blackrazor,
Why not build the 50 KM (California Magnum)? Go bigger... WAY bigger. It's the american way. I suggest necking down a 20mm to .50
Been done. It's called the .50 McMillan "Fat Mac".
The problems with it are several:
1) Obviously you can't load these things with any ol' setup.
2) 20mm brass is almost all electrically-primed. You need to use an adaptor to get a .50 BMG primer to fit the base of the flash tube. (On the upside, it has a flash tube; ignition is consistant and long-range accuracy is supposedly outstanding.)
3) A 750gr bullet at 3,425fps generates just a tad bit of recoil, blast, and boltface thrust. 19,545 ft/lbs of muzzle energy is nothing to sneeze at (that's half
again more than a .50 BMG. Or, think of it as a .50 BMG
plus a .458 Lott.
) The few rifles in this caliber tend to be monster rifles with 42" tubes, massive custom McBros receivers that dwarf a .50 cal rifle's, weigh a ton, and have prices in the early ionosphere.
If you're going to use 20mm shells as the base, why not go all out and resurrect .950 JDJ?
(Also, any .50 BMG rifle could be easily rechambered to .700 JDJ. It's just a .50 BMG case blown out to .70 cal, and bullets, dies, and cases are available from SSK Industries. 1100gr bullet at 2,200fps sound fun?
)