What is required, what press will work, who makes the die..Id like too make 348 bullets from Swift or whatever by this method..but need some advice..

LEE would probably make one for you.
10 thousands is a large drop in size,you want to make sure the core does not come loose in the jacket.
I use a Bonanza CO-AX press to size .338 bullets to .329 for my 8 M/M Siamese rifles and have had very good luck.

I have done .430 to .416 in one pass using a lee push through die.

If doing it again I would add a intermediate step.

Like 5 thou at a time. I would buy 2 lee 349 dies, polish one out to 354 or so.

I used a single stage press with the bullets well lubed.

Lee does not do custom work now. Last time I checked. But I routinely swaged 375 bullets down to 366; on a Rockchucker. Back before we could get 9.3 bullets. Like in the dark ages of the 1970s.
But if Swifts have thick jackets then, no. Call Corbin.
What they are talking about above is that the copper will spring back and lead doesn't; then you have to use a nose punch and squash the core back to match the jacket.

Problem solved itself..ET and Brownells solved that problem..Got 200 rounds of 350s 45-70 Swifts from Brownells, and 400 348s 200 gr. Hornadays from ET..that should hold me for awhile anyway..

For future reference the best way to achieve this is to use swage dies and make your own 348 bullets. You can get bullet reducing dies from Corbin but they only recommend them for .006 reduction. Any more and you may have issues.
(greater reduction can cause extrusion of the base over the punch, distortion of the bullet, and excess springback of the jacket).

https://www.swagedies.com/mm5/...tegory_Code=SDRAWDIE

Shopped around and got me a supply put in, its out there but you have to work for it...