Help understanding bullet calibers

[RangerK]

Damn. 70mm is almost 3 inches...Thats a huge cartrige. What would you fire that from?

[pistol fixer]

well as this thread shows there are many different ways that bullets and cases have been discribed over the years .
in the 1800 when most carts, were loaded with black powder a 45-70
ment .45 bullet dia. loaded with 70 grs. of black powder. a 38-40 as a pistol cal. .38 dia. bullet and 40 grs. of black powder. colt and winchester used several carts. in both rifle and pistol. so a cowboy could carrie one shell that fit both his rifle and his pistol. now to complicate it more the 70grs of powder in the 45-70 and the 40 grs. of powder in the 38-40 were both black powder but were not the same size. the rifle( 45-70) was may be 2 or 3xblack powder and the pistol (38-40) was going to be finer 4x. so since
the powder in the pistol was finer it burned faster.

Later carts. after the interduction of smokeless powder.

carts. were called by there cal. and case length. example 7.62x39 = 7.62mm bullet dia. and 39mm case length. 7.62x 51= 7.62mm bullet dia. and 51mm case length
(also called 7.62 nato and .308 winchester)

if you notice in the 45-70 the - indacates that when the cart. was first made it had black powder. when the cal. is seperated by an x it means
length of case.

in shot guns the gage (20ga,28ga,16ga, 12ga,10ga.ect) is the number of lead balls of bore dia.that it takes to = one pound of lead.
so in a 4ga. each lead ball =.25 lbs. and the recoil is very tough on the shoulder.

this is just the tip of the iceburg. when you get to really big cart. it changes again. for example a 5" 50cal. naval gun has a 5" bore and a barrel that is 50 cal. long. 50 cal. barrel length means that the barrel is 50 times the bore dia. so 5"x50 =250"or 20.833ft. long
now that all this is as clear as mud . go forth and learn more.