I guess the two piece tacks are upholstery tacks with a brass top and a stainless pin. They would probably work OK, but...
I am just not a fan of upholstery tacks on powder horns. I rather prefer the escutcheon pins myself. The 14 ga pins are what I always used. Lately, I have been using 1/8" dowel.
Silly me, I never thought about Amazon as a supplier for brass tacks, I always bought mine through Clendenin Bros, but they have changed policies and only supply distributors. I just didn't care to do that, so I have held off from ordering anything.
I did order some off of Amazon just now. They should be here on Monday. Evidently they are foreign manufacture, so probably made in China. Oh well.
Your Tansel style horn looks pretty darn nice. A job well done, my friend. I think I tried one once many years ago. I didn't finish it. Way more trouble than I had time for. Depending on what I was working on, I needed to finish about 8 - 12 horns per day. It was a grueling process. Fortunately, I did have a guy working for me who handled all the sanding and such.
John