Just to add to this, you can take a steel plate, and cut a U-shaped groove in it, and then file bevels on one side so that the groove becomes a round bottom " V ", The top, or opening can be left at the maximum diameter of the stock you are using to make the ramrod. The bottom of the " u " can be the final smallest diameter you want to achieve, with the sides sloping like the letter " V ".
The other thing that can be used is a spokeshave, which has an arched blade to use to cut a round surface. You can find spokesshaves at antique auctions, and farm auctions, or you can buy them new from Woodcraft, harbor freight, and other tool companies. As long as you have a vise, or clamp to hold one end of the ramrod stock, and you take your time, you can produce a ramrod with that bell-shaped nose without too much trouble. I have done it with nothing more than a draw file, although that is doing it the hard way, and I will not recommend that to you. Scrapers would work faster and produce a more uniform round rod. I know one man who I watched over a couple of days at a BP demonstration our club was holding, make such a rod with only a knife to use as a scraper. It was well into the second day before we figured out what he was doing! ( being ornary, he wouldn't tell us!)