This is a weld-on tank bung; a means of attaching pipe threads to a vessel. It is intended to be welded onto the wall of a tank or pressure vessel, providing solid pipe threads in a material typically too thin to be tapped for pipe threads. On some occasions I’ve used them for that purpose. On others, I’ve found that they make a great component in pneumatic cannons. More on this obscure part after the jump.
There are several ways to put NPT (National Pipe Thread, which is a tapered, sealing thread used in plumbing; it is the US standard for pipe threads) in a drum, tank, or pressure vessel. The obvious is, of course, to use or adapt any threads already provided on the vessel. Another is to drill and tap a pipe-threaded hole, although this only works if the wall of the vessel is sufficiently thick to create enough threads; often it is not. A third is to use what’s known as a through-wall fitting, which assembles around a hole drilled in the tank and does not require welding. These are rated for mild pressure and can be useful. And finally, there is the item pictured, a weld-on tank bung.
Assuming that you can weld an airtight seam, these are a great way to add threads to a tank or vessel. However, a strong note of caution applies: Do not even think about using these fittings unless you are a competent welder and versed in the risks of welding on pressure vessels. A pressure-vessel failure is not an event you want to be around.
If you are building a pneumatic cannon, you can pick a suitably sized pipe and weld a tank bung into one end of the pipe to create the perfect firing chamber. All of the pressure vessel risks and warnings stated above apply as well as the new risks of exploding windows, police, etc…