At the field I worked at back in the 90s, when we were bored, we would play "Blow ball", which like you said, we could only use barrels and had to blow the paint out.
With the paint we used at the time, we only had brakes on masks. It was rare to have brakes anywhere else. Depending on barrel length, we could usualy hit 150-175fps. Looking back, we were probobly lucky that nobody SUCKED IN a ball, and choked!
Likewise, we would sometimes use sling shots. The cheap ones could get up to 210-230. That was around the limit where we could get the paint to brake on "soft" targets like people. Not always, but the majority.
The difference between thin and thick shell paint is everything though. I remember back 20 years ago, RP used to make special "blowback formula" called RP Super-Shell. It usually bounced at 300fps!!
