Study of Setback for Hornady Critical Defense 9mm Ammunition
Setback of the bullet in a cartridge is a known issue that can affect the firing characteristics of the round. Setback is caused by the impact of the bullet on the feed ramp as the round is chambered. Adverse effects include overpressuring the cartridge when fired. Damage to the weapon is possible when a cartridge produces too much pressure. That is why some gun makers recommend against using +P ammunition in some of their guns. All rounds are chambered at least once before use. So, the ammo maker must take steps to ensure that any setback on the first chambering is small enough to prevent any adverse effect on performance. The question here is whether multiple chamberings lead to a problem.
Self-defense rounds, such as the Hornady Critical Defense 9mm, are considerably more expensive than ordinary range/practice ammunition. So, self-defense rounds are not often used as practice ammunition. This can lead to a situation where a self-defense round may be chambered multiple times before use. A person who carries his weapon concealed will load self-defense rounds for general carry purposes. When he goes to the range for practice, he may unload his weapon and load his practice ammunition instead. After practice, he reloads with self-defense rounds, possibly putting the same round back in the chamber that was there when he arrived. Similarly, when he cleans his weapon, he unloads and then rechambers the round. With frequent practice and cleaning, this round could be rechambered over 100 times per year! Even small amounts of setback could accumulate and ultimately have a major effect on performance.
There are a couple of questions to be answered:
1. How much setback is acceptable?
2. How many times can a round be rechambered before setback becomes unacceptable?
To begin this study, I took a box of Hornady Critical Defense 9mm ammunition and measured the length of each round in the box. The lengths varied from 1.074” to 1.097” with most of the rounds falling between about 1.08” and 1.09”. The following plot shows how the lengths were distributed within the box.
I picked a round close to the middle of the range of lengths (1.086”). The test consisted of chambering the round in my XDm 9mm Compact (3.8” barrel) and immediately ejecting the round. After each chambering, I measured the length of the round. After 10 single chamberings, I speeded up the process by chambering 5 times between measurements. So, I measured after 1, 2, 3, … 9, 10, 15, 20, 25, … 50 chamberings. The results are shown in the graph below.
The length scale on the left is set to match the range of lengths measured for all the rounds in the box. As you can see, after 15-20 chamberings, changes in the length of the round have slowed down a lot. More importantly, after 50 chamberings, the length of the tested round is still longer than the two shortest rounds in the box.
This test does not fully answer either question. Sorry, but partial answers may still be useful.
“How much setback is acceptable?” To answer that would require experiments that determine how much the pressure increases with different amounts of setback. But, by noting the range of lengths in the sample box we can get a partial answer to this question. The range from 1.074” to 1.097” is 0.023”. Most of the rounds in the box could tolerate about half this much setback and still be as long as the shortest round in the box. In my opinion, we could reasonably accept 0.010” setback with little risk on most of the rounds in the box.
“How many times can a round be rechambered before setback becomes unacceptable?” To answer this question we need a good answer to the first question. If we use the idea that 0.010” setback is acceptable, a valid answer to this question would be “More than 50 times”. In fact, the total setback after 50 chamberings was only 0.008” and the length did not change for the last 20 or so.
Different ammunition and different weapons may both produce different results than this. Canebrake has provided a photo showing the effect of a single chambering of a 460 Rowland with “little-to-no crimp”. (See the thread on Chambering same round…). The setback is very visible. The Hornady website description of the Critical Defense ammunition states “Bullets are cannelured and crimped to avoid bullet setback.” Apparently what they are doing to the cartridge does minimize setback. So, different ammo makers and different ammo can produce dramatically different results.
The results of this test suggest that, for the XDm 9mm Compact and the Hornady Critical Defense ammunition, multiple chamberings are probably acceptable. However, large numbers of chamberings do produce some setback. If the round happened to be the short one, we don’t know how much more setback can be tolerated. The results of this test DO NOT indicate anything about any other combination of weapon and ammo.
For me, I plan to take Tackleberry1’s advice and use the chambered round as the first practice round to get a measure of first round proficiency. Based on these results, I’m not going to worry about rechambering once after cleaning the weapon. If I want to unload the gun for dry fire practice, I won’t worry about even 5 or 6 more cycles. Chances are I won’t get over 6 cycles on a round before my next practice session.