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This is the "case" for the 10x24 caseless ammunition used in the USCM M41A Pulse Rifle. It is based on a design by Matsucorp, used with his written permission. In this particular model six cases are strung together on a sprue; it is plausible that in-universe the rounds are supplied like this, in a similar manner to NATO 5.56 on a stripper clip. If desired you can seperate them with diagonal cutters and a quick bit of sanding will clean them up.
To assemble you will need 10mm FMJ reload bullets similar to IK-215350 at Cabela's (Magtech bulk SJSP .44 mag). Also, the case is hollowed out to allow it to be weighted for balance; I used cut-off bits of solder for this, with a drop of epoxy. Green or OD seem to work. I'm experimenting with Krylon "stone" paint for the slightly pebbled look.