Don’t shoot high pressure loads

Handloaders will find high pressures pose no problems if they learn how to recognize the symptoms, both true and false, of pressure build-up.

Correct charge weight of the wrong powder  blew the head off the case and completely wrecked the Model 70’s bolt and receiver. Cartridge cases are not intended to withstand high pressures by themselves. They must be firmly supported by the rifle’s chamber walls and by the breech mechanism. But there are weak points in the system that sometimes allow gases to escape. The most common escape route is around the primer’s edge. This can occur with pistol and shotgun cases, but is most often seen in rifle handloads when the powder charge exceeds the correct level or when the wrong powder is used. With a maximum reload, the primer pocket can expand enough to allow gases to leak around the edge of the primer. The shooter usually isn’t aware that this has happened until he goes to reload the case and notices a black ring of soot around the primer left by leaked gases.

If you look at the bolt face of a firearm that has seen long use with hot reloads, particularly bolt-action rifles chambered for high-intensity cartridges, you’ll notice a ring of shallow pitting around the firing pin hole which matches the diameter of the primer. This is caused by tiny pinpricks of hot gas that cuts away steel just like an acetylene torch.

This is nothing to worry about. It is caused by normal leakage and results from firing all types of ammo, not just handloads. I’ve seen this kind of pitting on new rifles where it was probably caused by the firing of a proof round. Most types of firearms are designed to vent escaping gases safely. Check the bolt of a centrefire bolt-action rifle, and you’ll probably see one or more fairly large holes on the underside where they direct pressure down into the magazine, but some also have a hole in the receiver ring which facilitates the release of gas out to the side.

There are several symptoms that the handloader should be able to recognize being so evident. Cratered primers used to be thought a reliable indicator. If the fired primer looked to be flattened enough to be flush with the edge of its pocket, and the indent of the firing pin was cratered around the edge, that meant it was a hot load. This is still true, but only if you use primer appearance in a comparative sense, in relation to other loads using the same primer and already fired in your rifle. If load A looks normal and load B shows cratering, then B is the hotter load.

But not always. Sloppy firing pin holes, weak firing pin springs and sharp firing pins can cause misleading cratering. Also, primers of different makes and sizes have outer cups of different thickness. My experience is that some standard primers will flatten or crater less than others, while a load has to be really hot to crater a magnum primer. Remington’s 7-1/2 primer was brought out because their original 6-1/2 primer was too thin in the cup to prevent it being pierced in some rifles with certain .222, .222 magnum ,.223 and .17 Rem. loadings. The pellet of compound is identical, it was simply given a thicker cup, to resist cratering. CCI’s #450 small rifle magnum primer also has a thicker cup. So you can believe cratering only in relation to other comparable loads, as you work up.

A serious level of excess pressure is indicated when the solid brass case head expands so that the primer pocket becomes greatly oversize. When this occurs, the expanded case head may wedge so tightly between the chamber and the breeching mechanism that the action will not open normally, and considerable force is necessary to open the action. This gives a clear indication that the strength factor of the case has clearly been exceeded. The shooter may even notice a wisp of smoke issuing from the action.

Anytime extraction becomes difficult and bolt lift gets “sticky” operating pressures are definitely too high. Excessively high pressure with that case, load, bullet and rifle has expanded the brass beyond the limits of its normal elsticity so that it hasn’t shrunk back,
as brass normally does, to release its grip on the chamber walls.

When gas pressure rises to really dangerous levels, the brass case ruptures allowing gas to flood back through the action. Usually the bolt remains locked; although the lugs may crack, they seldom shear off, and the actions built-in gas venting features direct gas down into the magazine, reducing both the amount that flows to the rear and lessening the chance of personal injury. But the magazine box is so violently expanded by gases that the stock splits and the floorplate gets bent like a banana. In a worst case scenario, the locking mechanism lets go and the action blows up. When this happens, the shooter may suffer serious injury. This is why shooting glasses should always be worn. Even at normal pressures, a faulty case or excessive headspace could cause a case to rupture and release a blinding wave of hot gases.

The case head itself can be very revealing. If it is scraped by the ejector slot of a Mauser-type bolt, or if it shows a little circular raised and polished-off section indicating that pressure has pushed metal down into the hole in the bolt face that houses a plunger ejector, or if the case head carries in reverse any marks from your bolt face – you’re loading way too high. Either drop the load back a grain or two or look for another cause of added pressure, such as thickened or overlong case necks, differences in capacities
of mixed brass, differences in bullets.

Next time you fire a factory cartridge examine the case. You’ll notice that the forward section has lost its high polish. This is because that part has been expanded tightly against the chamber walls. Somewhere between a quarter and a half-inch up from the case head (it varies according to load and rifle) you’ll see a line below which the brass retains its original shine. To your fingernail that line may even feel to have a very slight bulge. Now if after several neck-sizings the same case is fired with really hot loads that line will be pushed steadily downward, back toward the head, and your eye and fingernail will clearly spot the bulge at that line. Why?

Cartridge brass is drawn to tapering thickness with walls thinner at neck, heavier toward the case head. Since higher and higher pressures will expand heavier and heavier brass, they push back the expansion point closer to the solid head section. The greater the expansion the farther back the line; the bigger the bulge and the tighter the case fits in the chamber. That causes extraction to become stiffer. Ultimately, of course, if pressure is pushed high enough by a serious overload, the entire head of the case can be over-expanded, the primer pocket is blown out and we may have a blown-up gun. Observation of that expansion line in cases that are neck-sized only, (as is carried out by a majority of reloaders) will give a comparative indication as to pressure build up.

Stiff extraction may also happen when cases are over- stretched lengthwise, though usually preliminary signs should already have appeared at the head end. Brass overlong to the shoulder is difficult to bolt in or out of the chamber. When only neck-sized, this can happen after as few as four or five reloads. Full-length resizing will fix the problem at least temporarily; after only two or three times tired cases should be junked and replaced with a new batch.

Case-head separations, usually associated with excess headspace can also occur at the expansion line area as the result of too much full-length resizing of cases fired at high pressures. The die squeezes the thick part of the case body down and then pressure blows it out to chamber dimension. As in flexing a piece of wire, eventually this makes the brass brittle at this flex point.

As I have written on numerous occasions, the surest way for the handloader to be warned of dangerous pressure is by use of a micrometer.

A perefctly adequate one can be bought quite cheaply. A skilled machinist may read it to tenths; the average reloader needs only to be able to read to thousandths or half thousandths. Anybody can do that.

The head of the case, be it belted, rimless or rimmed is of course the strongest part, since it forms the basic seal between high-powered gas and the shooter. Regardless of whether an individual make or lot of brass is thick-walled or thin, heavy or light in the web or floor of the cavity that is pierced by the flash hole; regardless of the presence or absence of the magnum belt, so over-touted as a strengthener, the head dimension will reveal risky pressure growth.

If you keep a once-fired factory case as a sample, and if you observe that in subsequent reloadings of that batch of brass you work up to a load that significantly enlarges the outside diameter of that case head (taken across the belt or the case wall just above extraction groove or rim) by .001” or more, you’re loading too hot. Don’t bother to mike the rim – it is seldom uniformly round, especially after a few extractions. Makes no difference what handbooks say or whether your brass is hard or soft, new or old, if such comparative mikings show that much enlargement you’re straining the combination. By how many pounds per quare inch? Who cares? You’ve crossed the red line. Back off
or take the consequences.

Every symptomatic red flag mentioned here appears only when operating pressures are already too high for regular use. The red flags won’t tell you the actual pressures. But every symptom mentioned here is comparative in that it applies to progressive loadings in your own rifle with your own components. Take your time when reloading and watch what you are doing yourself – and wear shooting glasses on the range at least, or better still, every time you shoot your rifle.

This article was first published in Sporting Shooter, July 2011.




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Nick Harvey

The late Nick Harvey (1931-2024) was one of the world's most experienced and knowledgeable gun writers, a true legend of the business. He wrote about firearms and hunting for about 70 years, published many books and uncounted articles, and travelled the world to hunt and shoot. His reloading manuals are highly sought after, and his knowledge of the subject was unmatched. He was Sporting Shooter's Technical Editor for almost 50 years. His work lives on here as part of his legacy to us all.