M&S study actual police shooting reports and autopsy reports and cull from these reports all cases where the person shot received only a single gunshot wound, that wound being to the upper torso (center of mass, or "COM"). All other shootings are discarded and are not part of the analysis. This is an important fact, and one that I’ll address at length later.

M&S then take all of these single-COM-hit cases and determine how many of them resulted in a "stop," which they define as a cessation of hostility within a certain time period. That number, represented as a percentage of all the single COM hit cases, is the official "one shot stop percentage" for that round. In cases where less than five shootings have occurred with a particular load, M&S have a statistical prediction model they use instead.

The effect of this should be obvious to anyone who has even a passing familiarity with defensive handgun skills. When you shoot someone who is attacking you, you keep shooting until the person stops attacking you. So the odds are very high that if you only shoot someone once, it is because he stopped attacking you after one shot. In those cases where only one shot is fired and the attacker did not stop, that is likely due to some event which kept you from shooting more than one round (e.g., the Bad Guy took your gun away, or shot you).

Other variables involved like specific shot placement, firearm used, angle of shot, etc., are also ignored by M&S. However, it is these variables which are determining whether the round fired actually stops an attacker.  Some folks have argued that, over a large enough data set, all of these variables will be self-correcting.  The problem is that very few of the loads which M&S discuss have been in a significant number of "acceptable" shootings under the M&S criteria.

Of course, this shouldn’t surprise anyone. In fact, it demonstrates just how biased the numbers are. Once again, you see that people shoot until an attacker is stopped. The numbers are not significantly different because they don’t represent anything about the ammunition.

1. One round fired, COM hit, target dies.
2. One round fired, COM hit, target incapacitated.
3. One round fired, COM hit, target doesn't stop and in fact kills his shooter, but then 2 seconds later hears his ex-wife yelling at him and so surrenders just to get away from her.
4. One round fired, head hit, target stops.
5. Two rounds fired in quick succession, target stops.
6. One round fired, left pinky shot off, target starts crying and surrenders.
7. One round fired, COM hit, target doesn't stop, two more rounds fired, COM hits, target stops.
8. Subject takes 14 rounds in the COM, head, and pelvis, doesn't stop, kills everyone within four square miles.
9. Six rounds fired, 3 COM hits, target doesn't stop.
10. Four rounds fired, 2 COM hits, 1 head hit, 1 pelvis hit, target doesn't stop.

According to the M&S criteria, only the first three (one round fired only, COM hit) meet their criteria and are part of the study. In this case, the Zenwolf round has a 100% OSS (the target surrendered within the M&S time limit in #3, and that's all that matters — unless you’re the deceased person who wasn’t saved by your One Shot Stop). In actuality, only five out of the ten (50%) were stopped AT ALL by the ammo, and only three out of ten (30%) were stopped by the ammo in one shot.

Case #5 is a big question mark: there's no way to know whether the first round alone would have been enough.

Cases #3 and #6 obviously have nothing to do with ammo selection. Nevertheless, one counts towards the study and one doesn't.

Cases #7-10 are all cases where one shot DID NOT STOP the attacker, but these don't count toward the M&S study because more than one hit was taken by the subject.

In particular, cases #8-10 show that even after multiple good hits, this ammo did not stop this particular attacker.

Nevertheless, this round gets a 100% OSS rating.

So it is possible (though by no means certain) that the .357 owes some part of its OSS success to the fact that .357 shooters tend to be more skilled than, say, .32ACP shooters. This could also account for the unusually high OSS rates of many .22lr munitions, since many people who use twenty-twos are accomplished pistol shots.

It has also been suggested that revolver shooters (and therefore revolver calibers and loads) are by necessity more careful with their shot placement, which results in greater likelihood of good stopping hits. Compared to the spray-and-pray so common with high capacity 9mm semiautos, this may be one reason there is a disparity between these calibers.

I would like to state that I’m not suggesting M&S are charlatans, nor do I think they are fools. They explain their methodology, and are then made victims by gun writers, editors, and gun shop commandos who misunderstand what M&S are trying to present.

The M&S OSS system makes perfect sense until you take a long critical look at it. However, I hope that my short discussion, above, has helped to dispel the myth that these numbers represent anything statistically significant about handgun ammunition. The average defensive handgun student desperately wants an objective way to measure his gun and ammunition, so he can know he has The Best. But, as M&S themselves frequently say, it’s all about shot placement … train to hit your target in the vital COM, and the difference between one caliber or another and one load or another are going to mean much less in real life than in any on-line debate.