Originally Posted by david shone
Is it possible to get the same result with very soft primers, rather than the load being over pressure. the reason being.....
"... S&B load is very sedate. 40.5 grains of S060/2.....which is the same powder as AA2015 behind a 167 grain Sierra MK.
. S&B primers are known to be soft.....when CZ tested a couple of hundred CZ 700 rifles they had "light stikes" with Norma Diamond line and no problems with S&B. I have tried CCI BR primers and found the same thing as the Norma Diamond line in my CZ700 - S&B work without a problem, hence my belief S&B primers are much softer.
The difference in primer hardness will show a difference in the point where pressure "signs" begin to appear.
Also Brown Dogs rifle is a AI AW......chamber is set to max headspace.....if it makes a difference.
It makes a difference.
When "we" started to hand load 150 years ago, there were no concerns about pressure - you filled the case to the top with black powder, and stuck a bullet on top.
When we started loading high pressure smokeless loads, where we could get in trouble, the only indication of "pressure" was the primer.
So it has traditionally been the thing we look at to know when we are approaching the limits.
But the reading of primers is a black art, and there are a number of variables that effect the appearance of a primer, even with the same load.
Primers are not a good indicator of pressure, but in most cases its all we have.
For a serious shooter, take a once fired case (from that rifle), and prime it. Then pop the primer with no charge in it and look at it.
You will see the edge at the pin strike is rounded and soft looking.
And the edge at the outside of the primer (the edge of the primer pocket) is rounded.
What you are seeing is the fired primer with no other effecting pressure.
This is your "base line" primer.
Now, when you look at a fired primer with a real load, any difference in appearance is due to pressure of the cartridge.
If you were take a pistol primer which has a very soft metal cup so it will fire with a blow from a 25 auto pocket pistol.
Put it in a 223 case and fire it in a Rem 700 with a 22 pound firing pin spring - it will show pressure signs at a low pressure - somewhere in the 30 to 40 Kpsia range.
In the same rifle, a Fed small rifle primer will typically show pressure in the 55 Kpsia range. The Rem 7-1/2 and the CCI 400/BR4 will show pressure signs in the 60-65 Kpsi range... all with the same spring!
Consider this. If the case pressure is 55 Kpsia (55,000), that the pressure AT
the pin strike is ~165 pounds (not PSI, but real pounds). Pressure on a dome is not the same as pressure on a flat surface, so the 165 pounds is reduced. I have long forgotten my calculus (my teacher said I never learned it ;) ) but for a rough estimate, lets take a figure of 1/2.
So lets say that the pressure pushing back against the firing pin spring is ~82-ish pounds (real pounds).
Now, if the pin has a 22 pound spring, and it 0.055" diameter, the force at the pin tip is 7300 psi (and 22 pounds absolute).
It doesn't take a rocket scientist to see that the pressure in the case that is pushing to get out is higher than the pressure of the pin to keep it in.
This is where the strength of the metal in the primer cup comes in play - it is the spring AND the primer cup that holds the whole thing together. And if you change the strength of the cup, you will change the pressure point that metal starts to move out and show "signs" of pressure. But if you change the strength of the spring, you ALSO will change the pressure point that metal starts to move out and show "signs" of pressure... but the pressure is the same, it's the cup and spring that determine when you see this flow.
If you increase the spring strength, the point that the primers start showing pressure will increase accordingly.
Ideally, the spring and primer cup will be strong enough so that the only sign of pressure you will have is when the case starts to flow into the ejector hole, or extractor cuts... because the only REAL
limit on pressure is the case head strength. But for the beginner, this is working too close to the edge.
David Tubb's advice on loading his match cartridge, the "6mmXC" is:
"I use the ejector marks as my pressure indicator -- when I see a shiny mark on the case head, I automatically back off the charge"
But this only works if everything else is up to peak. Sloppy firing pin holes, and weak springs will show pressure signs at ~35-40 Kpsia.
So here are the variables.
1 - Headspace. If there is space in the chamber, the primer will force the loaded round forwards to the shoulder, and the primer will protrude from the back of the case. When the powder reaches full pressure, the case will be forced back to the bolt face, and the primer (which is now stuck to the sides of the pocket under pressure) will get smushed instead of being reseated. This shows "flattening" at the outside edge, but normal appearance at the pin indentation. This will happen on the first firing of the case - after that, the case will fit the chamber, and this should not be seen again under normal circumstances.
2 - Light pin spring. If the spring is light, from age or other causes, primers will show "normal" cratering at lower pressures than they should. They will also show "shallow pin strikes", pin dents that look normal, but are not as deep as the should be. In the properly set up rifle, the primer should NOT show cratering before the case head shows shiny ejector marks.
3 - Over sized firing pin holes. If the pin hole is over sized, the cases will show cratering at low or normal pressures. These craters will have an odd look - like the top half of a donut. the outside wall of the crater will be rounded, instead of being straight and sharp.
4 - Soft primer cup. When the primer cup is soft, the case will show cratering at a lower than normal pressure. Changing the primer to one with a harder, thicker cup, will make the cratering go away, even thought the case is running the same pressure.
This is where the wives tale about "Brand X
primer gives higher pressure than brand Y
The pressure is the same, but the primer cups are different. You CANNOT change the pressure by changing the primer - there isn't enough compound in the primer to make a difference, or effect the pressure.
The purpose of the firing pin (other than the obvious) is to hold the pressure in the case from coming back out of the case at the primer. If it were not, we could use rebounding firing pins like auto pistols.
If the pin wasn't there, or the spring is too light, the pressure will find it's way out the back of the case through the bolt face, via the pin hole.
For a rifle that is designed for long range target/game shooting, we are working (or should be) at the limits of what is safe, but we are not letting any performance go because of sloppy or careless design or equipment.
So let's put it all together.
A solid action, minimum head space, or a case that has already been fired in that chamber.
A minimum clearance firing pin hole.
A pin spring that is strong enough to hold maximum pressures withOUT
A primer with a cup to hold it all together.
With a rifle set up as this, the first "sigh" you will see is a shiny spot on the case.
I hope this makes sense, cuz I haven't had coffee yet.