I`ve seated a lot of flat base bullets, no problems. BUT, I do champfer the inside of the necks more than "normal". Try a few cases..see what works. You may also have to adjust your finger positions when feeding the case neck/bullet into the die.
Yup. Chamfering the case mouths has been all I ever needed for seating flat base bullets. Don't expect to just set them on the case and let go though; you want to hold them in place as the case goes up into the die. Technique does matter.
I use a case flaring die (the Lee Universal die) for cast bullets, but if you need that for jacketed bullets I think there's something else wrong.
I have heard people insist they don't need to chamfer case mouths, but that's not true. At least a small chamfer makes a big difference in seating.
