Horn wall thickness definitely plays a roll. As well as how much you have to round it. If your rounding a mostly round base, a heat gun will work. If your making a flat horn in a press, or rounding a thicker or more oval base, it’s important to get beyond the “memory” heat point (roughly 325 to 350F). I prefer hot oil in these cases. Some stubborn horns I have had to “round” twice. Initially a rough rounding to work the horn down further. Followed by a final rounding to set the base shape and turn a base end. I also leave the more stubborn oval horns on a sizer when I’m not working on them. Until I finally get the bass plug installed. And a sizer shaped more like a bullet works better for these ovals, as it doesn’t leave a hard edge