I don’t use Boiling water unless the horn is already basically round, because it only gets to 212°. Horn needs to get to 325-350 to permanently change its memory. I have used a heat gun for horns that didn’t need a whole lot of rounding. I have also put them in the oven on a damp towel and a cookie sheet at 350 for the same purpose.
However, if they are more oval shaped, and need more adjustment, hot oil (or lard) is your best bet in my opinion. Sometimes I have even sized horns twice that were really stubbornly oval. A “presizing” before I take the body or ridge thickness out - before final sizing. It’s best to drill the pour hole in the tip first to let air escape as you dip the horn base in the oil.
A good sizer is also important. I prefer those that are a little more bullet shaped for more oval horns- as they will go up inside the horn farther, without leaving a noticeable bulge in the sides after heating. There is a sweet spot in heating the horn in oil to get it hot enough, but not overly hot and fry it. I dip and swirl it in 5 second intervals until its soft enough, but not cooked. You can also dunk a portion of the horn that is thicker for a few seconds first before putting the rest of the horn in the oil. Wash with Dawn when your done. Leave it on the sizer when your not working on it. My lessons learned on the topic, FWIW.
A pic of a more oval horn that was rounded up as described.