Posted a few minutes ago about re-barreling my little C/F Francotte Martini with a 308 barrel. but hadn't put my mind into gear when I wrote it.
Now with a glass of wine in my hand I think I am now thinking more clearly.
After reading about the Marling guy using the 32-20 case with cast bullets in his Martini with a 308 barrel I thought that's the way to go only with jacketed bullets as I stopped casting years ago and as I was going to use a standard 32-20 die set I would have to get another die to get the case to hold a 308 bullet and Just wan't to keep everything simple so will have a word with my gunsmith to see if he has some cheap 303 Brit barrels under his bench.
As soon as I was thinking more clearly I removed my post but saw some people had already been there and probably thought what an idiot. Would still like some advice if any body has gone this way.

Too much wine?

Nero.
As I suspected:
I sized a fired 32-20 case, but did not have the expander plug in my Lee sizing die. A 30 caliber jacketed bullet is a no go tight fit in the sized case. One could use a 30 cal expanding plug with the die and load simply for a "30-20"
rifle with a std 300/308 bore barrel. Think of all the jacketed AND cast bullets bullets available in 30 cal that you can choose from. I would go with a std 10 twist barrel.
beltfed/arnie

I came pretty close to what you are thinking.  I wanted to try a heavier bullets with a 32-20 cartridge, so I lined a Handi rifle with a 30 cal Lugar liner from TJ's.  It has a 1:10 twist with a .303 bore and .311 groove.  The problem was chambering, since the 32-20 reamer was set up for a standard .305 bore.  I had to put a strip of fine sandpaper on a small dowel and use that with a hand-held drill to gradually grind away .002 for the floating pilot to cut.  Took a little time but worked well - shoots 123 grain coated bullets very accurately.  The .300 bore of a 30 caliber barrel will make the 32-20 reamer issue more pronounced, unless you have a custom pilot made.  Pacific Tool & Gauge will make a custom reamer pilot for around $20.

Nothing wrong with the 30-20, works perfectly accuracy-wise.  My reamer has a .100 freebore at .309, followed by a 3° angle after the freebore.
And yes, the chamber is just a minspec CIP .32-20 chamber.  Still wondering if The limitation is myself, or the load.  I think it's myself, seeing that I'm at about 2 MOA with BP and cast bullets, sling only, no rest...