Best caliber for home defence: the overpenetration question first

Start with the uncomfortable fact, because it decides everything else. No round is safe through a wall. Two sheets of half-inch drywall stop nothing that is worth keeping in a house: pistol FMJ, pistol hollow point, rifle ball, buckshot and slug all pass through several interior walls in testing, and most of them still have enough energy left to hurt someone on the other side. There is no caliber that solves that. There is only a caliber you can shoot well, loaded with a bullet designed to dump energy into what you aimed at rather than through it.

For most people that is 9mm with a quality hollow point. It is not the strongest round on this list — it is the one with the best combination of controllable recoil, magazine capacity, affordable practise and proven terminal performance, and those are the factors that survive contact with the actual event.

What actually exits drywall

Interior walls in American houses are usually two sheets of gypsum board over a stud cavity, and they are close to irrelevant as a barrier. What testing shows, repeatedly:

• Pistol FMJ (+9mm, .45 ACP ball) punches straight through multiple walls with its full profile intact. It is the least likely load to deform, which is exactly why it keeps travelling.
• Pistol hollow points often yaw, fragment slightly or partially expand after hitting the first wall — a meaningful reduction in energy, and nowhere near a stop.
• 5.56 rifle ball tends to yaw and shed velocity faster than pistol calibers do once it is in the wall cavity, and it still exits. It is the best of a bad set on this metric, not a safe one.
• 00 buckshot and slugs go through, with nine pellets or a one-ounce bullet to account for afterwards.

What the FBI protocol rewards

The standard the industry tests against is the FBI protocol: penetration of 12 to 18 inches in calibrated ballistic gelatin, through clothing and intermediate barriers, with reliable expansion. That window exists because a bullet that stops in six inches has not reached anything worth reaching, and one that travels forty inches has carried its energy past the target.

Two practical readings. First, a good defensive hollow point is designed to dump energy quickly — which is the only mechanism available for making a round behave better after it misses a wall. Second, the protocol assumes a torso; it says nothing about drywall. Do not read “FBI approved” as “safe indoors”.

Pistol, rifle, or shotgun

A shotgun deserves its own row and it has one: it is a separate conversation, and the load you pick for it changes the arithmetic more than the gauge does.

The .45 is in the table because people ask. It throws a heavier, slower bullet that has less tendency to overpenetrate an unarmored body than pistol ball does — and it comes with two-thirds the capacity, more expensive practise, and a recoil impulse that a great many shooters handle worse. “Bigger hole” is not an argument that has survived contact with the gelatin data.

The training argument

Capacity and terminal performance are decided at the factory. What you contribute is shot placement, and shot placement is bought with practise rounds. At $0.27 a round against $0.43 a round, the same range budget buys roughly half as much .45 — and a shooter who practises twice as often with a gun they control completely is the one who puts rounds where they belong. This is why the pistol-caliber answer keeps being 9mm, and why the reasoning in the 9mm buying guide puts practise and carry on separate lines of the receipt.

The rifle case is the mirror image. A 5.56 carbine is easy to shoot well, holds thirty rounds, and — with a light, fast load rather than heavy hunting bullets — is often the most controllable gun in the safe. Its costs are different: it is louder indoors, its flash is considerable, and a defensive rifle in a hallway has to be handled by everyone in the house who might reach it.

The decision rule

1. If you shoot one gun well and practise with it often, that is the gun. In pistol caliber it will almost certainly be 9mm with a bonded or barrier-tested hollow point.
2. If you are choosing from nothing, a 9mm pistol with a weapon light, or a 5.56 carbine you have trained with, are the two answers with the most evidence behind them.
3. Regardless of choice: 100 to 200 rounds of the defensive load through the actual gun, confirmation of point of impact, and a backstop plan for every wall you might shoot toward. Then buy practising ammunition by the case — here is when that is worth it.

Three guns will be suggested to you this year, and every one of them will be argued for on the basis of what it does to a target. Judge them instead on what you can do with it, and on what is behind the wall you cannot see.