1st off, a laurel, and hearty handshake to HDW for amusing the Armorer!
This is a cutaway of the XM576- the funny part is that Rey, who essentially got it right (though his correction made his answer worse), did so for *all* the wrong reasons. It is a hi-lo pressure grenade, meaning that the launching charge goes off in the cavity in the casing and bleeds off the gas (under much lower pressure) into the chamber to launch the grenade. This is how you can square having such large, complex grenades made out of lightweight materials, fired out of an aluminum-barreled gun and not be unsafe nor pound the gunner and gun to pieces with the recoil. The pic I posted yesterday was the shot cup, not the cartridge case. So, Rey - you got it right - if incomplete- and cited all the characteristics that *weren't* on display!
The is the initial version of the cartridge which was made in test lots only for lab and field evaluation. It consists of the open-mouthed plastic sabot seen here with the shot cup in the center. The shot cup contained 20 lead pellets, each 18-19 grains in weight. In it's final configuration, the M576, it has 27 pellets.
This is one of two rounds developed for the M79 so that gunners would have a close-in capability (the explosive rounds all required several meters of travel before they armed) for both personal defense and for close-quarters combat, such as clearing buildings or dense vegetation.
The first was a flechette or "Bee Hive" round which fired several dozen small darts. This was later replaced by the M576 buckshot round. With 27 "00" (aka "double aught buck") buckshot, this round was devastating at close ranges. The pellets spread in a cone 98 feet wide and 98 feet high at 300 feet and zip along at 882 feet per second. This round is olive drab with black markings.
We don't make them anymore, nor have we for some time. Early this year the Product Manager Crew Served Weapons at ARDEC (Armament Research and Development Center, aka The Armorer's Funhouse), at Picatinny Arsenal put out a sources sought notice for availability of a non-developmental 40mm short-range antipersonnel shotgun type cartridge that is compatible with the M203 Grenade launcher and one that can be fired quickly without precise aiming, with a high probability of producing casualties. In other words, they're looking for someone who is already making this round or something like it.
On a related note - I went out looking to find some pics and ran across this... idiot. The Armorer *strongly recommends* if you don't know what you're doing, stop doing it. Of course, most people who are like this idiot *think* they know what they're doing. If the story isn't just a fantasy - though the detail is pretty good if it is. This guy posted it in at least two places - the comments are worth reading, as is the peek into the minds of people who are clueless as to how dangerous some things are - even for people who do know what they are doing.
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