Pressure Signs vs Pressure

[steve4102]

Steve, if you're thinking of using this information to "push the limits", there's a much more reliable, safer (relatively) and fairly inexpensive way to do it than eyeballs and micrometers.

No not at all. What prompted this question was a discussion I had with a fellow reloader about "pressure signs".

He is loading for his 223 and loading it hot, very hot. He tried to tell me that he is still within the MAP of 55K because there were no "pressure signs". He is convinced that "pressure signs" will show up as soon as chamber pressures exceed SAAMI MAP, in the 223 that would be just over 55K.

With my limited knowledge of chamber pressure I tried to tell him that the chamber and the brass do not know what SAAMI MAP is and that most pressure signs do not appear until well above 65K no matter the cartridge. But, I could not convince him as I have no real proof of this.

Thanks

[Brian Pfleuger]

Ah.

Well, you're absolutely right and he is dangerously wrong.

If he gave more than a cursory glance to the informational section of most any load manual he would know better.

[mehavey]

...when the firing pin smacks the primer, that drives the case forward hard into the chamber shoulder centering the round very well in the chamber up front. There's no "rattle space with either one; both are held very repeatable in centering up front.

I'd be in your camp (at least thoretically) were it not for one nasty piece of empirical experience back in `85.

I'd gotten/been given my father-in-law's captured Type 38/6.5Jap that year, and immediately gotten some Norma (¿best-there-is?) brass to load up. Bang! went the rifle; thwock! went the bullet in the cardboard backer; and Wow! went the shooter when he looked at the case's lower half when ejecting.

"bulge" is incorrect spelling.
"BULGE" is the corrected term.

As in many a physics experiment, sometimes the subtitles get lost in fine-grained data -- so you increase the differences until you get obvious 'boundary value' results. In this case the oversize Jap chamber/undersized Norma case was obviously pushed to one side of the walls by the extractor even after firing pin strike. Even moderate pressure gas expanded to fill (chamber) space available ...and the rest is history.

Bottom Line 1: I finally went with 243Win cases to fill the chamber and things began to behave themselves.
Bottom Line 2: Extractors -- especially big hulking Beautiful Mauser claws -- will definitively push (and keep) overly-sized cases to one side of the chamber during ignition.

[Clark]

CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.

steve4102
What prompted this question was a discussion I had with a fellow reloader about "pressure signs".
He is loading for his 223 and loading it hot, very hot. He tried to tell me that he is still within the MAP of 55K because there were no "pressure signs". He is convinced that "pressure signs" will show up as soon as chamber pressures exceed SAAMI MAP, in the 223 that would be just over 55K.
With my limited knowledge of chamber pressure I tried to tell him that the chamber and the brass do not know what SAAMI MAP is and that most pressure signs do not appear until well above 65K no matter the cartridge. But, I could not convince him as I have no real proof of this.

1) Accurate pressure measurement matters if one is going to sell ammunition.
2) Accurate pressure measurement matters if a gun is weaker than the brass.
3) For me, considering myself an advanced reloader, with a strong rifle [huge amounts of safety margin due to thick steel], knowing the pressure is red herring. What matters is the effects of pressure.
a) If the weakest link is the primer piercing, the load needs to be backed off from the threshold of primer piercing.
b) If the weakest link is getting loose primer pockets, the load needs to be backed off from the threshold of loose primer pockets.
c) If the weakest link is stiff bolt lift, the load needs to be backed off from the threshold of stiff bolt lift.
d) If the weakest link is the brass flowing into the bolt face in the extractor or ejector cuts, the load needs to be backed off from that threshold.
e) If the weakest link is a non adjustable gas system and the bolt carrier is slamming into the receiver and the brass is ejected a long way, the load needs to be backed off from that threshold.
f) If the weakest link is a recoil operated system that the bolt carrier slams into the receiver, the load needs to be backed off from that threshold.
g) etc.
It has been my experience with Ruger #1 .223s the the weakest link is loose primer pockets.
One can put in another primer to see if the pocket is loose, but I have found a better way. I measure the extractor groove before and after firing. I measure all the way around. In a load work up, the change in extractor groove is easily measured precisely enough with dial calipers. Any change at all may not make the next primer seem any easier to insert, but the threshold is very close. Hoping to see a primer fall out as a pressure sign in a work up is more crude than feeling insertion force.



pic left to right: unfired, 28, 29, 30, and 31 gr.
Hodgdon website: 223 Rem, H335, 25.3 GR. 55 GR. SPR SP, 2.200", 24"barrel, 3203 fps, 49,300 CUP
Test: Ruger #1, CCI 400 small rifle primers, LC brass once firedprocessed from Scharch and prepped by me, 55 gr Vmax moly, H335 unfired, extractor groove .329"
28 gr, extractor groove .329", 11% overload 69 kpsi [QL]
29 gr, extractor groove .329", 15% overload 80kpsi [QL]
30 gr, extractor groove .3295", 19% overload 92kpsi [QL]
31 gr, extractor groove .3320", 23% overload 106kpsi [QL]

What does it all mean?
I predict that the threshold of loose primer pockets is somewhere between 29 and 30 gr. I would do another more detailed work up and find the threshold more accurately. I would then subtract a safety margin for temperature variations in that powder, my reloading process variations, component variation, etc.
Vernon Speer said in 1956 that he backs off 6% Powder charge from brass change. I have found i can use a 4% margin for Hodgdon extreme powders.