GMX stands for "Gilding Metal eXpanding." The GMX is a monolithic bullet with a red plastic tip to initiate expansion. These are very sleek, boat-tail bullets with high ballistic coefficients. Like all such monolithic bullets, they are longer for their caliber and weight than conventional lead core, jacketed bullets and intrude deeper into the cartridge case when seated for the same cartridge overall length. Hornady's GMX is made from gilding metal, a 95% copper + 5% zinc alloy devised long ago to reduce copper fouling in gun barrels. Gilding metal is slightly harder than pure copper and superior to copper for bullet jackets.