M576 40mm grenade
The M576 is a US Army designation for a type of 40mm grenade cartridge. The M576 is a buckshot load for the M79, M203 M320 and M32 MGL grenade launchers. It was designed to give the soldier carrying a grenade launcher a powerful cartridge for close quarters combat such as found in clearing buildings, bunkers, and trenches, as well as thick vegetation.
When the 40 mm M79 grenade launcher was first developed, the weapon was to be the primary weapon of the infantryman carrying it. It was quickly found that in most engagements (while the grenadier gave the squad a decided force multiplier) the grenadiers were decidedly exposed if presented with an enemy within the arming distance of most high explosive rounds. Even rounds with a shorter arming distance presented significant danger to the shooter if used at those ranges. Prior to development of grenade launchers attached to standard infantry rifles, development commenced on non-explosive cartridges to allow those armed with launchers to engage targets at shorter ranges safely.
The M576 contains twenty 24-grain metal pellets. The XM576/XM576E1 was standardized to become the M576. Normal dispersion pattern of the M576 will put 13 of 20 pellets in a 1.5 meter circle at 40 meters. The remaining 7 pellets could be anywhere. Another test variant, the XM576E2, which had twenty seven, 24-grain metal pellets without a sabot within the shot cup was deemed to spread too quickly for effective use. Both types had a muzzle velocity of roughly 880 ft/s (268 m/s).
For comparison, a single pellet of lead alloy no. 3 buckshot also weighs 24 grain. A common 20-gauge 2.75 inch (70mm) shotshell loading of no. 3 buckshot (e.g. Federal F203-3B  ) pushes 20 of those no. 3 buckshot pellets to 1200 ft/sec at the muzzle from 18.5 inch barrel length. Compared to the M576 this has the same projectile mass, but exhibits 36.36% higher velocity at the muzzle for 36.36% more momentum at the muzzle, and 85.95% more energy at the muzzle.
- US Army Technical Manual TM 43-0001-28 Ammunition Data Sheets page 6-33