Although riot police have been using rubber bullets for years, the idea of mounted machineguns is more than a little unnerving to me. Getting hit in the chest with a rubber bullet from a rifle is one thing, but being struck possibly multiple times from a .50 caliber military grade machine gun is quite another.
Also, rubber bullets can easily cause death when misused, and misuse is not uncommon. It is impossible to be accurate when you are firing into a churning crowd, and deaths happen. Also, there are plenty of reported cases, I found this after a fifteen second google search, from a government website no less.
Death following rubber bullet wounds to the chest:... [East Afr Med J. 2005] - PubMed result
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"You know what, World? You got me cornered again,
I'm gonna role another joint."