The barrel marking isn’t necessarily controlling because some manufacturers (Colt and Bushy, for two) only use barrels chambered in 5.56 NATO even though some of them are stamped .223 Rem. Probably a liability thang since there’s no added risk in firing .223 Rem in a 5.56 NATO chamber, but the opposite isn’t necessarily true. Check the mfgr’s full specs to confirm.
Leade/foyer Barrel/home
They make a gauge to check your barrel’s leade. If you have a .223 leade there is a reaming tool to make it 5.56.
The leade is the non-rifled portion of the chamber where the bullet makes the “jump” into the rifled lands and grooves of the barrel.
The 5.56 has a slightly longer case neck than the .223. Think of the leade as the foyer to your home/barrel. If the cartridge is a 5.56 and its bullet is jammed into the barrel, that isn’t groovy. Too much gas pressure. 5.56 barrels can take more pressure than .223.
Like us freepers, the bullet needs a short period of time to think about making that trip down your barrel and it does that just after ignition in the leade. Shoot enough 5.56 in your .223 and you may start blowing primers.
Just my two cents. Don’t hurt me.
Another question
Can 22 LR be used in a 22 magnum gun?
And still not a single pic of ammo or anything that fires the calibers in question. Smh. Slipping.
The only risk for using 5.56mm in a firearm chambered and marked “.223 Remington” is in vintage firearms typically manufactured around the time before 5.56x45mm NATO was adopted by the US military. The difference refers to SAAMI specifications for cartridge case maxiumum pressures.
If you own a vintage bolt-action rifle (made in the late 1950s through the 1960s) that has “.223 Remington” rollmarked, engraved, or stamped on the barrel or receiver, you shouldn’t use 5.56 in it. These will almost always be domestic US-made rifles.
If you own a vintage military-style semiautomatic rifle marked for “.223 Remington”, it’ll almost always be OK for using 5.56mm in it. Many foreign and domestic semiautomatic rifles of recent manufacture still have “.223 Remington” stamped on them, but you can be confident they’re safe to run 5.56mm ammunition through as they’ll have a milspec 5.56 chamber and a barrel that can safely handle 5.56mm pressures.
If you have any concerns, ask a gunsmith to inspect your firearm.
And you are asking because someone got a new Christmas toy? :)