Twenty some years ago, I purchased Earl Cureton's powder horn business. He had used boiling water to soften horns from the beginning of his horn working time, which would have been about 30 years. I continued with that method for maybe 8 or 10 years after I bought the business.
But, I really didn't care for doing things like that. It sure steamed up the horn shop and I wasn't really happy about the potential damage to the insulation in the shop. However, it did help warm the shop up on a cold morning.
I had heard of using hot oil or lard to soften the horns, but for some reason I did not care for that thought. I was considering asking the manager of the local McDonalds if it would be all right if I came down at night and heated up some horns, but decided he probably would not go for the idea.
Then I thought about using a commercial heat gun to soften the horn. That worked just fine for rounding, for flattening, and for pressing pieces of horn.
Bottom line, I do not know how many thousand powder horns I made after switching over to the heat gun, but it was several. To this day, whenever I make a powder horn, after I drill the tip, the heat gun comes out. What I do is insert the nozzle into the base of the horn, and at high heat, I run the nozzle around the horn wall for 45 seconds to a minute. Then insert the baseplug into the horn and seat it with a dead blow hammer. Easy peasy, and you don't have to wait for the water or oil to warm up.