An M855 round’s 62 grain bullet performs identically to an FMJ until it makes contact with something. If it hits steel plating within about 300 yards, the seven grain steel tip beneath its jacket facilitates clean perforation. If it hits soft tissue, that same tip promotes penetration up to a depth of 20”. The penetrator bullet may even break into two pieces inside of soft tissue, thus increasing its chance of striking a vital organ. Effective stuff for self-defense – hence why the Army uses it – but just as useful for happy times with an AR-15 as well!
This ammo’s green-tipped bullets are loaded in new production brass cases. The cases are only gray around their necks and shoulders because they have been annealed, i.e. heat-treated to help them expand and reduce fouling within the action during ignition. Winchester’s consistent primers and propellant charges deliver satisfying target grade accuracy – just take note that a 62 grain bullet performs most accurately in conjunction with a 1:8 or 1:9 twist barrel.