Thread: How to tell the caliber of projectiles?

I could probably google this but hey it’s a forum right? But how do we find out what caliber projectiles are?
I’ve found some old projectiles and am wondering what they are.
My best ill informed guess would be 50cal as they are from an old WW2 military firing range.
One weighs in at 668grains and another at 684grains. This could be due to corrosion, sediment etc
Photo of them next to 130gn 308 projectile.

Yep, standard .50 Browning.

Vernier calipers or micrometer. But yeah, at a guess those would be 50cal bmg going by the picture and information you provided.

Yep 50 cal

but measuring isnt always that simple....
you have the 7.62mm also known as .30 calibre which is .308..... and the .303 which isnt as its usually .312 BECAUSE the barrel is measured from the inside of the lands not the bottom of grooves like the .308

7.62x51 means 7.62mm diameter projectile and 51mm case

the .22-250
is a .224 calibre projectile in a 250-300 case necked down

and the 250-3000 was .257 calibre whos projectile went at the then unheard of speed of 3000fps

.45/70 is different again...its old school black powder name
.45 calibre popelled by 70 grns of black powder

the .45/70/405 is the same with 405 grn projectile

so its all simple but confusing untill you know what system is used and why....

oh and then you have the likes of the .307...which is a rimmed .308 the same in all other respects.

hope that cleared things up somewhat.