EVERY week, tens of thousands of Australian shooters assemble live cartridges at home. That is, they handload.

We looked last month at how to tell when the internal gas pressure produced by each shot is getting too high.

If this pressure pushes the cartridge case's brass past its elastic limit, we're in trouble.

The brass will fail to seal the breech and escaping gas will surge to the rear.

Result: anything from gas cutting of the bolt face to an eyeful of super-hot gas and powder residue.

Nasty. If the case ruptures, however, we must add a splintered stock, a wrecked magazine and hand and arm injuries to the list of possibilities.

Now let's turn to a common but often-misunderstood ritual: inspecting fired primers.

Spent primers can't tell us specifics, such as "the pressure peaked at 55,000 psia". Nor can they tell us when the cartridge case itself is overstressed and at risk of failing.

However, they do offer two broad clues to peak pressure levels.

These clues are the extent to which the primer cups have been flattened and the degree of cratering (a raised ridge around the firing pin indent).

The greater the flattening and the more evident the cratering, the higher the pressure - all else being equal.

But there's a catch. Appearances can be deceptive: it is possible for one primer to encounter less pressure than another, yet show more flattening and perhaps more cratering.

Time to study our photo. The rounded primer at the left encountered 49,200 psia (pounds per square inch absolute) in a .17 Mach IV.

But the flattened primer at the right met only 34,400 psia, or 30 per cent less pressure, when fired in a .222-28 Short.

So the difference in the appearance of the primers is the reverse of what many would expect.

How come? A fired primer's appearance is affected by variations in the thickness and hardness of its cup.

Softer or thinner cups tend to show greater flattening than their more robust brethren - and the more flattened Rem 6 primer in our photo had a thinner, softer cup.

Hence the rule: spent primers can only be compared when they are the same brand and type.

Provided we follow this rule, the amount of flattening and cratering can show us when one load produces higher pressure than another.