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Discussion Starter · #1 ·
There is a considerable amount of misunderstanding about reading pressure signs, particularly in an auto pistol, so perhaps a review would be helpful.

The most important thing to remember is that pressure signs are relative. You cannot judge pressure by looking at a fired case and its primer without a baseline for comparison.

For instance, a "flattened" primer can only be read compared to the same case and primer fired in the same gun with a known max load.

Many things affect the "flatness" of a fired primer. The hardness of the primer is one most people have heard about. Less understood is headspace. If the headspace in your pistol is slightly on the long side, the primer can back out of the case slightly on firing, and then be reseated as the case moves back to the breech face when pressure builds. This will result in a very flat primer with normal pressure. Chamber roughness can affect the degree to which a primer can back out before reseating, as can chamber lubrication. So can a short case, which, in effect, increases headspace.

Primer extrusion into the firing pin hole may indicate high pressure, but it can occur with normal pressure if the hole is slightly larger than the firing pin or the hole or firing pin are slightly out of round. And the amount of extrusion is relative. There is often some degree of extrusion.

Breech face marks on the primer may indicate high pressure, but may indicate a rough breech face at normal pressure. Breech face marking is considerably increased with long headspace and may be affected by chamber lubrication. And the degree of marking on the primer is relative to start with. There is always some.

In a revolver, stiff extraction may indicate high pressure, or it may indicate a slightly rough cylinder while pressure is normal. Conversely, an extremely smooth cylinder (chamber) can eject cases easily all the way up to catastrophic pressure.

If you reach seriously high pressure -- unlikely in a .45 with proper loading procedures -- there are obvious signs such as the primer dropping out of a fired case or a bulge in the case head where the case has flowed into the feed ramp. Short of such gross indications, everything is relative.

The most common cause of pressure problems in auto pistols is the bullet being forced back into the case, or seated deeper, during feeding into the chamber. This can result in extreme pressures, and must be guarded against. A tighter crimp will not solve the problem, and may in fact actually loosen the case's grip on the bullet. Expander plug diameter is the key here. The expander plug or button must be smaller than bullet diameter by at least .002". My Dillon .45 ACP expander plug ring had to be reduced to .448" before case tension was adequate.

Since few of us have pressure testing equipment, learning to read pressure signs is important. The best way to start is to closely inspect cases and primers from factory ammunition fired in your pistol. (Federal 230 Hydra-Shok seems to be close to maximum, and I suspect Cor-Bon is, too.) Then compare those to reloads fired ONLY IN THE SAME PISTOL. This will give you a starting point, recognizing that your reloads use different primers and may vary some. From there, it is mainly experience that will guide you.

Several of the better reloading manuals have pictures that may help in learning to read pressure. They are highly recommended.

Stay safe.

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The commonly discussed pressure indicators can be deceptive and you shouldn't rely on them exclusively. It is not unusual for the first excessive pressure indication to be a case head seperation which is bad news to the gun and possiblly the shooter. Be sure all your reloading equipment is in good repair, clean and working properly. Do not exceed maximum loads as listed in the reloading manuals. Inspect your cases to be sure they are within specs (you will need a good instrument to measure tolerances). Discard any suspect cases that do not meet specs and/or show signs of excessive wear or corrosion. Be wary of "range" cases as you never know the history of them or what they were shot in or why they were discardid in the first place. When reloading be sure to verify that you are using the correct bullet, powder charge, case, etc. It is very easy for a page to accidentally turn in your reloading manual. You may be looking at 10mm data instead of 40 S&W for example. Keep good records as to what components you have tried and write up results to refer back to later. Above all, develope good reloading safety procedures and do not deviate from them.
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