For my readers who don't know what I'm talking about: setback is what occurs when a cartridge is chambered so many times that the bullet actually backs into the casing. (See video below for how this happens.)
Setback is a bad thing because ammunition is (ideally) a finely calibrated explosion, and so by pushing the bullet back into the casing you are reducing the volume in which the propellant ignition occurs. This results in a greater overall pressure in the remaining space and could result in the cartridge walls rupturing, and in a worst-case scenario could result in damage to your gun or even to your hand.
As a point of interest, when I load my pistol I always insert the round into the barrel and then release the slide. I'm not sure if this actually reduces setback (although it should, because it avoids that whole "nose hitting the feed ramp" problem entirely), but I tell myself that it does and that makes me feel better.
How It Happened
This is the curious part: these rounds weren't chambered. Instead, I had put them in my SGM 50-round Drum for use as home defense ammo in my Sub-2000, because A) I had 50 rounds, 2) my Subbie can't handle +P ammo like put in my Glock 26, and III) it seemed like a good place to put them.
However, when I decided to take the drum to the range and unloaded it, I noticed that several of the bullets were severely set back into their casings. Here's an example:
|Left: some 115gr. round nose I bought at Wal-Mart, probably Winchester.|
Center: Speer Gold Dot +P 115gr JHP that I use in my carry pistol.
Right: PMC Bronze 115 JHP.
- The average overall length was 27.4x millimeters. I figured anything above 27.00mm was fine.
- Of the acceptable cartridges, the longest was 27.45 mm and the shortest was 27.30 mm.
- 10 of the 50 were lower than my limit. They were, from highest to lowest, 26.56 mm and 24.24 mm.
- I couldn't eyeball the 26 mm ones, but the 25 and 24 were definitely noticeable.
Let me state for the record that this ammunition was not exposed to moisture or excessive temperature change. It was kept in my bedroom, where the temperature ranges from 69° F in the evening to at most 75° F in the daytime.
I'm not sure if this is just a freak occurrence in manufacturing, or if this is indicative of low quality assurance at PMC, but I will tell you that I'm not certain I can trust this ammunition any more.
The next time I'm at the range, I'm dumping these in the misfire bin.