I shoot the old 303 Savage in model 99 Savages. A freind up north (I' m from Texas) has shown me how to do this sucessfully from Swift brass. I have shot over 2000 rounds and no failures. Glad to relate it if any interest.
To begin with the 220 Swift cartridge rim is smaller than the 303 Savage. I've four old Savage 99s in 303 Savage and they all extract the 303 made from resized 220 Swift brass. The brass does not swell at the base and is as good to use as the Winchester 303 Savage brass I purchased at a premium.
First, you need virgin Remington brass to begin. I was told to start with Winchester virgin 220 Swift brass and when resized it did not work. Why, I do not know but the originator of this method could not make it work either
when I sent him my resized and loaded ammo. Win may be thicker than Rem. Don't start by resizing 100 cases as I did. Those cases from Winchester refused to chamber. They may have if the necks had been reamed. With a .308 bullet seated they would not chamber. GOOD BARRELS
All you need to make the transition from 220 Swift to 303 Savage is a set of 303 Savage full length dies, an intemediate size expanding neck sizer and a case trimmer to shorten the looong neck when the 220 Swift is forced through the full length sizing die.
I was originally told to use a Jap 6.5mm neck sizer to minimize crushing the neck. With this I often crushed the neck of the brass. So I called Lee Precision and explained my problem to an engineer. A week later (at no charge) I received two decapper sizers that were neither 30 cal or 6.5mm or 220. The decapper sizers were tapered from .308 to .22 over a 3/4 inch bevil. One pull instead of changing the expanders in the process worked. Lubricate well.
I have lots of dies (probably 50 sizes) from Lee, Dillon, RCBS, Hornady, CH,and some real old timers for an old True Line press. For my money Lee is as good as anyones. My 303 Savage setup is Lee 90756.
So much for the resizing history. Again, force the virgin Remington 220 Swift brass through the full Length die set with a deprimer sizer tapered and leaving a .306 - .307 or so inside diameter neck. Trim the LOOOOng neck and chamfer the neck to 2.015 ( I think). Load, seat a .308 bullet and enjoy that old 99.
I did this from memory but it should all be here. If you like another brand of dies, have at it. For me the proof is in the results and sitting here at the bench I can count 12 sets of Lee, 27 sets of RCBS, 1 set of Dillon, and 6 or 8 Hornady. If they aren't Lee, I found them used and cheap.
It is after 2030 Hrs so past my bed time.
As Red said, "Good Night and God Bless"