Maybe some 40 years ago, I decided that priming horns seemed just a bit too bulky for my personal taste and I surely did not care for the push type valves on primers that were made by Treso. So, I came up with the idea to press fit a horn bushing into a 150 grain fixed powder measure and then screw a regular horn valve into that with a 15 grain measure installed. I felt that was THE answer to the problem.
A year or so later, Linda and I were displaying at the SHOT Show and I had one of those on the table. So, one morning, Don Minor and Andy Fautheree (owner and manager of House of Muskets) came walking by and stopped to visit. Linda and I were chatting with Don about something and Andy was checking out stuff on the table. All of a sudden, he calls out, "Don, look what they are doing to our products!!!" So Don looks at the primer and says, "I can make these better than that. How many do you want?" I said, "If you can make them markedly better, I will take 100 of them. When will they be ready?" And so it went. I have no idea how many of them I have sold since then, but it has been a bunch. I think they now have a 135 grain capacity body and a one piece valve/nozzle. The nozzle holds about 3 grains, so it is pretty right for priming and 4 Fg flows out of the valve like water. Priced at the factory in Thompson Falls, MT, they are $16.00. I would guess Fffg would also work in the primer, but Ffg would probably be a little coarse. It is called the Free Flow Primer.
Over the years, I have used 4Fg, 3Fg, 2Fg and 1Fg in various flinters and they all work pretty good. I can see using any grade of powder to prime with, and I don't think you really loose much if anything by priming out of the horn as far as ignition time goes.