Originally Posted by boomtube
You're probably overly FL sizing your cases and setting the shoulder too far back. That's the major cause of flat, or 'muffin top", primer cups, not pressure.
I strongly disagree with this premise. Here's why.
A normal, max, safe charge in a rimless bottleneck case whose head-to-shoulder datuim distance (otherwise called case headspace) is several thousandths short will do the following when fired:
1. Upon primer impact by the firing pin, the case gets driven hard forward into the chamber shoulder and typically this sets the case shoulder backi a couple thousandths.
2. As pressure builds up the case expands in all directions and the bullet gets started out of the case neck. The thinnist part of the case body, the front part behind the shoulder, expands hard against the chamber wall first. And at the same time, the case body stretches back from pressure. And it sucks the shoulder and neck back as it moves back.
3. With pressure increasing, more of the case body presses against the chamber wall and contact works its way back on the case body. And the primer gets pushed out of its pocket a few thousandths.
4. At some point the case stretches back until the head stops against the bolt face. This is as far as it goes. And this pushes the primer back into its pocket. This happens even with case headspace several thousandths shorter than chamber headspace; the case just stretches more. If case headspace is too short, head separation starts as cracks start in front of the extractor groove at the pressure ring.
5. As the bullet goes down and leaves, pressure drops and ends up a zero. The case shrinks down a bit but its head to mouth length is now shorter.
If too little powder is used, the primer will not get pushed back into its pocket.
Proof of this can be seen by loading 10 rounds of rimless bottleneck ammo marked with the charge weight. Each has its case headspace at 3 thousandths shorter than chamber headspace. (Use new cases!!!) Starting with a max load then dropping 1 grain for each one, you'll end up with a series of rounds each with one grain less powder. Shoot the full charge firstl. Note how far its primer sticks out. Then fire each one in succession noting primer protrusion from the case head. Somewhere at 10 to 15 percent below max load, the primers will end up being pushed out of their pocket 'cause they ain't pressed back in with higher pressures. Use a case headspace gage to measure fired case headspace and, like I did when I did this test with new .308 Win. cases, cases ending up with their primers pushed out have a shorter case headspace than those whose primers were even with the case head.
Wanna see "muffin top" primers? Start loading over max with new cases and you'll see those appear when pressure gets high. Note those cases are not too short headspace wise.