Anti-Personnel Cartridges
When 40 mm grenade launchers were 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, they were also decidedly exposed if presented with an enemy within the arming distance of most high explosive rounds. 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 grenade launchers to engage targets at shorter ranges safely.
Similar rounds have also been developed for weapons chambering the higher velocity 40x53mm cartridge as well.
These anti-personnel rounds fall primarily into what could be considered the "canister" category, and are designed in most cases to turn the grenade launcher into what many would consider a large shotgun.
40×46mm types include the M576 and the XM678.
The M576 contains twenty 24g metal pellets. The XM576/XM576E1 was standardized to become the M576. Another test variant, the XM576E2, which had Twenty-seven 24g metal pellets without a sabot within the shot cup was deemed to spread too quickly for effective use.
The only 40×53mm type is the M1001 and is a canister round filled with 115 17-grain 2.0-inch long flechettes. The choice of fin-stabilized flechettes over pellets or similar projectiles for the high velocity ammunition is most likely a product of their greater accuracy over slightly longer ranges.
Read more about this topic: United States 40 Mm Grenades