Flat Primers

[LoboGunLeather]

The individual firearm involved can have some effect. Even at moderate pressures it is possible for the primer to be forced to the rear early in the ignition/combustion cycle, then the cartridge case head is slammed back against the recoil shield (or slide breech face), reseating the primer in the primer pocket with the flattening effect.

The primer cup circumference being peened out into the radius-cut of the primer pocket entrance may support this theory. I can't detect it from the photos, but I suspect that the primer cup, around the firing pin indentation, may show markings from the milled surface of the revolver's recoil shield.

Obviously, firearms with greater clearances are more prone to this than those with closer clearances. Ammunition loaded for military use will usually have the primer crimped in place to reduce or prevent this from happening, particularly in automatic weapons firing from an open bolt (Browning MG, M60, Thompson, M3, and others).

 

[old&slow]

Kind of late to the party. but I thought I'd throw in my two cents.
I had a S&W 929 and noticed I was getting extremely flat primers.
( see attached picture)

9mm, 147gr coated or plated bullets, Federal primers and 3.5 grs. of Bullseye ,,
850 fps average. Not really a barn burning load.

I didn't have flat primers with the same load in my 9mm autos.
And I never noticed the problem with my other moonclip guns.

FWIW,, 14.8 grs of 2400 with a 158 jacketed bullet is a bit warmer than I run my 357's.

 

[steelslaver]

Revolver primers kind of lie because of the nature of the beast. In order for a revolver to revolve there has to be some headspace to allow the cylinder and case head to turn. When fired the case rim is first driven tight against the cylinder, then on actual firing the case is slammed back against the recoil shield (the primer against the firing pin bushing). This movement causes more punishment to the back of the case than something like a bolt action rifle where the headspace can and should be at a minimum and the caming action of the bolt locks the case head tight against the bolt face. Because of tolerances between case heads your going to need some extra headspace in a revolver. Plus a revolver with endshake is going to add to this as the whole cylinder is shoved forward by the firing pin striking the primer then slammed back by the explosion.

Take a bunch of S&W revolvers without recesses to keep it simple. With the cylinders pressed all the way forward stick a .060 stack of feeler gauges between cylinder and recoil shield at firing pin bushing. That is the minimum for case to get into position. Now add thickness to the stack and get your true headspace. The results will vary some gun to gun and the more endshake you have the more its going to be. You could also use a chambered fired cases and thin gauges behind the case head. There has to be some clearance for the case to turn into battery, just like a very tight barrel to muzzle gap binding with a little crud a .001 final headspace is going to be a problem at some point. The actual head space with .000 endshake or cylinder pressed back is held by the center of ratchet. This can actually be stoned down to reduce headspace (but increase endshake which can be shimmed out increasing barrel to cylinder gap)

Point is the necessary larger headspace needed on a revolver causes the case to get slammed into recoilshield/firing pin bushing and that makes looking at primers way different than rifles with their much tighter headspace. Revolvers with max headspace are going to show more primer change than those with minimum at the same pressure. Poor way to read pressure in a revolver. a chronograph on the other hand will tell you way more. Takes pressure to get velocity.

A 357 with moon clips should still headspace on the outside area of the cylinder face. That is how my 45 cold cases headspace in the ones I have cut for moon clips. 45 acps and 9mms in cylinders might still head space on the case mouth, but maybe on the moonclip. Slightly thin clip and a bit of a loose cut on the extractor groove= more headspace, thick clip, tight groove=min headspace

 

[rwsmith]

Maybr I'll check...

Yeah, my old and newer Lyman books says 14.9 but my Sierra 3rd shows 15.3 and my Speer #10 shows 15.9. However, my Hornady 4th Edition lists a max of only 13.5.

Regardless, the signs reported scare me and that Scandium frame is probably not the best platform for particularly hot loads anyhow. A Super Redhawk might be more appropriate.

...my old Sierra book. It probably sez 16.8 gr max.

Let's also remember that ALL GUNS are different.

 

[rwsmith]

It would be worse...

Regardless. I made a rookie mistake and thought I knew everything. I appreciate the heads up and the great information.

Give me some props for being aware enough to stop shooting them and ask questions. Did you all notice I shot very few out of my box. This was first time out with these loads. One of the first things I do is check for pressure signs and bullet jump. I was second guessing myself, but threw the moon clip in. They are flatter on the moon clip.

It would be worse if you make an 'old pro' mistake by thinking you know eveyrthing like I do..

You checked it out. Kudos to you.

 

[Nevada Ed]

I sort of agree..........

Some of those old data books did have loads that make you shutter.

Make me wonder if they did not smoke some of that "Wacky Weed"
while they were doing some of those 1970's test??

 

[S&WIowegan]

I won't try to critique your ammo or gun. I will simply tell you that I shoot S&W Model 627s in competition frequently using moonclips and 38 Short Colt brass from Starline. I load very mild loads using Vihtavouri N310 (3 grs) and Bayou Bullets 160 gr LRNs. Primers are always Federal small pistol. My primers look like yours after firing. I suspect the flatening/dishing of primers is purely a function of the physics of moonclips and frame-based firing pins.

 

[fredj338]

Most likely a headspace issue with the moonclips.