SLIDE a cartridge into the chamber. Lock the action. Squeeze the trigger. With these motions, all shooters call on a fundamental of physics: pressure.

Powder inside the cartridge case ignites, decomposes and turns into lots of gas, all in a few milliseconds.

The gas pressure surges to about 2500 to 4500 atmospheres and drives the projectile out of the barrel.

As a result, every cartridge case is more than a holder for the powder charge, primer and bullet.

It also has to seal the breech, thus stopping hot gases from escaping to the rear, damaging the firearm and causing nasty injuries.

Many of us are handloaders - that is, we assemble our own live cartridges.

Safety depends on our ability to tell when the gas pressure is becoming excessive, so that we can stop shooting and take steps to reduce the pressure, such as using a lighter powder charge.

Published maximum charges serve as a guide but, in practice, we must use a variety of pressure indicators or warning signs. One is the ease with which fired cases can be extracted.

An often-reloaded brass cartridge case may not contract enough after firing.

Result: the bolt handle becomes hard to lift, whereas the same load in a new case would extract perfectly. It will happen sooner with "hotter" loads and minimum-taper cartridge cases.

Hence this basic safety rule: When extraction difficulties occur early in the case's life, and the firearm is clean and in good order, reduce the powder charge - usually by 5 per cent.

(Experienced handloaders may judge when a smaller reduction, such as 2 per cent, is feasible.)

Non-rotating bolts and some vintage single-shot actions tend to have weak extraction.

But because their action strength is often less than a common turnbolt's, we can't always assume that they can be safely handloaded to a point just short of extraction failure.

Next point. Because breech sealing relies on elasticity within the cartridge case's brass, a load is only safe for as long as the brass stays within its elastic limits.

But how can we tell when those limits are being exceeded? Answer: watch for physical changes in the cartridge case head (the case's rear end).

Any sign of brass flow in this region means the brass is being abused and could fail. In the above photo, the right-hand case, a .270W, is bad news. Five features should be noted:

Brass has flowed into a recess on the bolt, leaving a bump on the case's base. (Such extrusions often look polished, like this one.)

The letters and numbers in the headstamp have become shallower and less distinct. Scars and machining marks on the bolt face are now echoed on the brass.

In addition, the primer pocket has stretched so that new primers fall out, and the base has expanded, too.

To see how much, compare with the left-hand case, which matches the .270s dimensions as they were before brass flow started.