In use for more than 60 years, the M4 and it's variants are the main sidearm for the USCM. It is a recoil-operated semi-automatic handgun. It holds 12 rounds of 126 grain 9mm ammunition in replaceable magazines. Due to limited effectiveness of the ammo against body armor, newer sabot ammunition is being introduced that has a hardened metal core surrounded by a high impact resin bullet. Upon impact, the resin body breaks apart allowing the inner core to penetrate the armor.