You don't have a pressure issue. You have case to chamber adhesion creating the flat primers.
I assume you sized the case around 0.003" less than the chamber. That's what I do, full length resize, bump the shoulder 0.003" from chamber. On ignition, the primer backs out of the primer pocket and pushes the case forward. If the cases are clean, the chamber is clean, the front of the case adheres to the chamber. But the primer is still backed out. Then as pressures build, the case sidewalls stretch, the case head touched the bolt face, and the primer is stuffed into the pocket, and because it was stuffed into the pocket when pressures are around 27,000 psia (according to Prof Boatwright, that is when 243 case sidewalls stretch) the primer is quite flat. So you see that flat primer and think you have pressures problems. You do not.
I regularly lubricate my new cases on their first firing. On firing, the case slides to the bolt face, the case shoulders fold out to fill the chamber, the case is stress free and perfectly fireformed.
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And, primers are nice and round with below max loads. I in fact lube during load development, looking for the transition from rounded to flat. Which only tells me, I am above a maximum load, probably a grain above maximum load. I learned to lube my cases from a Distinguished HM gunsmith who took his cases all season in his M1a. The rule of thumb at the time was to shoot LC cases five times and toss them due to case head separations.
these FAL cases show the type of case head separations you get with dry cases in dry chambers. FAL's are brutal on cases. I recommend to FAL shooters, lube those cases!
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those other cases, I fired them in my M1a coated with Johnson paste wax. R stands for number of reloads, and if you notice, 23 reloads, 23 firings, no case head stretching, no internal ring, no sidewall stretch out of a M1a. Beat that. And, once I started lubing them, the primers went from flat to rounded. Obviously pressures had not changed within the case, so, something else was going on.
So, if the primers spook, take a tiny amount of case lube, and rub it on the case shoulders and neck. Or the whole case body. I have been using vasoline or hair gels, which are perfumed vasoline with thickeners. A thin amount is all you need. I used a lot of industrial greases and decided since fingers get in the mouth or eyes, I might as well use human rated greases. For match ammunition I spent the night before rubbing Johnson paste wax over the whole case, and then polishing the rapid fire rounds smooth. Slow fire, globs and swirls did not matter. But I had bolt over rides in cold weather during rapid fire stages with globby cases. Once I polished the case, no problemo what so ever. I shot out several M1a barrels and one M1 barrel with lubricated cases.
As a blast from the past, Pedersen used a ceresin wax mixture on his 276 Pedersen ammunition. The Pedersen rifle was a retarded blowback, and needed lubrication to function.
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Future General Hatcher well knew the need for lubricated cases, this is from 1933, and most of this is in his 1947 Hatcher's Notebook.
Army Ordnance Magazine, March-April 1933
Automatic Firearms, Mechanical Principles used in the various types, by J. S. Hatcher. Chief Smalls Arms Division Washington DC.
Retarded Blow-back Mechanism………………………..
There is one queer thing, however, that is common to almost all blow-back and retarded blow-back guns, and that is that there is a tendency to rupture the cartridges unless they are lubricated. This is because the moment the explosion occurs the thin front end of the cartridge case swells up from the internal pressure and tightly grips the walls of the chamber. Cartridge cases are made with a strong solid brass head a thick wall near the rear end, but the wall tapers in thickness until the front end is quiet thin so that it will expand under pressure of the explosion and seal the chamber against the escape of gas to the rear. When the gun is fired the thin front section expands as intended and tightly grips the walls of the chamber, while the thick rear portion does not expand enough to produce serious friction. The same pressure that operates to expand the walls of the case laterally, also pushes back with the force of fifty thousand pounds to the square inch on the head of the cartridge, and the whole cartridge being made of elastic brass stretches to the rear and , in effect, give the breech block a sharp blow with starts it backward. The front end of the cartridge being tightly held by the friction against the walls of the chamber, and the rear end being free to move back in this manner under the internal pressure, either one of two things will happen. In the first case, the breech block and the head of the cartridge may continue to move back, tearing the cartridge in two and leaving the front end tightly stuck in the chamber; or, if the breech block is sufficiently retarded so that it does not allow a very violent backward motion, the result may simply be that the breech block moves back a short distance and the jerk of the extractor on the cartridge case stops it, and the gun will not operate.
However this difficultly can be overcome entirely by lubricating the cartridges in some way. In the Schwarzlose machine gun there is a little pump installed in the mechanism which squirts a single drop of oil into the chamber each time the breech block goes back. In the Thompson Auto-rifle there are oil-soaked pads in the magazine which contains the cartridges. In the Pedersen semiautomatic rifle the lubrication is taken care of by coating the cartridges with a light film of wax.
Melvin Johnson knew too
Army Ordnance Oct 1936 What Price Automatic?, by Melvin M. Johnson, Jr.
Several methods have been devised to retard the unlocking of the block or bolt mechanically. The most appealing point in such a system is consolidation of the “automatic” parts in the breech. However, there is one serious difficulty. The conventional cartridge case does not lend itself to such a system unless adequate lubrication is provided, such as grease or wax or oil on the cases or in the chamber. Thus, the Schwarzlose machine gun has an automatic oil pump: the caliber 30 Thompson rifle (not the caliber 45 T.S.-M.G.) had oil pad in the magazine, and special “wax” was needed on the cases designed to be used in the Pedersen rifle.
There were several "standard" loads in the M1a when it ruled the firing line in NRA Highpower. The most common was 40.5 to 41.5 grs IMR 4895 with a 168 SMK. The second was 42.5 grs IMR 4064 with a 168 SMK. At the time the CCI #34's were not on the market, but in time, everyone heard about the slamfires and out of battery slamfires that Federal's caused, and the nickel plated WLR primer was considered the least sensitive primer, and that was commonly used. Now, the only real appropriate primer to use in a M1a or M1 is the CCI #34 because it is the least sensitive primer on the market. This is for those who may be looking to duplicate the match loads of the past in gas guns. Don't use sensitive primers, such as the brass colored WLR or Federals!
