Fun valve stem project. My father and I had a bunch of fun with this project before he passed 3 years ago from turbo cancer. He taught me a lot in life that I'm so gratefull for. This was a fun one that put smiles on all our faces. Start with a heavy cast iron 2" cap and drill a hole large enough for a valve stem to nestle in the top, with the valve pointed inward. Use your favorite waterbottle brand, and put a hole in the lid so that you can snugly insert the valve stem into the lid like pictured. At this point this build is reusable. You only need more empty waterbottles. Screw one on and use your bicycle foot pump to inflate the waterbottle to about 35 psi. This can be a little scary till you get used to the crackle sound.(ALWAYS USE PROPER PPE) Set them up at desired distances and have fun with your BB gun target practicing.