These devices are very dangerous, WE. powder residue build up on the " gate" that separates the powder in the horn from the outside. The residue also eventually builds up enough to prevent the gate from closing properly. That horn is a large grenade, you are holding, and if a spark gets in their, part of you is going to be over here!
ALWAYS LOAD FROM A POWDER HORN OR FLASK INTO A SEPARATE POWDER MEASURE. Then close the powder source up, and put it aside way from fire, heat, etc. then pour the powder from the powder measure into the gun barrel. IF a charge of powder is ignited in the barrel as you pour it in, at most you will suffer some minor burns. If that powder horn ignited, you probably won't be around to worry about the lawsuits that will be filed against your estate.
For these reasons, I discourage friends from putting spouts, or measures on their horns or flask. Its too much a temptation, and that is how you get hurt.