Shooting Illustrated:

High-pressure symptoms are probably the most commonly encountered stop signs our rifles provide. Inspecting spent cases whenever you try new ammo, changing a handload recipe or shooting in drastically different atmospheric conditions can help you spot potential issues. Primers warn us in the form of “blowing” completely out of the brass, cratering (a slightly raised lip around the firing pin indention), flattening out or being pierced by the firing pin. Minor flattening or cratering is not uncommon, especially when firing heavy bullets. Either condition needs to be watched, but neither is cause for alarm so long as it is minor and limited to one ammo type.

But, when multiple factory loads are showing these signs consistently, it is time to have the rifle looked at by a qualified gunsmith or the firearm’s manufacturer. The same goes for blown primers, damage to the case head with semi-automatic rifles or brass flow into a bolt’s ejector-pin hole on any gun: A minor amount is not abnormal for heavy loads, but excessive or consistent damage is cause for concern.

Consistently pierced primers warrant more immediate action. Checking your brass will reveal piercing, but sometimes you get an earlier warning from a blast of gas in your face and the contents of an external magazine littering the ground under your rifle. This is your cue that it is time to stop and find the source of the problem. Piercing can force small discs of primer material into a bolt’s firing-pin hole, eventually causing other problems. A jammed up firing-pin bore can stop pin movement and possibly fix the firing pin in the forward position. That may lead to a “runaway” machine gun, firing every time the bolt slams forward on a fresh round or worse, firing out of battery. A full-auto surprise is bad enough, but a gun that fires out of battery can be catastrophic for the shooter and anyone nearby.

At any sign of primer piercing, stop using the suspect ammunition and ensure the firing pin and bolt are closely inspected for proper function or damage. Just a few pierced primers can erode a firing-pin tip, leading to greater primer-piercing frequency, leading to more firing-pin damage, and so on.

Other harbingers of pressure trouble include bent firing-pin-retaining pins, broken extractor pins, bent or broken extractors, a sudden increase in recoil/muzzle blast/gas without a change in ammo, case-head separation or spent cases that are split or ruptured. In each case, it makes more sense to stop what you are doing and look for the source of the problem(s) than it does to just keep banging away.

Read the rest at Shooting Illustrated.  I would have thought most of this is common sense, but perhaps not.  There are the harder to find issues as well.  I once went shooting where the Range Officer was a gunsmith, and he took interest in the brass my pistol was ejecting.  Close inspection of it, along with me, showed that it was heavily charred on one side, while the rest was clean.  It turned out that my barrel was out of round and had to be replaced.  To this day I thank that man for his attention to detail (if he happens to be reading this).

The point of this is that we all have to be mechanics if we’re going to be good shooters and sportsmen.