OzFclass

Bruce,
I am getting the same sort of case head / web expansion on Remington cases on first firing in the RSAUM. They have a lot to make up as are several thou under chamber (SAAMI Spec) to start with and seem very soft. If I fireform with a light load 45,000 psi approx.) they seem to have this initial expansion with little effect on primer pocket then the second load can be upped with very little further expansion. Talking to a few of the other RSAUM owners this seems to be a common result. (All done with pressure gauges strapped on whilst undertaking the testing.)

I don't disagree with your statement as it is the safe approach. Case head expansion is an important measure of safety as it indicates bolt head thrust, chance of blown primers and case head separation. But here in lies the dilemma - If you lube your cases you will see pressure signs start to impact on case head at say 45,000 psi (or less depending on case head hardness and cartridge design). No doubt very safe and also in another way - you will get very little case wall thinning so no dangerous case head separation either. I believe this happens by having good case wall grip and over bumping headspace so that case head is pushed back by pressure whilst case wall holds front of case forward. This is a very safe way to load but your 284 cases (worst case as has relatively small head plus steep case wall taper) will be very limited in pressure and speed. Very safe but maybe not very competitive????

If you dry your cases and walls well then case head pressure will give pressure indications at or around max recommended chamber pressures but there are 2 catches. 1) Case head separation if regularly bumping significant amounts and 2) beware the difference of quality cases - (e.g. Lapua small primer cases) can hold together well so may exceed max recommended chamber pressure.

Interesting info Dave!

I know with my reamers I always order min spec saami, except for throat and neck specs which depend on what brass and bullets I will be using. It also highlights the need for headspace measuring gauges which are based on the datum, (or close to it) to minimise shoulder set back as you have already pointed out. The less we work our brass the better condition it stays in.

Interestingly my new toy in 300WM shows brass that looks like brand new after firing 230gr hybrids at 2850 - 2900+fps. I have measured in the extractor grooves of new and once fired brass and find they are the same to within 0.0005" which is at the accuracy limit of my "Limit" Japanese made digital calipers. The first firing did expand the head measured in front of the belt out to 0.513" out from a new case measured at 0.509". This matches the reamer dimension of 0.5136" at that point in the chamber. The primers after firing still have the BR logo looking brand new and are without any signs of flattening. Without your pressure testing equipment it gives every indication that 3000+fps should be possible in my chamber.

The Nosler Custom brass (Norma) I am using looks to stand up to the loads I want to use with the heavy bullets easily. Its a lot different to the Lapua 284W and Shehane variant which needs a bit of babying to get a load worked out and reliable.

Watching the pressure when working up loads must be interesting, and thanks for sharing some of the things you have found!

Ian

ian,
my measurements were on the belt just in front of the groove.
possibly 0.003 movement is sufficient to harden the brass such that it ceases to move further.
the brass always lasted well and strangely the primer pockets did not seem to open up.
with rimless cases, 0.0005 can be a bit of a worry.
whatever you shoot, as pressures go up, brass will move permanently, firstly causing looser primer pockets, then case failure.
dave,
there is no question that the more grip a case has on the chamber walls the better is the situation.
apparently absolutely zero friction there can stress locking lugs.
this is one reason Ackley cases have closer to parallel sides than many originals. to reduce bolt thrust by increasing chamber grip.
the old h&h cases had a lot of taper, as did the 7x57, to make the function of chambering more reliable, but this increased backthrust.
the 308 and modern belted cases have less taper, more chamber grip, and gain less from ackleyizing.
I am surprized that modern saum cases expand that much.
keep safe,
bruce.

Bruce, I measured the cases again, this time on the belt as you had done previously with your cases and found the unfired case to be 0.528" and the once fired case to be 0.529" The fired cases picked up 0.001" which is not much at all when you consider its launching 230gr projectiles.

Ian

If you have zero headspace or say .001" case crush, then how does case/ chamber grip enter the equation? The case cannot move laterally as it's locked in, --therefore no friction.
Considering the amount of backthrust generated by a chamber pressure of say 60,000psi I would imagine that with a locked up case most of the load generated would be transferred to the bolt lugs anyway as very little of it would be absorbed by friction between case and chamber.
The modern rifle has lugs designed to withstand stresses far in excess of most anything that could be generated, ( using the correct powder ) by a large factor of safety.
If you were operating with some headspace then I could see where case
/chamber friction could reduce backthrust, but considering the polished finish in the chamber and the smooth case exterior I rather doubt this reduction would be much.

Try it Barry - Lube and try.

I could do that but I have no way of measuring Backthrust with and without lube. I can calculate it.
Either way the lugs will survive if designed correctly.

It was not my intent to debate action design, but in Bruce's last post he made a point of saying that reduced backthrust is better.
Quote " Apparently absolute zero friction there ( case/chamber) can stress locking lugs"
Does it matter? or does this less stressing lead to better accuracy?
Then he states the Ackley cases are closer to parallel -- to reduce backthrust.
And the old H&H cases had a lot of taper - functioned better but had increased backthrust.
From this ( thinking like somebody who does not know better ) I gather that backthrust is a bad thing.
My point is that unless more backthrust is detrimental to accuracy then it is a non issue as modern actions are well and truly overdesigned.

Barry,

The main concern I have with this is brass life, in particular primer pocket expansion. In the past I have ruined brass when cases are wet, particularly when all lube has not been wiped off. At Brisbane Q day 1 I shot in the rain like everyone else, but religiously kept my ammo and breech dry, and the brass was fine. Its not so much the safety of the shooter as the risk to the bank account of replacing the brass !

Alan

The original post was about case issues and whether it was pressure related or other. Barry I have no doubt modern actions are over engineered and will easily handle the pressures we are talking about but still challenge you. Even with crush fit try lubing some cases (some with oil, some with water) and see if you cannot get measurable case head expansion differences (use acetone cleaned and dry cases and chamber as the control). Water seems to be terrible and I wonder whether the water forms a "veneer" between shell and chamber - Something akin to aquaplaning??

I have no reservations that reducing friction increases back thrust, but Ackley's experiment did not ascribe the deleterious effects that we saw the first day of the National Queens to it. Why were those cases popping primers? I assisted one shooter who, if he did not hold the case against the bolt as it cleared the chamber, would find an ejected primer in the loading chute every shot.

Barry makes a valid point by questioning why the coefficient of friction of the case is an issue. We know that it takes a gross overload to free the primer from its pocket normally, so what is the circumstance involved with wet ammunition and/or chambers that mimics that overload condition, or why does a wet chamber & ammo lead to grossly higher ignition pressures?

It can't be just a tad of overpressure involved in this phenomenon. I'm surely not alone in having exceeded a prudent load on occasion & have the ruined cases to prove it, but I have never popped a primer out of the case in 45 years of handloading. Apart from the June Queens, the only time I've viewed primers that dropped out of the case was two occasions when the shooters contaminated their powder resulting in irregular/delayed ignition.

I guess that there is another possibility to consider, viz that some shooters are loading right on the ragged edge of disaster & these conditions are enough to tip them over the edge a mite. Even so, why & what is the condition that is invoked?

and rimmed non bottlenecked cases lengthen in wet, or oily chambers.,
while they don't in dry clean chambers.
this happens at blackpowder pressures (approx. 25000 psi), so what happens at 50,000 + psi?
keep safe,
bruce.

John,

Maybe the standard of finish in chambers has improved since Ackley?

Alan

Alan,

Are you suggesting that the finish of modern chambers is resulting in less grip on the case than those in Ackley's era? I don't think that that is a viable explanation, because on the one hand, manufacturers the likes Remington bore their chambers these days (clamped barrel & spun reamer, rough as guts output) & on the other, all the advice exchanged between gunsmiths on forums that I frequent acknowledge that a high degree of chamber finish is deleterious to functioning.

Perhaps somebody who has loosened primers to the degree that they drop out of the extracted case under normal weather conditions might be able to shed some light on this. The only reference that I could Google up opined that the issue was incompressible water getting in front of the projectile & thus causing extra pressure.

John

John,

As is often the case, there is probably more than one factor at work here. Not everyone at the Qld Queens was popping primers. But I have been keeping things dry for years since experiencing case head expansion in the wet, and from everything that's been said on this thread, I remain convinced that it is because of reduced friction due to the lubricating effect of water on the body of the case.

But whatever it is, the preventive measures are : keep breech and ammo dry, and remove all case lube.

I use what I call a roller towel - it is a long towel with ends sewn together to form a loop. It goes over the scope, over the ammo box, then back underneath the rifle. The loading hand, breech and ammo stay dry.

Alan