Thursday, December 23, 2021

Setback and KABOOMS!


Out of curiosity, I loaded 124 grain, Rocky Mountain Reloading hollow-points into two un-primed 9mm Luger cases. I fiddled with the C.O.A.L. (Cartridge Over All Length) until it seem to chamber reliably. That O.A.L. was 1.11 inches.

I zeroed the scale with the loaded/empty/unprimed round on it. I inserted a 14 gauge needle and syringed in water until I saw water in the primer pocket. I did not take heroic steps to eliminate the bubble that may have been trapped at the base of the round. Then I reweighed the water-filled round.

The difference in weight for each example was 0.47 grams which suggests a powder volume of 0.47 cubic centimeters. 

A little bit of math suggests that the swept-volume of the base of the 9mm bullet is 0.636 cc for every centimeter of travel.

Converting to inches and percentages: starting with that bullet at 1.11" C.O.A.L. then every 0.010 inches change in C.O.A.L. changes powder volume by 3.8%

Less powder volume means significantly more pressure for a given amount of powder.

For the purposes of visualization, four sheets of ordinary "copy" paper adds up to about 0.010"

In a similar way, a LONGER bullet (projectile) reduces powder volume by the same percentage versus a shorter bullet with the same C.O.A.L.

If you look at some loading manuals, you will see C.O.A.Ls. for 9mm Luger loaded with various 124gr and 125gr bullets that range from 1.060" to 1.169". 

Assuming the same length bullet (projectile), a 1.060" C.O.A.L. offers 17% less volume than the 1.110" C.O.A.L. while the 1.169" C.O.A.L. (The S.A.A.M.I. maximum) offers 20% more volume.

All other things being equal, loading longer C.O.A.L. offers some breathing room in the event of bullet setback, shooting ammo that is thermally "hot" (perhaps the case was sitting in the sun), if supply constraints force you to use a primer with more brisance or should you encounter a slightly faster burning lot of a cannister powder.

"All other things being equal..." means that your bullet is the correct size for the firearm (don't laugh) and the bullet is not already engaging the rifling in the barrel, that is, the bullet has some amount of free-travel before it encounters the resistance of being engraved by the rifling.

As with all things on the internet, this if offered for entertainment purposes. Pack your own parachute. Collect your own data. Do your own math. It is your own fingers and eyeballs that are at risk.

The failure followed the bottom of the "E", "C", "S" and "O" in the manufacturer's stamp. Unfortunately, those features aligned with the thinnest part of the barrel around the chamber.

Aside: This combination of overloading and a less-than-optimal location for a manufacturing stamp is not unique.

Back when I was a wee-sprout in engineering school, I remember seeing a textbook showing an airplane propeller that experienced the same failure. All of the testing was done with an un-stamped propeller. It sailed through the testing with no failures. It went into production WITH the stamp and a plane lost a blade while in-flight.

I have no knowledge about this particular product (the barrel). They might have tested it with the manufacture's stamp.

I included the photo to show that casual disregard for details like C.O.A.L. put you at the mercy of variations in heat-treat and steel chemistry, exposure to hydrogen, location of stamp marks and even how "sharp" (or the age of) the actual stamps used on your barrel.

This KABOOM! might have never happened if the bottoms of the stamp-marks were more rounded (i.e. older stamps). It is unwise to place yourself in a position where microscopic details like the geometry at the bottom of a stamp-mark might decide whether you go home with all of your fingers. 


  1. I used to reload quite a bit when I was young, 40 - 50 years ago, but quit because I didn't have time. Ammo was cheaper than time. The past couple of years I have been thinking about getting back into it but as I read a lot of articles like this I think I'll pass on doing it again. Too much risk for the benefit.---ken

    1. .38 Special and .357 Mag are much friendlier rounds to reload.

      The bullets have grooves to crimp into. 9mm Luger headspaces on the rim of the case so you cannot crimp it too much.

      Also, the .38 Special was designed for black powder and has gobs of spaces so small changes in seating depth don't impact it to the degree it impacts 9mm Luger.

      Finally, bullet setback is a risk in semi-auto pistols because the slide impacts the round in the magazine and the nose of the projectile hits the ramp on the barrel. That can cause the projectile to move back in the case.

      Revolvers are more likely to have the bullet walk forward due to recoil. The failure mode is to have the cylinder not be able to rotate which is embarrassing but not far less kinetic than a KABOOM!

  2. Also, how many rounds did the barrel have on it? They are usually designed for a couple to several thousand rounds; beyond that, behavior is unpredictable.

    1. Even though the typical handgun might only six rounds go through it in its lifetime there are many thousands of service handguns that exceed 25,000 rounds in their lifetime.

      I base that on chatter on a Glock forum where armorers were discussing which Glock components are most likely to fail and when they will start failing.

      MOST police don't come anywhere near 25,000 rounds through their weapon but that is not an uncommon number for a cop on the squad shooting team when the Chief is buying the ammo. 1000 rounds a month is not out-of-line for that kind of shooter.

      The rule-of-thumb for fatigue failures in the steeper part of the curve for steel (less than a million cycles) was that the life would double for every 15% reduction in strain. For fatigue failures over a million cycles life would double for every 10% reduction.

      Different grades of steel and different heat-treatments have the knee between steep and flat at different numbers of cycles. There is data but obviously every grade and heat-treat is not available.

      I do not know how many rounds were on the barrel.

  3. Spot on. Few years back, I made a purchase decision based on similar observations (of online photos). I was looking for an 18 inch AR barrel from a particular maker. They offered one in stainless steel that was fluted, their sales blurb pitching it as their most aggressively fluted barrel. I was tempted, but the flutes appeared to end very close to the chamber, which made me nervous. I opted for the non-fluted version. The company shall remain nameless, but to be fair, I have no reason to suspect there has ever been an issue with any of their barrels. The accuracy of the barrel I did buy is superb.

    There are also revolvers out there with the notches for the cylinder bolt cut into the thinnest part of the chamber wall.

  4. ERJ - Thanks. I understood about 10% of it, but it is a good piece of knowledge to have access to.

  5. Thanks for detailing why I don't reload!

  6. Joe, based on your detailed description of the considerations, I would assume that was not your kaboom, correct?

    1. Not my KABOOM!

      Just a picture I grabbed off the internet

  7. Number of possibilities..... the powder charge was increased inadvertently, there was an inherent weakness in the metal, the case was weak and ruptured. Without more data and subjecting the firearm to competent scrutiny the cause can only be guessed at. However the shooting sports are not without risk....including harm caused by the rare but not unheard of KABOOM.

    1. I sort of wrote-off case rupture. The chamber pressure drops like a rock when the brass lets go. If the brass blew-out then I cannot visualize the barrel fracturing in that way.

      Yep, absolutely a lack of details for that particular KABOOM! Placing the stamped manufacturer's mark in that location is just plain stupid.

  8. Rental Glocks ran hundreds of thousands of rounds, with nothing more than recoil spring and magazine replacements, at ranges in the 80's and 90's. When there is a catastrophic failure, it's unusual and unlikely. My 1991 g19 is still my EDC.

  9. Interesting post. COAL is an issue, but there is also a significant lack of background on the actual failure. # of rounds, cleanliness of the weapon, etc. Often, especially with the Glock 40s, the failures have been due to bullet set back due to constant recycling of the first round in the mag. That significantly raises the SAMMI pressure and the oddity of the Glock design is that the rear of the barrel is, for all practical purposes, unsupported.

  10. As a general rule don't have sharp internal corners, or even unneccessarily small corner radii in brittle or sensitive-to-stress-fatigue stuff.
    And don't add extra stresses by distortion post manufacture.
    Super ore carriers, pressurised aircraft, right down to some silly plastic bit in a toy will suddenly fail. Broken toy not so bad, the others........

  11. @Old NFO:

    Yes, bullet setback is a problem that you never forget once you have seen it in action.

    Many moons ago, a relatively new acquaintance, my DW, another buddy and I went to the range to do some casual shooting and to allow one another the chance 'show and try' some of each other's various weapons. The acquaintance was having some failure to feed issues with his class 3 H&K MP5 related to some of the aftermarket magazines he was using. By itself, that would have been a simple annoyance, but this guy was also in the process of learning how to load 9mm on his newly automated RL1050, and this batch of ammo had somewhat loose case necks that had been over-expanded, allowing the projectiles to be pushed a significant distance into the case. I commented on this but since this guy was a very experienced shooter with more training than I, I stopped short of saying- "You really ought not shoot this thing any more today!"

    I will spare you the details, but eventually he touched off a round which probably had the bullet pushed back so far that the tip was likely close to the case mouth. I was impressed at the engineering of the H&K, which was clearly designed to protect the shooter against just such extremely high pressure rounds, which pressure welded the case to the remains of the bolt face, which we did recover. We figured that the pressure was probably in excess of 120,000 psi.

    Bullet movement after loading is never a good thing....I've also had revolvers tied up by bullets jumping forward under recoil.


Readers who are willing to comment make this a better blog. Civil dialog is a valuable thing.