I read after posting that there may be a few 10's of thousands but it does not sound right to me.
There was a bit of a craze for a while but I think most actual firearm users who do not get free ammo soon got over it.
Actual serious shooters (in my opinion) want to hit a target not spray randomly so a high rate of fire is not very relevant.
Take a 3-gun or other action sport shooter; 30-40 round stage and they stop. Do they want to shoot fast, heck yeah but they also must hit in order for it to mean anything.
A 600 round per minute statistic is only relevant to a gun-grabber in the context of a semi-auto firearm.
Yup.