Corbin Mfg does make the a die set to make small jackets from a shotgun primer cup....but I don't have a source for shotgun primer cups!!! As talon mentioned, I think they are for a lighter bullet, unless ALOT of exposed lead is okay.
What quantity of bullets are you looking at here? Is 6S okay? 60 gr?
I tried the "let's run some Hornady flat points through the point form die" to try to make a round nose. I wasted quite a few bullets trying that. I'd guess trying to go to a 6-S would be even more catastrophic! Corbin Mfg sells a "special" die to reform/"fix" deformed bullets, but I think it runs $400+ for a "H" die. Also, as he writes, it's sorta defeating the purpose of bullet swaging when you go out a buy already made bullets and then sink some more time in the process. Even so, I just had to try to make some round nose.
Why a round nose? I have one of those M-1 carbines chambered (from the factory) for the .256 WinMag. The flat points won't feed at all. The other available bullets are too long to fit into the magazine. So, in order to shoot a cheap, inaccurate m-1 carbine, I went out and spent $1K+ for dies, press, and supplies (I have the dies for 6-S and 1-E). A false sense of economy!
I'm still learning about the swaging process (didn't get much further than trying to cram flat points in a round nose point forming die), but I have the stuff to do it with....I just need the time!
So....is 6S okay? 60gr?
BTW, someone in one of the varmint hunting mags was looking for .257 bullets, too, in this weight range. Was that you, Leftoverdj?