This is 280 Rem. Hornady Superformance 139gr. GMX Ammo. The GMX bullet uses Gilding Metal Construction. Gilding Metal is harder than solid copper which delivers devastating terminal performance, deep penetration and 95% plus weight retention. This is Hornady's brand new line of hunting ammunition. Superformance is a cutting edge technological advancement in ammunition design that transcends convention and achieves the highest performance of any ammunition on the market today. Superformance ammunition is 100 to 200 fps FASTER than any conventional ammunition on the market today, and achieves this performance in EVERY gun, WITHOUT increases in felt recoil, muzzle blast, temperature sensitivity, fouling or loss of accuracy. This ammo is brass cased and boxer primed, it comes packed in 20rd. boxes.