I suspect many of you already know this, but I didn't see anything in the archives. My apologies if it's already common knowledge

The really small Maglight, "Solitaire", works perfectly as a bore light in .54 barrels. It's the one that uses a single AAA battery. Just twist it on, and slide down the barrel, lights up all the lands/grooves perfectly. Only costs about $6, and replacement batteries & bulbs are readily available.

Tomy chagrin, it won't quite fit down the barrel of my .50 caliber, but it has a fairly tight bore. If your .50 is a bit on the loose side, it might fit.