I recently acquired a very useful tool I had no idea even existed (I'm relatively new to guns): A bore light attachment.
I got it from a friend who didn't need it anymore; I'd guess he bought it as part of a bore light maybe?
Anyway, I stuck it on the end of an inexpensive HF AA LED $1.99 flashlight. The fit is great, and I was really surprised how handy having one of these was for inspecting the bore of a pistol. The head fits right in the chamber and the diameter of the fiber-optic rod that bends the light is about 9mm/.38.
Not exactly a "hack" but it is pretty cool: