Federal® XM855 Ammo features the classic M855/SS109 penetrator projectile, but is technically NOT mil-spec because it features a lead core instead of the traditional steel core. That's why it gets the "X" designation. That said, Federal XM855 is great stuff, and highly affordable. These rounds are loaded on 10-rd. stripper clips.
PLEASE NOTE: While the 5.56x45mm round will fit in a .223 Remington chamber, 5.56 is a military round that is loaded to higher pressure than its .223 counterpart, and is therefore not recommended for use in rifles chambered for .223.