Speer sells plastic wad cutter ammo .. In the smaller sizes they come with a plastic case .. Larger sizes no case, brass case is used.
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Speer plastic bullets usually were fired with primer .. They were heavy enough in each caliber to reduce FPS to about 400 FPS ( under the magic 500 FPS threshold ), about as fast as a BB. I only ever owned the 45'acp version, hollow and wad cutter shaped to keep them slow, would most likely crimp but not really the shape you are going for
Melt straws ( drinking ) in a mold .. Will make a nice hard plastic round with the shape you are looking for and should take a crimp ...