I must say, that is the prettiest water buffalo horn I have ever seen.
The horn workers from India have used a technique that is "interesting."
So, what they do on the occasion that they cannot reach the cavity from the tip, is they drill down from the tip as far as possible, then marking where the drill stops, they go in from the bottom side of the horn and drill down into the cavity, hoping I guess that the two holes will match up. Then they fill the outside access hole with body filler. I guess it works, but nothing I would ever have done in all my horn working over the years.
And that's a FWIW, not a recommendation, for sure.
What is a recommendation is that the first thing to do when working on a powder horn is to: a) cut the tip off and b) drill the hole. If nothing else, that can save the frustration of doing a lot of work and then drilling out the bottom of the horn. Just my two cents worth, but what do I know?
John (Bigsmoke)