PLEASE, do not aim the Bazooka at people, animals, your neighbour's prize-winning tomato plants, or other objects that may be damaged. Only launch outside. Don't do anything stupid, like lighting the bottle on fire before launching it. Use only water as a propellant. Do not over-pressurize the bottle. Use under adult supervision. Keep small children away (regardless of my video!) Use the thinking part of your brain.
MOST IMPORTANTLY, do try to retrieve every bottle you launch. Let's not litter, okay? I'd also suggest using "reclaimed" water as propellant, such as rainwater, snow melt, or the like.
Step 1: Materials and Tools
3-4 feet of 3" diameter ABS pipe
One 3" to 1.5" ABS reduction adapter
One 1.5" to 0.5" ABS threaded pipe adapter
One 1" inside diameter metal washer
About 6" of scrap 2x4
About 6" of 3/4" diameter plastic pipe (any kind)
One #4 rubber stopper, solid core (cork won't work, it has too many holes!)
A bicycle brake lever
Bicycle brake cable housing (cut to fit)
Bicycle brake cable (cut to fit)
A brake cable clamp or similar
Any sort of glue that will adhere plastic to plastic, and wood to plastic
Water bottles - 500mL size, or anything that will fit in the launch tube
A long Presta valve - a used bike tire tube is a good place to get one!
And here are some tools that you'll probably need:
Something to cut metal - a Dremel or metal-cutting bandsaw would work best
Something to cut plastic - a handsaw or miter saw or something similar
Something to cut wood - really, any sort of saw would work.
A drill press (or a hand drill, in a pinch)
A sharp utility knife or X-acto knife
Assorted measuring and marking implements