Hit a branch with a launched dummy especially if it is tethered on a line and somebody is likely to get hurt.Re Gundog dummy launchers They are legal to own & use because they fire a dummy off a Spigot with three radial holes in the end, ie the barrel / spigot isnt drilled all the way through so it cannot fire bullets. They use .22 blank cartridges. The dummy is a rubber cylinder approx 2 1/2" dia & about 8" long.
They go a long way & recoil can be fierce. One old boy in the wildfowlers club used to keep one under his bed in case he got burgled!
Back in 1994 I shot a dunny through a 1/2 wooden shed door to prove the power.
IIRC in 2005 here in South Wales I had my launcher put on my shottie cert so I could buy blanks

You have whip the valve open sharpish or it was a bit less impressive but a PVC version would easily get something like a tennis ball over just about any tree.