Can You Shoot .40 S&W in a 10mm? And Other Dangerous Ammo Swaps
Some calibers look almost identical but aren't interchangeable — and mixing them up can range from "it'll probably work" to "catastrophic kaboom." The internet is full of people who've done it and survived, which makes it dangerously tempting to assume it's fine. This guide covers the most commonly confused caliber combinations, what actually happens when you swap them, and a clear compatibility reference chart.
The Golden Rule
Shoot only the caliber stamped on your barrel. Every other discussion is about what happens when this rule is broken — sometimes safely, sometimes not.
.40 S&W in a 10mm Chamber
Will it chamber? Yes. Same bullet diameter (.400"), same case head diameter. The .40 S&W is literally a shortened 10mm — the case is just 0.142 inches shorter.
Will it fire? Usually. The shorter case headspaces on the extractor instead of the case mouth, which means the round floats forward in the chamber. The firing pin has to travel the extra 0.142" of freebore before striking the primer, which may cause light strikes. If it fires, the case stretches slightly to fill the chamber.
Is it safe? It's not recommended. The case isn't designed to stretch, and primer ruptures are a real risk. Smith & Wesson's Model 610 revolver is specifically designed to fire both using moon clips — that's the exception, not the rule. In a semi-auto, don't do it.
.380 ACP in a 9mm Chamber
Will it chamber? Yes — same bullet diameter (.355"), same case head diameter. The .380 ACP is a shorter, lower-pressure version of 9mm.
Will it fire? It might. Same headspacing problem as .40 in 10mm — the shorter case moves forward, potentially causing light primer strikes.
Is it safe? The pressures involved (.380 operates at ~21,500 PSI vs. 9mm's ~35,000 PSI) mean there's no overpressure risk. But the bullet will travel through extra freebore, potentially losing accuracy and velocity. More importantly, the .380 case may not generate enough pressure to reliably cycle a 9mm pistol's action. Don't do it. There's no benefit, and the malfunction risk in a defensive situation is unacceptable.
.300 Blackout in a 5.56 Rifle — THE MOST DANGEROUS SWAP
This One Can Kill You
.300 Blackout will chamber in a 5.56/.223 rifle and will cause a catastrophic failure when fired. The .300 BLK bullet (.308" diameter) is forced into a .224" bore, generating pressures that can exceed 100,000 PSI — roughly three times the safe operating pressure. The result is a destroyed barrel, shattered upper receiver, and potentially lethal shrapnel directed at the shooter's face and hands.
This is the single most dangerous ammunition mix-up that occurs in the real world, because .300 Blackout uses the same magazines, same bolt, and same lower receiver as 5.56. The rounds look similar at a glance. If you own firearms in both calibers, never store the ammunition together, never load them side by side, and always verify your magazine and chamber before loading. Clearly label all .300 BLK magazines with colored tape, paint, or dedicated magazine markings.
Safe "Swap" Combinations
Some caliber combinations are actually designed to be interchangeable:
| Combination | Safe? | Notes |
|---|---|---|
| .38 Special in .357 Magnum | Yes | By design. .357 Mag was created as a lengthened .38 Special. Same diameter, same case head. Revolvers chambered in .357 Mag safely fire both. |
| .44 Special in .44 Magnum | Yes | Same relationship as .38/.357. The .44 Mag is a lengthened .44 Special. |
| .223 Rem in 5.56 NATO chamber | Yes | Lower-pressure ammo in a chamber designed for higher pressure. Always safe. |
| 20 Gauge in a 12 Gauge | NO — extremely dangerous | A 20 gauge shell can slide into a 12 gauge barrel and lodge partway down. If a 12 gauge shell is then loaded behind it and fired, the barrel obstructs and explodes. |
For more on the 5.56/.223 compatibility question specifically, read our detailed 5.56 vs .223 vs .223 Wylde guide. And for help identifying your caliber in the first place, our new gun owner's guide walks through the process step by step.