I think I'd better correct this:
'SS109' isn't a cartridge, but is the NATO standard for a hardened 62gr projectile; SS109 is just the bullet.
M855 is a loaded cartridge that uses the SS109 NATO-spec projectile and is intended for 1x7" twist barrels. There is another type of M855 that's lead-free using a projectile made of a copper-nickel and tungsten alloy, but it's generally only used on marksmanship ranges if it's even used at all anymore.
The milspec cartridge with the 55-56gr projectile that has no penetrator component and is just a FMJ lead slug is M193. That's the old Vietnam round intended for the M16/M16A1.
Thanks.