When I was at school they[0] brought round a rubber bullet (I presume it was a plastic bullet actually but they called it rubber). It was a light grey, featureless cylinder, flat at both ends, at least as my memory has it. It was surprisingly light, seemed to have barely any heft at all.
We were told it was the very bullet that killed a protester in northen ireland.
[0] I can't remember who 'they' were, an ant-violence group I guess.