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.