Re: deployed .30cal
 

Those groves are called rifling, hence the name rifle. They impart spin to the round as it travels down the barrel to stabilize it and improve accuracy.

Rifling is measured in twist rate, the units being 1 full twist / the barrel length it took.

So a .30-06 Springfield with 1:10 twist rate, the rifling makes a full twist in 10 inches of barrel travel.

And yes .50 caliber is 1/2 inch. Caliber with a leading .XXXX number was a term used to describe diameters inside an inch instead of saying "five tenths and sixteen hundreds caliber"... You can see why this was shortened. Sizes over an inch (And under it to about .6) tended to only occur in cannon or mortar pieces at which point the round or (ball) was simply measured in its weight. 2 pounder, 5 pounder, etc.