the neck.

I support the theory that the case is not sealing fully (maybe oil or filth in chamber), where some gas leaks back and “pops” a dent in the case.
But I also agree that this usually happens with too weak loads or too slow burning powder… not hot ones.


I think you may be correct. HK makes some of their firearms with fluted chambers . But I have never seen this on fired cases from a HK91. A bore scope will tell the tale.

I have an K&K and the whole side of the .308 case is fluted (all around the case) then when ejected it flies 40 feet into the field (don’t stand to my right when I shoot this rifle) .to me it looks like rifling marks on the neck of you brass, my guess back pressure going back towards your brass. My 2 cents.BB34

I ment to type H&K ….fat fingered that one.

H&K is also know as spent case throwing device. If this is my P7 my SL8 or especially the G3 do not stand on the right side of the gun when it is fired

But returning to the basic of the thread I made a quite similar experience with reloading 308. This type of marks occure if you lubricate the case to much. The excess of lubricant is collected on the shoulder when you recalibrate the case and as oil is a non compressible liquid the case gets deformed

This is occurring upon firing.

Hi guys,

attached a picture of the unfired, original cartridge. As you can see, the axial grooves in the case neck are before firing. They results from factory neck sizing.

I cleaned up the chamber and the cartridges with aceton. So there was no oil and soiling in the chamber. Same result. Getting dents in the case shoulder.

Best regards Chris

Chris, interesting!
I have not observed such obvious tool marks (on life rounds - there are Swedish 7.62x51 blanks which have similar ones and I always wondered if these are on purpose) and also not such unevenly distributed ones.

Below a cropped section of your image.
These tool marks are very uneven as stated, marked as dotted lines.
Also there is a dent visible in the case before firing (marked “X”).
Also there is more scratches and stains (arrows).

Maybe the reason for the dents is a combination of all togehter where the tool marks allow to pass gas back, that being uneven due to the uneven execution of those, also maybe influenced by the stains (material?) and then catching in the micro dents at the shoulder and allowing the gas pressure to build up there?

What puzzles me is that these toolmarks are present at all!
Can you get hold of the same cartridge without such tool marks and shoot some to check then for the dents?

Alex, I shot cases from Horneber too. These unfired cases doesn`t have axial grooves or micro dents. After shooting same result. Dents in the case shoulder.

I think that dent is result after bullet is leaving the muzzle. Because the pressure in the case and barrel is, while the bullet is still in the barrel, high enough, that the case is completly lids the case to the chamber. So a dent to the inside of the case couldn`t happened.

Best regards Chris

Chris, the cartridges you fired were hand loaded ones and factory made ones and all caused these dents?

Is it to be assumed that the dents are then caused by the rifle / chamber?

Chris - this is VERY wrong! There should NOT be tooling marks like that on NEW brass.

One last question before diagnosis: You purchased new empty brass from Peterson, and loaded them yourself. Your photo shows loaded, but unfired cases with severe (in my opinion) tooling marks on the necks. Can you please check the NEW UNLOADED brass for the same?

If the new brass has these marks, Peterson screwed up bad… which is VERY unlike them - but stuff can happen. Contact Peterson directly with photos - I am confident they will make it right.

If the new unloaded / unfired brass does NOT have these marks - they most likely came from either your sizing die or bullet seating die.

Please let us know… this is most interesting.

Kind Regards, RJ

I work at the Proof House in Birmingham England and this happens with some proof rounds on firing in test barrels during proof ammunition development.
There is no ‘carbon’ fouling around the neck of the case, so it appears that the case has sealed the chamber. One possibility is that there is a vacuum caused when the bullet leaves the barrel, causing the case to ‘collapse’ in on itself at a weak spot.
I have seen cases with ‘bumps’ down 2 thirds of length of the case body.
Hope this helps.

Spacegunner, do you happen to have images of such cases? It would be most interesting to compare them and view for reference.