Choosing Gear
Reading a Calibre Designation
Calibre names are historical accidents, not measurements. Two cartridges with the same number can be entirely incompatible, and that is the part that matters.
The number is barely a measurement
A calibre designation is marketing and history layered over an approximate bore diameter. .38 Special uses a .357-inch bullet. .380 ACP, 9mm Luger and .38 Super all use .355-inch bullets and none of them interchange. The number tells you roughly how wide the bullet is and nothing at all about whether it fits your chamber.
What actually has to match
The cartridge case: its length, its shoulder, its rim, and the pressure it generates. That is what the chamber is cut for. Two cartridges are interchangeable only when the chamber was designed to accept both, which is why the correct question is never 'is it the same calibre' but 'what is stamped on the barrel'.
5.56 and .223, in the direction that matters
A 5.56 NATO chamber safely fires .223 Remington. A .223 chamber should not be fed 5.56, which runs at higher pressure with a longer throat. The asymmetry catches people constantly because the cartridges look identical and the boxes sit next to each other. The same asymmetry exists between .308 Winchester and 7.62x51 NATO, and between .45-70 loadings for different actions.
Where a shorter cartridge does fit
Revolvers are the friendly exception. A .357 Magnum revolver fires .38 Special, and a .44 Magnum fires .44 Special, because the magnum case is simply longer and headspaces on the same rim. It works in that direction only — .357 Magnum will not chamber in a .38 Special cylinder, and forcing the question is how revolvers get destroyed.
Bullet weight, in grains
The second number on a box. 115, 124 and 147 grain are the common 9mm weights; 55, 62 and 77 the common 5.56 ones. Heavier bullets go slower, hit with more momentum, and need a faster barrel twist to stabilise — which is why a 1:9 twist AR-15 may shoot 55 grain beautifully and scatter 77 grain.
The one habit worth keeping
Read what is stamped on the barrel, then read the box, every time you buy ammunition for a gun you have not fed before. It takes four seconds and it is the only step in this whole subject that reliably prevents damage.
Common questions
Can I shoot .223 in a 5.56 rifle?
Yes. The reverse — 5.56 in a .223-marked chamber — is the one to avoid, because 5.56 runs at higher pressure with a longer throat.
Can a .357 Magnum revolver shoot .38 Special?
Yes. The .38 Special case is shorter and headspaces on the same rim. .357 Magnum will not chamber in a .38 Special cylinder.
Are 9mm and .380 the same?
Same bullet diameter, different cases and pressures. They are not interchangeable.
What does grain mean?
Bullet weight. Heavier bullets travel slower and need a faster barrel twist rate to stabilise properly.