this is mostly coming from memory, and guessing, so I could very well be wrong but it looks like either a 30-06 or a .270 to me
the 30-06 has a bullet dia. of .308 or just under 5/16", the .270 has a bullet dia. .277 or just under 9/32", measuring the neck ID will tell you the bullet dia
I did look this up, it isnt much but I found a little coumpany info...
HEADSTAMP- US
OTHER HEADSTAMPS USED- USC Co or US with $ sign, U.S.C Co.
COMPANY NAME- United States Cartridge Co
YEARS IN BUSINESS- 1864-1938, then purchased by Winchester/Western
a number like that usually indicates what type of round it is(bullet type) and is usually used in military rounds(I'd have to look it up to know what that # indicates)
I'm not much "up" on military rounds other than the basics so without looking it up I dont know if USC Co made rounds for the military or not but because of the "18" on the headstamp I'm guessing its a 30-06 military surplus round, alot of people buy them because they're cheap, you can pick up cases of ammo for little to nothing, I buy alot of surplus ammo myself(or used to anyway)
some surplus ammo is steel cased(and buy far the cheapest to buy), although without looking it up I dont know if USC Co ever produced steel cases, try sticking a magnet on it, if a magnet sticks to it its a military surplus round
I'd have to dig around to be positive about alot of this though so like I said, I could very well be wrong about some of it