I used a 3/8" 4-6" dowels to seat caps. I like wood rather than metal or horn as it is a little less hard and to me, less likely to set off a cap if pushed hard. When I shot in a "True Grit" type match (required 4 pistols, 2 rifles & shotgun each stage with approx 450 rounds sent down range) my palm was pretty sore from seating caps. So I just drilled a hole into another hardwood of 1" dia, rounded the edges and glued the 3/8" into the 1" part. In essence, made a "short starter" like is used in muzzleloading rifles only much smaller.
Hellgate (orygun)