Pressure Spike!

When A Cartridge Goes Off, You Only Want
Gas going In The Assigned Direction.

By Dave Anderson

Shooters like to speculate about which rifles have the strongest action. A better question is, how well does the action protect the shooter in the event a case fails and releases high-pressure powder gases? And what can we do to prevent gases getting loose?

Rifle blowups are rare enough it is hard to find any significant statistical data. Most of the information is anecdotal. From what I’ve read over the years, I’d say the most common cause of a blowup is a barrel obstruction, such as a bullet lodged in the bore, a forgotten cleaning patch, or mud, snow, or rain encountered in the field. Next most common is chambering and firing the wrong cartridge, such as a .270 Win cartridge in a 7mm Rem Mag chamber.

The most important component link in the chain, and the weakest, is the cartridge case. The case is not just a container to keep the other components—primer, powder, and bullet—in a handy, protective package.

The cartridge case seals the chamber so powder gases can’t escape, except at the muzzle. The brass in cartridge cases is thicker and harder in the case head area, and progressively thinner and “springier” toward the case mouth. When the shot is fired and gas pressure builds, the thinner, softer part of the case expands until it grips the chamber walls, forming a gas-proof seal.

Once the bullet exits and pressure drops, the elasticity of the brass causes it to contract. It doesn’t return all the way to original size, but does contract enough to release its grip on the chamber walls and allow the case to be extracted.

The quality, strength, and durability of cartridge cases have improved tremendously since the smokeless powder era began late in the 19th century. In those early days manufacturers had a lot to learn about cartridge case manufacture. Hunting stories from the era often had references to cartridge problems.

This 1950’s era Brno ZG-47 is built on a Mauser 98-style action.
A large flange on the cocking piece covers the left lug raceway
in the receiver to deflect escaping gas away from the shooter.
The Brno’s bolt (below) has two gas ports on the bottom of the
bolt so escaping gases coming down the firing pin channel are
vented down into the magazine well.

W.D.M. Bell, who did a lot of shooting with a variety of rifles, wrote, “… although I have used almost every kind of rifle, the only one which never let me down was a .276 with German (DWM) ammunition. I never had one single hangfire even. Not a stuck case, nor a split one, nor a blowback, nor a miss-fire. All of these I had with other rifles.”

The cartridge case alone is not strong enough to contain the pressures of modern loads. A cartridge case is a thin-walled pressure vessel contained within a much stronger thick-walled pressure vessel, the steel of the rifle’s chamber.

Ideally, the chamber would enclose the entire case. In practical terms this isn’t always possible since we also want a convenient means of extracting the fired case. With bolt actions using the Mauser 98-style large external extractor, a portion of the case head is unsupported.

They work because the unsupported part is the thickest, strongest portion of the case. The original Mauser 98 design has a flat breech and the case is supported almost to the extraction groove. Some of the Mauser descendants such as the pre-’64 Winchester 70 used a coned breech for smoother, more reliable cartridge feeding leaving a bit more of the case head unsupported.

In designs of this kind modern cartridge brass can handle pressures to around 90 to 95,000 psi according to most sources. No factory ammunition approaches such pressures, and about the only way they can be exceeded is if the bore is obstructed, or a cartridge is reloaded with the wrong powder.

When the Mauser 98 was designed, with ammunition not nearly as reliable as it is today, there was a very real possibility of gas getting loose. The design goal was ideally to contain the pressure, but if it did get loose, to manage the escaping gas so as to protect the shooter as much as possible.

Gases escaping on the right side are mostly exhausted on the open right side of the receiver. Any gases continuing further on the right into the right-side lug raceway of the receiver bridge are blocked and deflected by the bolt handle.

On the left-side receiver raceway, the thumb slot (on military actions) exhausts some of the gases to the left. A heavy flange on the bolt’s cocking piece covering the left raceway blocks and deflects gases away from the shooter’s face. Relief holes in the bottom front of the bolt exhaust gases coming down the firing pin channel down into the magazine well.

The current Ruger 77 incorporates many Mauser 98 features. A few years ago a friend was shooting his Ruger 77 in .22-250. Among his handloads was one with no powder. There was enough power in the primer to drive the bullet a few inches down the bore. Thinking it was a misfire he worked the bolt and fired a full-power round into the obstructed bore.

The effect, he said, was roughly equivalent to all the nuclear weapons in the world simultaneously detonating just as the sun explodes. The floorplate was blown open, the wooden stock split around the magazine box; the extractor blew straight up and buckled the tube of the scope.

The point is, the rifle performed exactly as its designers intended. The lugs held, the escaping gases were managed and diverted away from the shooter. A bit of gas and powder residue struck his shooting glasses, enough he was very grateful to be wearing them.

Another approach is to design the action so little gas can escape. This is the approach used by the very popular Remington 700. As its “three rings of steel” ads note, the bolt face is recessed to fully enclose the case head. The extractor is a spring clip in the bolt face recess, with no extractor cuts required.

The Ruger 77 uses a flange on the bolt, similar to the Mauser 98
design, to block the left locking lug raceway and protect the
shooter from escaping gas in the event of a case failure. Viewed
through the magazine well (below), if a case failure occurs and
high-pressure gases escape through the firing pin channel, they
are vented through these ports down into the magazine well and
away from the shooter.

Bolt designs intended to fully enclose the case with steel so it can
withstand pressures higher than the cartridge brass alone include (from left)
the Remington 700 with internal spring clip extractor and the Savage 110 with
sliding wedge extractor in right locking lug. The Savage also has a baffle
behind the locking lugs, which block the lug raceways in the receiver. The
Weatherby Mk V’s 6-lug bolt uses a hook-type extractor, and the full diameter
bolt requires only a round hole in the receiver so there are no locking lug
raceways. The Ruger American uses a sliding wedge extractor and also uses
a full-diameter bolt with no locking lug raceways needed in the receiver.

The post-’64 push-feed Winchester 70 also had a recessed bolt face and used a sliding wedge extractor built into the right locking lug. The Savage 110 originally had a spring-clip extractor similar to the Remington 700, but adopted the sliding wedge extractor about 1966. The 110 design also has a set of baffles behind the locking lugs as an additional block to escaping gas. The current trend seems to be a recessed bolt face with one small cut for a spring-loaded, hook-type extractor.

With these designs the case is fully (or almost fully) supported by steel and can handle higher pressures. If gas does get loose, say due to a bore obstruction, generally only a small amount of gas can escape down the firing pin tunnel or the extractor cut. Some gas escapes through relief holes in the bolt and receiver ring. If the bore is obstructed it isn’t unusual to see the barrel bulged or even split by the pressure.

It is possible for a problem to result from factory error; for example, improper heat treatment of rifle components, or a bad batch of ammunition. Such events are rare simply because manufacturers take great pains to prevent them.

With those exceptions virtually every other aspect is under your control. Here are some things I do to reduce risk of overpressure loads:

1) Check to see the bore is free of obstructions before shooting. If hunting, after visually checking, tape the muzzle with electrical tape to keep water, snow, mud and wasps out of the bore. 2) Keep the chamber, and the cartridges, clean and dry. Oil on cartridges or chamber walls makes it more difficult to get a good chamber seal and greatly increases the backthrust on the bolt face. 3) Make absolutely certain you’re using the correct ammunition. 4) If you are a reloader, make absolutely certain you are using the intended powder. Blowups don’t result from going a 1/2-grain over the recommend maximum, they result when you accidentally use the wrong powder. 5) If you reload, treat your cartridge cases as if your life depended on them, for it very well may. 6) Always wear protective glasses when shooting. No exceptions.

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