Effective Firearms Stopping Power Calculator — Smartphone App

By the time I started my law enforcement career in 1980 with the Licking County Ohio Sheriff’s Office, I already had a healthy interest in guns and shooting, ignited by my father and fueled by many hours spent on the shooting range at Boy Scout Camp. That healthy interest exploded when I became a deputy sheriff, and my biggest law enforcement goal was to become a police firearms instructor.

At the time, discussions of “stopping power” of various handguns reigned supreme, especially when one talked about defensive handgun calibers. “Stopping power” is generally understood as the ability of a particular cartridge to stop its target (man or beast) with one well-placed shot to the center mass of the body. It can be thought of — and is sometimes expressed as — a percentage of chance. Thus, the higher the numerical rating, the more effective the load is thought to be.

At the Licking County Sheriff’s Office, the late Colonel Jeff Cooper’s opinions on stopping power were the predominant ones that held sway in this debate. The colonel’s rock-solid belief in the superiority of the Colt 1911 semi-automatic pistol — and the much-vaunted stopping power of its .45 ACP cartridge — led to Licking County being one of the very first law enforcement agencies in Ohio to adopt it as their primary duty weapon.

The debate on stopping power is nowhere near today what it had been then, especially since the FBI declared that the 9mm, .40 and .45ACP are all equivalent in that category. Colonel Cooper has to be turning over in his grave — rapidly and multiple times — at that proclamation. (If you are new to defensive firearms and have not heard of the late Colonel Cooper, many of his works are still in print and are worth reading. He truly was a pioneer in the field of handgunning, developing the “Modern Technique” of defensive handgun and rifle use.) Despite what the FBI claims, the law of physics can’t be altered, and there is, in fact, a difference in the effectiveness of cartridges with different ballistics on animate targets. If this wasn’t true, moose and bear hunters could drop their .45-70 caliber rifles in favor of the .22 LR (after all, it is much easier on the shoulder, both in terms of carrying and shooting).

I recently ran across an app for my smartphone called “Effective Firearms Stopping Power Calculator.” The EF Calculator is extremely easy to use and automatically expresses stopping power based on four different evaluation formulas: the Taylor Knock Out Factor, the Thorniley Stopping Power Formula, the Hatcher Relative Stopping Power Formula and the Hatcher RSP Expanded.

The Taylor Knockout Factor was based on observations of John “Pondoro” Taylor, a big-game hunter, ivory poacher and guide in Africa who is reputed to have killed some 1000 elephants over his career. Taylor clearly had the opportunity to observe the effects of various cartridges on various forms of big-game animals during his career. The term “Knock Out” referred to the ability of a cartridge to “stun” a large animal like an elephant with a strike to its head, even if the bullet failed to reach a portion of the animal’s brain and cause instant death. While KO capability could mean the difference between life and death for a big-game hunter in close quarters, it is not the same as stopping power as it is currently understood. However, this does not mean that the Taylor KO factor isn’t valuable. It is simply a method of measuring physics that Taylor used to correlate and validate what he observed in the field. The formula used is expressed thusly:

Taylor KO Factor = (Mass of Bullet × Velocity of Bullet × Diameter of Bullet) / (7000 (Conversion of Grains to Pounds))

Fortunately for us all, the Effective Firearms Calculator does all this for us; no need to break out paper, pencil and calculator. And while the Taylor Factor doesn’t include bullet shape as part of the equation, the Hatcher Formula does.

The Hatcher Relative Stopping Power Formula (RSP) was developed by Major General Julian Hatcher of the U.S. Army Ordnance Training Center. His formula is most pertinent to handgun ammunition, as it is the only one that factors in bullet shape and type, which is certainly an important consideration when talking about stopping power.

Because Hatcher includes bullet type and profile in his calculation, the EF Stopping Power Calculator allows you to select from 11 different bullet types. Momentum of the bullet is also factored in:

RSP = M ÷ (2 × G) × A × F

M = Momentum of bullet in foot pounds per second. Momentum is calculated as Mass × Velocity
G = Acceleration due to gravity in fps²
A = Frontal area of bullet in square inches
F = Bullet form factor based on a list of bullet types by Hatcher

Peter Thorniley was another big-game hunter in North America and Africa who developed a calculation now known as the Thorniley Stopping Power Factor. The Thorniley Formula has nothing to do with stunning large game with head shots; rather, it considers stopping power in terms of well-placed body shots. The Thorniley Formula reads like this:

Thorniley Stopping Power = 2.866 (Constant) × Velocity × (Mass of Bullet ÷ 7000) × √ Diameter

If you are like me, your head is starting to hurt. My college algebra class was a long time ago, so I am so glad that there is the Effective Firearms Stopping Power Calculator. The EF Calculator also features an “expanded” Hatcher computation that adds in values for JHP and soft-point loads. Let me show you an example of how this all works out using the traditional 9mm vs .45 ACP debate (and SIG Sauer V-Crown JHP ammunition).

Formula 9mm V-Crown 124-gr. @ 1165 feet per second .45 ACP V-Crown 230-gr. @ 830 feet per second
Taylor KO 7 12
Thorniley Stopping Power 35 53
Hatcher RSP 35 75
Hatcher RSP Expanded 43 92

There is simply no comparison to the power imparted by the 9mm versus the .45 ACP. And guess what? If you calculate the values for a 165-grain .40 at 1090 feet per second, it falls right between the 9mm and .45.

Now I realize that there is so much more to factor in when talking about shooting dangerous game or people — muscle density, organs and structure damage, bullet expansion performance, adrenaline, drugs and mental instability, to name a few. All these factors will certainly contribute to the outcome of a gunfight.

What it all boils down to is this: Carry the caliber that shoots bullets with the most weight, diameter and velocity that you can consistently put on target. That will give you the best advantage when it comes to the elusive quality of “stopping power.” The Effective Firearms Stopping Power Calculator can assist you in that selection.