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Bullet Setback Fears

The Truth Behind Bullet Setback Fears

For most of us, bullet setback occurs when the nose of a cartridge strikes the barrel’s feed ramp, or another internal part, while the round is chambered. The primary culprit is when the same round is rechambered multiple times, as you load and unload your firearm. This is mostly a problem with pistols, especially a CCW that’s cleared every day, but it occurs in rifles, as well.

Slight projectile setback is common, and a few thousandths of an inch is nothing to worry about. It’s when the bullet recedes into the case by a hundredth of an inch or more that it can cause problems in semi-auto pistol rounds, such as 380 ACP, 9mm, 40 S&W, 357 Sig, 45 ACP, etc.

This is potentially dangerous because it reduces the cartridge’s internal case volume, producing higher than normal pressure when the round is fired. Depending upon the round in question, this pressure spike can exceed the established safety limits of the cartridge — especially if the load itself is already pushing the limits. Sometimes, this situation can exceed the ability of the host firearm to safely contain the pressure of the fired cartridge.

Many handguns are tested at the factory with intentionally overpowered ammunition, called proof rounds. In most instances, this is performed with double- or triple-charged loads. The test isn’t so much to protect shooters from an errant over-pressure hand-load, but to replicate a dangerous case of over pressure due to bullet setback. Not every manufacturer performs this test, nor does every pistol pass it.

rifle bullet setback

Setback is most often associated with pistol ammunition, but it’s been known to occur in rifle ammunition as well.

Why Is It Dangerous?

Most semi-auto pistol rounds are loaded with medium-to-fast burning powders because the cases are so short. These powders build pressure quickly when they burn. So, if there are only a few grains of fast-burning powder, there must be some space to provide a buffer for the pressure wave. The less airspace there is, the a greater risk of a pressure spike beyond safe levels.

Typical 9mm ammunition is a relatively high-pressure round, and the pressure can rise significantly if the bullet is seated a few hundredths of an inch deeper than normal.

With less internal volume in the case, high pressures develop before the forces created by the burning powder begin to move the bullet. In the 1990s, setback-pressurized rounds caused a near epidemic of firearms blowing up. The rear of the case separated, the polymer frame came apart, and the magazine blew out. Most of the firearms were chambered in 9mm and 40 S&W.

These aren’t exactly Magnum-powered handgun rounds, but small changes in case volume can make a big difference in small rounds like these.

Late firearms writer and one of the founders of IDPA, Walter Rauch, reported in a 2004 Police & Security News article that bullet setback caused by repeatedly chambering the same round could raise pressures exponentially.

It was confirmed to Rauch via an Austrian cartridge manufacturer, Hirtenberger, and a Glock representative that 0.10 inch of setback can cause pressures to double from 35,000 psi to 70,000 psi in a 40 S&W case. In addition, the Speer cartridge company reported as early as 1979 in their annual Reloading Manual that the chamber pressure of a 9mm round increased by 55 percent (28,000 CUP to 62,000 CUP) when the bullet was setback by 0.03 inch. These higher pressures are more in line with what’s generated by a 7.62 NATO cartridge.

Is It the Gun?

Poorly manufactured pistols, or pistols with improper feed ramp angles or poorly fitted barrels, are the primary culprits when it comes to serious and potentially dangerous mechanical bullet setback. Likewise, a bad magazine follower or spring contributes to the bullet hitting the wrong spot during the loading sequence.

The best way to test for a setback issue in a pistol is to measure the length of a cartridge after feeding it using several different magazines and see if setback appears. If it doesn’t, the magazine is probably not the problem. If the problem persists, try the ammunition in a different firearm.

I experienced bullet setback with 185-grain 45 ACP Winchester Silvertips and 180-grain .45ACP semi-wadcutters back in the early 1990s with a compact 1911. The solution was to replace the factory barrel with a Bar-Sto and have it fitted and polished by a gunsmith. The problem went away.


Bad Habits Hurt

Bullet setback can occur with the best firearms and the best ammunition on the market. Sometimes, it’s caused by human error while loading and unloading the same firearm repeatedly. Aaron Cowan of Sage Dynamics performed a test a few years back where he induced setback by press checking his pistol 500 times. The overall length of his 9mm round went from 28.82 mm to 27.87 mm — a difference of 0.95 mm. Or for those of us whose country went to the moon, 0.037 inch. That’s just from press checking. Admittedly a lot of it.

The suggestion here is to take it easy on press checks. As a rule, avoid rechambering the same round multiple times to limit setback-causing conditions. If you’re heading to the range, consider shooting a magazine of carry ammunition as part of your practice regimen. Then, reload your EDC with fresh carry ammunition.

If your situation requires loading and unloading the same pistol daily, there are ways to avoid bullet setback. First, rotate your carry ammunition on a regular basis, and place the first round at the bottom of an empty magazine to ensure every round takes an equal beating. If you notice setback occurring, put that ammunition aside.

Constant rechambering creates other issues besides bullet setback. An abused primer will break down and go “click” instead of “bang,” for instance, especially with guns with free-floating firing pins or strikers. The Gwinnett County Police Department in Georgia noted this back in 2011 when an officer’s sidearm failed to fire during a shooting incident. The first round was repeatedly chambered, and the primer became contaminated — whether by lubricants entering the case due to setback or the primer taking a beating from repeated chambering; the cause is still debated.

Hornady critical duty

Hornady’s Critical Duty FlexLock defensive ammo features a cannelured bullet and crimped case to prevent bullet setback.

What the Hell Else Could It Be?

If the rounds in question experience bullet setback through your entire range of firearms in that caliber, and even your buddy’s firearms, then the fault may lie in the ammunition. Factory ammunition has come a long way over the past few decades, but occasionally even the best ammo will have a bad lot slip through quality control procedures. If the ammunition in question was newly manufactured and exhibits the same behavior in everything you try, let the manufacturer know immediately.

One way that ammunition manufacturers keep bullet setback from happening is by using a powder charge that completely fills the case when the projectile is seated. When loading like this, the powder acts as a physical stop to prevent setback.

Oftentimes, the bullet will have a cannelure that’s visible as a segmented ring where the bullet is crimped in the case mouth. When you see this cannelure start to get smaller by moving rearward into the case, you’re going through bullet setback.

Some ammunition marketed as defensive ammo has an external cannelure midway down the case. This is formed into the brass approximately level with the backend of the seated bullet; it’s there as a deliberate stop to prevent bullet setback. This extra feature is something you may see on more costly defensive rounds.

Testing for Bullet Setback

Measure the overall length of a few rounds of your ammunition to make sure it’s in spec and will function in the firearm. Take a few of your freshly loaded rounds and hold them by the brass. Push them nose first into the wooden face of your work bench, or set them on a level working surface where nothing hard and sharp can strike the primer, and push down on the bullet. Measure the rounds a second time — if the measurement is shorter than the first, your ammunition itself is prone to setback.

As we mentioned earlier, a few thousandths of an inch of bullet setback is normal and shouldn’t cause an issue, but if the bullet moves back 0.01 inch or more, it’s a problem waiting to happen. In Cowan’s press check experiment, the round moved 0.037 inch and would be a candidate to cause catastrophic failure.

bullet pile 9mm

If you’re a reloader and your ammunition is exhibiting bullet setback, you may have a wrinkle in your handloading process, poor equipment, or perhaps the wrong components. First, check your projectiles. Using a pair of micrometers, measure the diameter of your bullets and ensure they’re not undersized. You shouldn’t see this with most jacketed ammunition, but it can, and has, happened before.

Usually we’ve seen this problem with plated bullets and fully leaded bullets. With lead bullets, it usually comes from the sizing die being worn out at the manufacturer level. Plated bullets can have the same problem if the lead or other core material wasn’t sized properly prior to the plating process, or the bullets didn’t go through a correct sizing process after they were plated.

If the bullets are the problem, see if you can return them. If you have no recourse on that front, you should probably scrap them altogether. Should the problem not lie with the bullets, it could be with your equipment. So, the next thing you want to check is that your reloading dies are in spec.

Loose Rounds
Setbacks happens. When it happens to your ammunition it can potentially ruin your day. If you notice it as a problem, take immediate steps to address it. A catastrophic failure can destroy your pistol and cause injury to the shooter.

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4 responses to “Bullet Setback Fears”

  1. Loyd says:

    I have reloaded ammo for over 45 years and anyone who loads their own ammo, 1 keep up to date reloading manuals 2 weight your powder every 5 rounds 3 buy only excellent bullets check bullet weights, check overall shell length with bullet installed.

  2. Sam says:

    No doubt bullet setback issues are real, however I think many articles like this generally assume that all gun owners allow their slides to slam the same round into chamber. Isn’t the risk greatly mitigated if one gently eases the round into chamber instead?

    • techs says:

      Setback may be reduced by gently easing the slide forward, but I think it increases the possibility of pistol ending up not fully in battery and/or not fully engaging the extractor — just substituting potential malfunctions. Pistols are designed to function properly with the full force of springs and slide in forward motion. Ammo is not really intended to withstand multiple chambering, although it probably will.

      I unload my EDC only for practice with reloads or a laser cartridge — 2-3 times weekly for live or dry fire, and I use dedicated practice magazines. I don’t unload the duty magazine, so each session just exchanges the top two rounds. Monthly, I measure cartridge length and, if bullets are set back appreciably (couple hundredths), I mark the cartridge base with a red Sharpie and move it to the bottom of the magazine. When the red fills the magazine, they all head to the range on practice day.

      My subjective sense is that’s enough to avoid compressed charge/overpressure and feed problems. It doesn’t evaluate primers breaking down from the battering. If I ever had a single round under that routine fail to function or show unusual pressure signs, I would develop a new plan.

  3. RugerSturmer says:

    In a 50-round box of Armscor 9mm 115 gr FMJ, six of the cartridges came set back about 1 mm.

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