So is it possible to get a nef handi-rifle with the 20 guage ultra slug barrel use 3" 20 guage brass and shoot 600 nitro bullets out of it?

If so what dies are available for loading these?
would the 2.45 inch magtech 20 guage brass work?

I think magtech brass is 2.5".
It is best to get 3" 20ga brass from Rocky Mtn Cartridge.
They also make a die I think.. And their cases fit chambers
tight so that a minimum of sizing is needed.And they make
cases any length and make the inside diameter to fit
.615" to .625" bullets. You can shoot heavy 600 nitro
bullets, but the light gun, and stock setup of a break
action the recoil will be a problem. And you can cast
hard cast 6-700gr slugs for a third or less of the cost
of 600NE slugs. Hard alloy cast takes high speed ok. Ed

Mostly I was thinking a hardcast like Beartooth bullets on a casefull of trailboss.

Maybe load it up a bit just till the recoil is still tolerable.