9mm Carbine vs. Shotgun in an Apartment—Which Doesn't Endanger Neighbors?

Good question, and I'll cut straight: the carbine-is-safer myth is mostly generational gospel. Here's what the ballistic testing actually shows.

Both 9mm carbine rounds and buckshot penetrate drywall and studs significantly. Federal's testing (which they published) shows 9mm defensive ammo through four layers of drywall at typical apartment distances—somewhere in the 8–12-inch range depending on velocity and bullet construction. Buckshot spreads, but each pellet still goes through. Neither platform is a "safe" choice for shared walls; they're just different tradeoffs.

The real advantage of a carbine in an apartment is handling, not penetration. Easier to aim in tight hallways, less muzzle rise, faster follow-ups if you miss. That matters. But your neighbor's risk doesn't change dramatically either way.

Honest take: if this is your actual concern, the platform choice is secondary. The primary issue is *placement of your shot*. A miss or an over-penetration from a miss is what endangers neighbors. That makes training—not gear—your real variable. A carbine might make that training easier to execute, but it won't save you if you're not accurate.

If your wife needs confidence this is safe, you need to be able to explain your shot placement and backstop thinking, not just the ballistic numbers. That's the conversation worth having.

kept.simple is right on the data, but let me break apart what's actually happening in your question—because this is an internet argument that mostly doesn't hold up once you separate gear from the real variable.

**What's the actual concern here?** You're worried about rounds leaving your apartment. That's legitimate. But the carbine-versus-shotgun framing is a proxy for something else: "Will I miss and hit my neighbor?"

**Does platform choice matter for penetration?** Sure, technically. But kept.simple nailed it: both go through walls. The spread of buckshot sounds safer until you realize each pellet still has lethal energy downrange. A 9mm carbine round goes further and faster. Neither is "apartment safe" in any absolute sense.

**What actually changes the math?** Your training level and your shot placement discipline. Full stop. I've seen people with shotguns who understand their home geometry and backstop positioning better than carbine owners who bought the gun because they read it was "easier." Easier to *handle*—yes. Easier to *shoot accurately under stress*—only if you trained it that way.

Here's my honest read on your situation: You've got kids, shared walls, and a wife who needs confidence. That tells me you're not looking for a tactical solution—you're looking for a *responsible* one. That means training before you pick the gun. Take a civilian home-defense course *first*, then choose the platform your instructor thinks you'll execute best under stress. Your wife's confidence comes from seeing you know your home, your backstops, and your limitations—not from a ballistic chart.

Start there.

All three of you are solid on the ballistics and training piece—I'm not going to relitigate that. But I want to push on something nobody's mentioned yet, and it matters with kids in the house.

You're all talking about what goes *through* walls and how to place your shot. Fair. But what about what comes *out* of the muzzle?

A 9mm carbine in an apartment—16 or 18 inches of barrel—is running around 1,100–1,200 fps depending on the load. That's *loud*. We're talking 155–160 dB without hearing protection. A shotgun with buckshot is roughly the same, maybe a decibel or two lower depending on the load.

Here's what I'm actually worried about: I fire a defensive round in my bedroom at 2 a.m., and my kids—who are 40 feet away in their room with the door closed—just got exposed to impulse noise that can cause permanent hearing damage. Not to mention the disorientation in an already terrifying situation.

So the question I haven't seen addressed: Are you planning to have hearing protection staged somewhere accessible? Because if the answer is no—which it probably is for most of us—then the choice between a carbine and shotgun isn't really about what penetrates your neighbor's wall. It's about what you're actually willing to do under stress.

I'm not saying don't buy the carbine. I'm saying if you're going to use it for home defense with kids in the house, you need to think about what happens the instant you pull the trigger. That changes the whole conversation.

Both of you are working from the same real answer—training and shot placement matter most—but I think there's a framing that's getting lost here that might actually help this person decide.

The carbine-versus-shotgun question gets cleaner if you separate two things: *what you're defending* and *what you're defending against*.

In an apartment, you're defending a hallway or a single room. That's maybe 30 feet of distance, tops. At that range, a 9mm carbine and a shotgun with buckshot have nearly identical practical effect *on target*. The carbine's ballistic advantage (velocity, energy retention) doesn't matter much. The shotgun's spread doesn't hurt you—it helps you. Both penetrate walls identically. kept.simple's data on that is solid.

So where does the carbine actually win in your apartment? Handling and follow-up speed in tight geometry. If your bedroom is 10 feet from your kids' room, a 16-inch or 18-inch carbine moves faster in that space than a 18.5-inch shotgun. Easier to present from ready, easier to manage recoil on a second shot if the first doesn't stop the threat, easier to navigate a corner without muzzling your family. That's real. That's not tactical theater.

The shotgun wins on simplicity and redundancy—multiple rounds in flight means a miss is less catastrophic—but loses on handling in confined space.

Honest take: if you're in a tight apartment and you've got the time and interest to train either platform to competence, the carbine is probably your lower-friction choice for *your* environment. But gulfcoast_ops is right that training comes first. Pick the gun after you know how you move in your own space, not before.

Take a course. Try both if the instructor has them. Then decide.