Considering these are for the US military, most of them will be "ball" made to US/NATO spec, M855 catridge, ball. (Assuming we're really buying loaded ammo, not just bullets, but you can't tell from the article, but even so, the bullets would be the same).
The NATO standard, M855 round is intended for use against light matériel targets and personnel, but not vehicles. Identified by a green tip, the 62 grain projectile is constructed of a lead alloy core topped by a steel penetrator, the whole contained within a gilding (copper alloy) metal jacket.
So, lead and steel.
More infomation in the manual (PDF file).
Thank you for the comprehensive answer. I’m going to read that pdf.