The XM678 canister was designed to replace the M576 series. I have attached some information and a drawing I made years ago. The data sheet shows the XM209 case based on information I had at the time. My actual round has the XM169E1 case. As far as I know, both used the M70 primer. Although mine has none, markings were stenciled in white on the projectile. The entire projectile body is made of OD nylon plastic. There is a metal push back ring and an aluminum pusher plate on the inside. Once fired all you would find is pieces of this projectile also.
Bob
Took a while to find it but I have an April, 1970 draft technical manual on the XM678 cartridge and can add a little more information.
This item has always interested me since first reading about it and seeing your example in a photo you sent about 15 years ago. This canister round was actually intended for use in high velocity 40mm launchers, not the M79 or M203, and not as a replacement for the M576 series rounds. It was developed for a single specific use, to provide suppressing fire for short ranges in jungle and low light conditions, especially to disrupt ambushes and engage unseen or fleeting targets.
I suppose you can do the same thing with a belt of M384 HE rounds, assuming they get far enough to arm the fuzes and not so close that all the friendly troops get fragged too. I suppose 54 30-grain canister shot pellets really are the way to go in such situations.
As noted in Bob's drawing and data sheet, it was to be used with the XM182 weapon system. This was the XM129 40mm electric motor driven grenade launcher with spade grips in a vehicular pintle mount. A special warning appears on the inside cover of the manual stating that the XM129 is the only launcher authorized for use with this round and that it will not be fired over friendly personnel. I assume the usual platform would have been the M113 APC and perhaps on armored gun trucks used for convoy escort duty.
The XM209 cartridge case used with the XM678 also seems to be unique among 40mm rounds. As noted earlier, it used an M70 primer with 52 grains of M9 propellant sealed with a cardboard wad. No high-low pressure system was used, presumably because of the 900 fps velocity and 150 meter maximum range. Also noteworty is the case length, .093" longer than the standard M169, and the ring of heavy crimps that can be seen on Bob's round.
A really interesting example of one of the specialized 40mm rounds of the Vietnam era. And with the fact that it was only used in one gun in a very specialized mounting, I'm sure production was very limited with most going directly to Vietnam. This would certainly explain why surviving examples are so scarce today.
