Both are used. If you see caliber after the number it’s in 100 of an inch. If you see mm it’s metric. (Though caliber refers to barrel diameter and can be expressed in imperial or metric). So a 45 caliber is 0.45 inches or about 11.5mm.
NATO has standardised on a few round types so they can share supplies between countries. The NATO 7.62×51mm round is very similar to the .308 and they can usually be interchanged.
Both are used. If you see caliber after the number it’s in 100 of an inch. If you see mm it’s metric. (Though caliber refers to barrel diameter and can be expressed in imperial or metric). So a 45 caliber is 0.45 inches or about 11.5mm.
NATO has standardised on a few round types so they can share supplies between countries. The NATO 7.62×51mm round is very similar to the .308 and they can usually be interchanged.
And when not, kablammo?!
You can fire .223 in a firearm rated for 5.56mm pressure
.308 is the opposite. It has more pressure than 7.62mm
Most any new barrel is going to be rated for both pressures now a days, but always good to doublecheck!