[4V50 Gary]

The "cap" is called "plug" or "butt plugs" by horners. They are permanently attached by glue and sometimes tacks or pins (could be iron, brass or even wood pegs). It varies according to the maker. Whatever means it is attached, the horn should be both waterproof and airtight. The test for airtight is to blow down the pouring spout and feel with your free hand any air leakage.

Some fancier horns have a finial which is turned to screw into plug. This allows for a bigger hole with which one may refill their horn. Otherwise, one used a funnel (even a rolled up paper one) and refilled the horn via the pouring spout.

Losing the plug can be a disaster, especially if it was the only powder a frontiersman or trapper had. It meant a trip back to camp or even to a sutler/trading post to refill the horn.

Traditional glue included stuff boiled down from hooves. I've never read it anywhere, but resin from trees may have been used too. One thing to consider, if your horn is an antique horn, you don't want to use something that will expand or contract drastically from the rate that the horn or plug expands or contracts. That can cause the horn to crack.

The best resource on horns is the Honourable Company of Horners.