I had a partial case head separation using a 1.8oz buffer

[LRRPF52]

Is that a crack forming in the 2nd half of brass from the right?



Also, notice the burn patterns of the propellant.

[LRRPF52]

I went through the same process when I first started loading for Grendel, without any failures though.

The shiny ring you see is the web area. The case walls forward of that actually expand into the chamber walls, then rebound a little.

Everything gets put back into place when you properly re-size the brass, using consistent case lube and minimal movement of the headspace to allow reliable feeding in your chamber.

You can see the same surface contrast on other cases too, including 6.5x55 Mauser, .260 Remington, 6.5x47 Lapua, 6.5 Grendel, 6mm-AR, and .308 Winchester:

(These have all been polished, so the presence of the ring varies in appearance.)

[stonehog]

What is length of a pre-kaboom but ring present case after firing?

Generally they are 1.515-1.522.

[BluntForceTrauma]

Off topic, but what are you guys using to section brass? I got a hacksaw or an angle grinder! What's the best method?

[stonehog]

Is that a crack forming in the 2nd half of brass from the right?

Also, notice the burn patterns of the propellant.

Yes - and there is a slight indent in the Hornady case, as well.

[stonehog]

Off topic, but what are you guys using to section brass? I got a hacksaw or an angle grinder! What's the best method?

I used a dremel type tool - it does a pretty clean job of cutting, but it is not easy to make a straight cut. The hardest part is holding the case - first try at this, so some learning to do here.

[stonehog]

I also pulled the remaining 9 that were left in the kaboom box, and segmented a Hornady case, and the 40th pre-kaboom case that had a ring. There is definitely a ridge inside the Lapua case.

I forgot to mention that the powder charges were all within spec on the 9 pulled bullets. They measured from 29.40 to 29.56 and averaged 29.47.

[BluntForceTrauma]

Been listening and learning and mulling this over and a technical question arises regarding brass materials science: What is the minimum pressure in PSI required to deform cartridge brass, to obturate it to the chamber walls? Can't find this answer by Googling, or my Google-Fu is weak.

This is relevant to what pressure might possibly be required to be residual in a case to cause the case web area to balloon into a "belted magnum" as it is in the process of being extracted from the chamber during an early unlocking event.

Apologies if this was asked and answered earlier.

[lazyengineer]

Off topic, but what are you guys using to section brass? I got a hacksaw or an angle grinder! What's the best method?

Dremel. Hacksaw works, but will leave uneven gouges and scratches, and obscure potential details. Ideally a couple moments with a fine file when done.

[Zeneffect]

Been listening and learning and mulling this over and a technical question arises regarding brass materials science: What is the minimum pressure in PSI required to deform cartridge brass, to obturate it to the chamber walls? Can't find this answer by Googling, or my Google-Fu is weak.

This is relevant to what pressure might possibly be required to be residual in a case to cause the case web area to balloon into a "belted magnum" as it is in the process of being extracted from the chamber during an early unlocking event.

Apologies if this was asked and answered earlier.

i dont think this is published information.. but based on everything demonstrated previously, using the exact same software and theoretical methodology of acceptable fudge factor, looks like somewhere between 12k-30k of chamber pressure. 30k being the upper end before rupture, belted magnum look occurring as low as 12k with an early unlock. if carbine length is 7" from the back of the extension, then its about 6-6.5" (guessing) of actual pressurized barrel to the port. either way, if this were to occur on a rlgs like with some 6mm arc folks, the resultant pressure is 1/2 and still occurs.



yes this is quickload. details entered just for irritation factor and to see what comes next.

[LRRPF52]

Off topic, but what are you guys using to section brass? I got a hacksaw or an angle grinder! What's the best method?

I’ll ask my buddy what he does. He’s the guy that does high magnification failure node analysis of exotic alloys in the medical device industry.

I’ve never seen rifle brass sectioned and cleaned at the cut line as good as his. He did a virgin Lapua 6.5 Creedmoor piece for me several years ago, mainly because of his own interest and duplicated one for me.

To say he’s a gifted machinist would be an understatement.

His threadwork on muzzles and actions feels like it was CNC machined and polished.

[LRRPF52]

Been listening and learning and mulling this over and a technical question arises regarding brass materials science: What is the minimum pressure in PSI required to deform cartridge brass, to obturate it to the chamber walls? Can't find this answer by Googling, or my Google-Fu is weak.

This is relevant to what pressure might possibly be required to be residual in a case to cause the case web area to balloon into a "belted magnum" as it is in the process of being extracted from the chamber during an early unlocking event.

Apologies if this was asked and answered earlier.

Bill answered that:

Bill Alexander: Brass will move under a residual pressure of only 1600 psi. For a fully plastic movement it will obviously depend upon the composition of the brass but I would use a pressure offset between 5400 psi and 7000 psi while running pressure/velocity measurements. This was derived by static hydraulic measurement.

With that said you can now look at your rifle set up. The Grendel, (I have no experience of the 6ARC) we could time a gun to drop to the correct residual bore pressure with rifle length gas systems up to 24" barrels. This did however demand that the gas balance was rather finely set. Carrier had to be full weight and a minimum of an H2 buffer (with the tungsten nickel iron weights not the tungsten carbide weights). Hydraulic buffers created general mayhem and only the 308 type was suitable with shorter barrels or longer gas systems, if not both. I would expect from the expansion ratio that the 6mm would be more sensitive.

Statically, the breach has to be machined correctly. The case must remain supported to a point behind the start of the case web, seen as where the case walls begin to thicken into the case head proper. This was achieved by reducing the 120 feed cone diameter on the barrel which was in turn made possible as the round would lift higher on the feed ramps. The projectile diameter placed the centerline higher when it sat on the feed ramps. Again not sure how this would look with a 6mm. Note that the support must not coincide with the case wall to web transition or a shear line is created.

If your gun has the correct dimensional set up then my immediate suspicion would be residual bore pressure from the buffer allowing the breach to open too early. Hope this helps.

[LRRPF52]

If we had a 6.5 Grendel-specific internal ballistics engine, populated with all the available data points we have from actual pressure test breaches, as well as down-barrel pressure trace data to map out port location pressure and exit pressure at the muzzle, it would be a useful tool for sure.

There is already a mountain of industry data for the chamber peak pressure inputs, but it would have to be funded/valued properly.

I would still separate it by the different published test breeches from AA, Lapua/Vihtavuori, Hodgdon’s, Hornady, and Western Powders.

[Zeneffect]

Too easy.

Anyways... looks like the force equivalent to 800x sneezes to move thin brass since we will be using tested pressure chambers as references.

Does dykem work on brass? If op is willing we should be able to visualize dimensional accuracy of feed cone by witness marks and referencing the cross section.

Per Bill Alexander's comment it appears that both are still suspect to occur. It appears as if a bit of both issues may be in play and the contention point is which occurs first. Fissure, or early unlocking. Looks like fissure first then exacerbated by early unlock event to me after seeing the sectioned almost blown out case.

Regardless I no longer feel this is a single point failure and we are seeing multiple issues.

[LRRPF52]

Yes, Dykem works if you want to register some witness marks.

The fact that there are incipient case head separation symptoms on both Hornady and Lapua pieces of sectioned brass indicate something wrong with the pressure-containment system and/or timing.

A combination of on-the-edge chamber support from the feed cone, CLGS and its high port pressure/mass flow, and lack of appropriate carrier and buffer mass are a recipe for case head separation.

This is mostly what happened on the Colt 602 (2nd AR-15 production model) in Vietnam with that really lightweight Edgewater spring guide/buffer, and excess port pressure from ball propellant.

[LRRPF52]

And another post directly addressing Lapua brass and how it articulates with the chamber under initial peak pressure, as well as during extraction:

Bill Alexander: The Grendel chamber geometry and bolt was designed in concert with the Lapua case. The chamber "if" correctly dimensioned and with the correct feed cone geometry will allow chamber pressures well in excess of 68k psi without the case deforming. Obviously this is for Proof loadings only and should NOT be used as the basis of any load development. It will damage the rifle! The absolute maximum is 51,800 psi and MAOP pressures are held around 50k psi. Pushing loads to 52kpsi allows zero envelope for pressure excursions, which will and do happen even with the most carefully assembled loads.

Stretching of the case above the web results from either an incorrectly machined barrel or excess residual bore pressure as the case is extracted. I do not have direct experience of the 6mm ARC but simple expansion ratio calculations indicate the timing will need attention. This is not just the size and position of the gas port in the barrel but also the reciprocating mass which includes the impulse from the fire control group.

Regardless, the 6mm ARC is an exceptionally well designed cartridge with an excellent balance between case volume and primer flash volume, which is something that is frequently missed. It benefits from some excellent powders and projectiles.

[stonehog]

A combination of on-the-edge chamber support from the feed cone, CLGS and its high port pressure/mass flow, and lack of appropriate carrier and buffer mass are a recipe for case head separation.

Would I be correct in eliminating the carrier/buffer issue by turning off gas and firing a few to see if I get the ring? May be worth a try.

[Zeneffect]

Yes. This was something I had mentioned earlier, you will be removing early unlock from the equation. I would only do 1 though due to safety concerns

[lazyengineer]

Would I be correct in eliminating the carrier/buffer issue by turning off gas and firing a few to see if I get the ring? May be worth a try.

My expectation is yes. Though be aware one of the 3 Kabooms in the last year was with a bolt action rifle with that same brand brass. That said, it was with a casing that had been run 20 times or so, and one would think that to be a notable potential factor. A high-fire/resize count rerun factor that you are not facing. To do the refire test without the gas on, and see if that change alone fixes it with everything else the same, it would be ideal to run same load, same type brass, etc. Not sure if you really want to do that, or where your comfort level is on this.

Personally, after a catastrophic failure, I'm more along the lines of changing a whole bunch of stuff that might be contributors all at once. Your sectional tests and posts were helpful and appreciate you going to the effort. No real surprises there, TBH, but good to see confirmations all the same. From a scientific analysis standpoint, I'm curious to see what your no-gas test result looks like for sure, if you're willing to do it. My expectation is everything will come out looking fine, as the issue in question seems to arise when there is a combination of factors:

-too fast bolt-unlock (timing), plus
-excessively chamfered chamber entry (Odin and PSA perhaps?), plus
-brass with a more shallow head-design and so the first to be affected by the above two (apparently Lapua; though Starline also seems to have a high count of belt-formation complaints - though no kabooms there that I know of).

Mitigate any one of those and it seems to help mitigate the issue. Or so it looks like from the posting patterns on this issue. Curious to see what yours looks like when you eliminate the timing factor, by turning the gas off. You likely will need to fire more than one to get a lot of meaningful data, since only a portion of the brass in the original run exhibited the phenomena.

[Zeneffect]

More than 1 is definately needed for an appropriate sample size, but as there is a risk factor of losing something irreplaceable, I think even 1 may be pushing the envelope of safety. Got a string and some cover?

Anyone got a picture of some cross sectioned starline? If not I can cut a case open and share its guts for the fortune telling. If its already available no point in me wasting the time and a case.

*edit - found it

https://www.65grendel.com/forum/showthre...post291053