Ok. Uncle it is.
Borelight.
That is a green led at the bottom of a muzzleloader barrel, four feet deep and half an inch across.
At camp this year, we got a chance to learn all about firing a muzzleloader, and part of that process was cleaning them. At the end of the cleaning procedure, before reassembly, the barrel was inspected with this tool. Since a muzzleloader's barrel is more like a cup than a tube (ok, a really long tall thin narrow cup with a tiny leak at the bottom...nevermind), the borelight had to be dropped in from the muzzle and when it slid to the bottom, we were able to inspect it with the light it cast on the inside of the barrel. This is a picture of what I saw.
How to roll your own.