In FL they are considered firearms, even the air powered ones. That being said, I've built one combustion powered and two compressed air powered ones here without issue using them. They are super fun! My advice is that if you're going to spend the money building one, go for the air powered version. The last one I built utilized two fill valves, which I attached two 12v car air compressors to fill ($10 a pop at Wally World). It would charge in about 5 minutes but easily was 10X more powerful than the combustion cannons I've played with.
Imagine launching a potato (or orange, etc) about 90 yards with a hairspray/ether/propane cannon.
Now imagine launching SIX potatos at once, nearly 200 yards with the air powered cannon. And not buying propellants!