The qty marking (1200) is IMO the giveaway. Imported ammunition, very heavy, 1200 rounds to the crate would likely make it 7.62 X 39 caliber, my guess, used in the venerable AK-47s and Chinese SKS rifles.
7.62 X 51, our common .308 caliber is way too heavy for one crate. 5.56 X 45, our common .223 Remington, is a possibility, though, but I think the weight shown, is it 24KG(?) that's over 50 lbs., so not likely .223.
I bought plenty packed that way, just before the Clinton arms bill, folks were hauling crates like yours stacked 8-high on refrigerator dollies out of the gun shows to their cars! Mountains of them were offered for sale, back then.
Open a box up! The stuff will be useable for 50 years, likely. imp