Flattened primers are not necessarily an indication of high pressure. Let me recount my experience, some of it is considered heretical by the shooting community.
When I started shooting NRA Across the Course Highpower, the M1a ruled the firing line. Most competitors loaded their brass five times and tossed it, to avoid case head separations. No wanted a malfunction at a match because malfunctions ruin your score.
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However one shooter, a Distinguished High Master, who incidentally is a great gunsmith, he lubed his 308 Win cases and ran them all season. HM sized his cases with RCBS water soluble lube and then trimmed, primed, charged, and seated the bullet without removing the case lube. His cartridges felt slightly greasy. However that tiny amount of retained case lube was enough to prevent sidewall stretch. HM never had case head separations.
I copied his technic, but I did not like the greasy feel, so I experimented with many lubricants. The best for matches, was rubbing Johnson Paste Wax on the loaded cartridges. The cases were nice and clean, I tumbled them before lubing and then sized and trimmed them. After trimming I washed off the lube in hot soapy water, and then dried the rinsed cases in a toaster oven at low. Only after sizing, priming, dumping the charge and seating the bullet, did I apply Johnson Paste Wax. Why I like Paste wax was because it was dry and not sticky. Dirt did not adhere, I could drop the case on the ground, wipe it, and it was good to go. My fingers were not greasy from case lube at the range. Rubbing paste wax on did take longer than just leaving the case lube on, but that is something I did from match ammunition. Most of the time. Sometimes I did not have enough time between matches and ran cases with the case lube still on.
One of the first things I noticed was, with dry cases in dry chambers, the primers were always flat. However, once I lubed the cases, the primers were rounded. Same everything else. Humm. Something going on.
A dry case adheres to the chamber, and then the sidewalls have to stretch to when the case head moves to the bolt face. Paste wax melts and turns into a lubricant under the temperatures and pressures of combustion. So the cases were sliding to the breech face. That was what prevented sidewall stretch.
R stands for number of times reloaded. All these cases experienced case neck splits or a body split. These failed cases where then sectioned to see if there was a stretch mark.
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So why were my lubricated cases not showing flattened primers? This is the clue:
Sako fired brass
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Sometimes shooters encounter fired cases with the primer sticking out. This phenomena happens under a couple of conditions. The first condition is good adhesion between the case and chamber. The case is locked to the walls of the chamber during combustion. And, the internal pressures in the case are insufficient to stretch the case sidewalls. But the primer backs out to the bolt face. The end result are these high primers.
I figured out that the primer backs out of the pocket during ignition. If there is clearance between the case head and bolt face, the primer will back out to the bolt face and push the case forward. And then, if pressures are high enough, and case adherence great enough, the sidewalls stretch, and the primer gets stuffed back into the primer pocket. However, the primer has slightly expanded with pressure, so it does not go in rounded. It will look flat.
Since then, I regularly develop loads with lubricated cases. One reason, I want to see the transition from rounded primers to flat primers. I am usually a grain to a couple of grains above a “max load” when I see flat primers. But this is not 100%. Also, I fireform new cases lubricated, so I don’t experience any sidewall stretch.
Such as this. These are 300 H&H cases, at the time about $2.00 each. Probably more now. The base to shoulder distance is not controlled in the chambers of belted magnum rifles, and I did not want to experience case head separations due to excessive clearance between the shoulder of the case, and the shoulder of the chamber. So I lubed the cases on first firing.
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Shot well too.
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When I lube a case, after firing I have a perfectly fire formed, stress free case. When I resize, I just bump the shoulder back 0.003”.
Many in the shooting community consider the practice of firing lubricated cases heretical. It is a very long story why they believe this, but they were taught this. It is one of the longest and most successful coverups in shooting history, over a century old. And I have written extensively about this, and am not interested in repeating it now.But it starts with the Army, low number Springfields and cupronickel bullets.
With regular loads and lubricated cases, my primers come out rounded. Unless I have some horribly hot load with lubricated cases. Which has happened during load development, and even afterwards when I thought I was under a maximum load. I do not trust primer appearances to show anything unless the cases are fired lubricated. And then primer roundness or flatness is still an unreliable indication of pressure.
The only pressure indications I trust are blown, leaking, or pierced primers. Some of these primers fell out when the case head was turned down, or tapped on a table.
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Primer cupping may or may not be a pressure indication, because some manufacturer’s have made bolt faces with extra large primer holes. And those always cup the firing pin.
If your primers can be pushed into the pocket with your thumb, after a couple of firings, the load is way the heck too hot. That is an absolutely reliable indication of excessive pressures.
A sticking bolt is a positive indication of over pressures. I have seen the bolt handles broken off Rem M700 when reloaders use blocks of wood, or their boots, to open the bolt.
This would a positive indication of high pressures
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And regardless of the data in a reloading manual, if you ever experience real indications of over pressure conditions cut your loads. I have read a lot of denial on this and have made posters unhappy when I state this and I have received angry replies. Confirmation bias can be an ugly thing. Manuals are just a guide. Powders vary by lot, chambers vary by gun, and it is very possible to get over pressure conditions under the maximum loads shown in manuals. Overpressure means too much powder in the case, so cut the loads.