This is a topic with a lot of mythology around it, so here’s what the evidence actually shows:
The concept of “stopping power” with handgun rounds is largely a myth. No common handgun cartridge reliably “stops” a determined attacker instantly. The FBI, military, and trauma surgeons have studied this extensively, and the conclusions are often surprising for many gun owners.
What Actually Stops an Attacker
There are only two reliable mechanisms:
Psychological stop — the attacker decides to stop (most common). This can happen with any caliber, or even a miss.
Physiological stop — the brain/central nervous system (CNS) is deprived of blood/oxygen, or the CNS is directly disrupted. This takes seconds to minutes, even with a perfect shot to the circulatory system — long enough for an adversary to continue an attack.
A CNS hit (brain stem, upper spine) is the only immediate stop, and it’s an extremely small target to target under stress.
What the Research Shows
The FBI’s landmark studies (especially post-1986 Miami shootout) concluded that penetration depth (12–18 inches in ballistic gelatin) and reliable expansion matter far more than raw caliber size.
Caliber differences are smaller than believed. The FBI’s 2014 handgun ammunition study found that 9mm, 40 S&W, and 45 ACP produce similar wounding when using quality modern hollow points. The wound channels are not dramatically different.
Shot placement dominates everything. A well-placed 9mm round outperforms a poorly placed 45 every time. Under stress, most people shoot worse — which is why higher-capacity, lower-recoil guns (9mm) are now preferred by most law enforcement.
Number of hits matters. More rounds fired = more chances to hit vital structures. This is another argument for 9mm over larger, lower-capacity options, assuming good performance by the 9mm ammunition.
Modern Consensus (Law Enforcement & Military)
The FBI switched back to 9mm in 2015 after years of using 40 S&W — citing equivalent terminal performance, less recoil, higher capacity, and lower cost for training.
Most major police departments and the U.S. military use 9mm.
Quality hollow point ammunition (HST, Gold Dot, Critical Duty, Ranger-T) performs similarly across 9mm, 40, and 45 in testing.
What Actually Matters (in order)
- Shot placement — hits on vital structures
- Reliability — a gun that doesn’t malfunction
- Capacity — more rounds available
- Penetration — quality hollow points that reach vital organs (12–18″ gel)
- Your ability to shoot it accurately under stress — which means managing recoil
- Caliber — genuinely the least important variable among common defensive cartridges
Common Myths Debunked
“One-shot stops” — largely anecdotal, often psychological, not a reliable metric
“45 ACP is slow, so it hits harder” — not how terminal ballistics works; kinetic energy and wound channel matter, not momentum alone
“Bigger hole = more damage” — a few mm difference in diameter is clinically insignificant vs. penetration depth and placement
“Magnum rounds are better for defense” — Magnums typically cause overpenetration risk and have more recoil, making them worse for follow-up shots.
Practical Takeaway
Choose a reliable handgun chambered in a common defensive caliber (9mm is the most evidence-backed choice today), use quality hollow point ammunition, and invest heavily in training and practice. That will save your life far more than caliber selection.
Here’s a list of comparison testing for self-defense rounds we’ve conducted at Gun Tests. All the links are in front of the paywall.
9mm Luger Short-Barreled Ammo Shooting for Concealed Carry (February 2026)
Standard-Velocity 38 Special Loads Pony Up for Self Defense (September 2024)
9mm Luger Personal Defense Ammunition Testing (May 2023)
Heavy 38 Special Loads Tested (March 2022)
For Defense: Shooting 45 Colt In Wheelguns and Leverguns (October 2021)
40 S&W Ammunition Test: We Pit Eight Self-Defense Rounds (August 2021)
38 Special Loads: Federal’s Punch Is a Best Buy Bullet (July 2021)
380 ACP Ammunition Testing: Bullet Development Continues (January 2021)
9mm Barrel Lengths Compared In Range Performance Testing (September 2020)
Deep-Penetrating Heavy-Bullet 9mm Loads: Pretty Good Picks, November 2018
