[pokute]

A lot of folks, me included, got comfortable with the notion that the degree to which primers appeared flattened or deformed was an indication of pressure. Specifically that flat primers meant excessive pressure.

The first inkling I had that maybe this wasn't exactly true came when I ran out of the Federal primers that I normally used, and finished a bath of ammo with Winchester primers. After shooting, the W primers were all nice and round, and the F primers were the usual pretty much flat. Hmmm....

Seemed that everybody knew that F primers were very soft compared to W primers, but nobody clued in to the fact that this would affect the appearance of the primer?!

Well, guess what. As a few brave gunwriters have said over the last 80 years, but not emphatically enough, is that primer flatness has nothing at all to do with relative pressure.

How do primers get flattened?

Here's the short sweet life of a (center-fire) primer from ignition to to the end:

• Firing pin strikes primer, producing a centered or nearly centered indentation.
• Primer is driven back against the breech-face riding the firing pin back through whatever head-space and/or cylinder free play exists.
  If the firing pin hole is oversize or counterbored, the primer get's cratered when it hits the breech-face.
• The cartridge is driven back against the breech-face (just as the primer was initially) by the initial charge pressure, forcing the primer back into it's seat. If for some reason this takes a relatively long time, the primer may be partially blown out of it's seat by high charge pressure, resulting in a flattened primer if the primer cup is compliant enough.
  In an auto-loader with a weak spring, the action may begin to open at some point early in the firing cycle, producing a comma shaped firing pin drag mark on the primer. If so, you need a stronger spring.
• If head-space and/or cylinder free play is excessive, some combustion products may be forced out around the edge of the primer, producing a black line around the primer (or in extreme cases - A prime blowout).

The thing to note here is that the final appearance of the primer is determined by a combination of factors that are unique to each gun, and not constant over the lifetime of the gun. As peak pressure increases, and as it occurs earlier due to faster powder, primer deformation will increase, but not in any deterministic way.


This is why anyone who has actually blown a gun up in a controlled way always mentions that the primers looked normal right up until the time the gun blowed up. It doesn't help that they usually experiment with a gun that's already been shot to nearly the point of failure.


Load developers like Brian Pearce use new, tight, proven guns for load development. They are able to detect case sticking when the pressure increases to the point that the cylinder starts to deform, and stop as soon as sticking is observed. Of course, by the time the cases are sticking, the cylinder is already deformed, and can never again be used to detect over-pressure.

 

[pokute]

Back in the 1900's when I lived on top of the mountain, I was an avid varmint hunter. I had an old Ruger M77 in .25-06 that was my favorite long range blaster. When I worked up a load for it, the hotter the load, the tighter the group. I stopped when it consistently cratered the primers. It didn't last too long as I shot the throat out of the barrel pretty soon. Sure was fun, though.

I guess I should have mentioned that super extremely high pressure crazy mountain man loads can send the primer scooting back so danged fast that it doesn't just ride the firing pin back but beats it to the finish line - Producing cratering or a pierced primer.

And of course, bottle necked cartridges can go wrong in a whole lot more ways than a straight-sided pistol cartridge.

Lucky you stopped before getting a pierced primer followed by the whole charge back through the action!

 

[pokute]

Also, during the winter months, I would shoot wax bullets in the basement. I would take .38 Special brass and prime them and then push the case into a block of food grade paraffin wax, the kind used for candies and canning. The I'd load the loaded ammo into my old S&W Model 10 and shoot across the basement. The primer was the only propellant and the ammo was very accurate. But I remember that it flattened the primers very badly. Not sure why, couldn't have been pressure.

I never tried this, but a friend of mine is using this technique to learn aerial shooting. Did you drill out the flash hole?

As for the flattening - You got me. Pretty interesting. I need to try this and check it out!

 

[pokute]

No basements here at all. I did once find a little hole in the lawn behind a building we lived in and when I looked in with a flashlight it turned out to be a tiny little opening into a buried bomb shelter! Pretty strange. Basically the backyard was hollow. 4 inches of dirt and then half of a railroad tanker.

 

[pokute]

I am getting senile. I forgot to mention what led me to start this thread.

I was shooting heavy loads in a 44M Super Blackhawk that I had shimmed to reduce the endplay to about a thousandth. I had a bullet jump crimp, and when I removed the cylinder to clear it, I lost the shim. The primers with the shim in place looked normal. The primers with the 0.002 shim missing were flat and a half. Only takes a couple thou to screw things up.

Pics this evening, if I don't fall asleep as soon as I get home like last night.

 

[pokute]

I should be so lucky.