with typical APDS rounds from 20-30mm, or a .50BMG, i had a couple of questions

what is the typical velocity, does it ever go as high as 4700FPS

if so , what is the barrel wear like for these rounds?  

if you scaled down the APDS to .338, and created a fast-firing machine gun with 4700FPS, would it have much barrel wear problems?

have there been any NON-SABOT rounds that were very fast? i recall the soviet 6mm Unified had 3800FPS.... could a round like this be successful in machine guns today?

nobody got any ideas?

4700 fps (1,400 m/s) is beyond the typical 20-30mm APDS launch velocity. 1,300 m/s is a  more "typical" velocity. 

Barrel wear with APDS rounds is generally less than with conventional rounds. Discarding-sabot projectiles are lighter and the barrels see less time with the projectiles 'inbore''. The projectiles usually have plastic rotating bands and bore-riding surfaces; reducing frictional and engraving forces. Initial propellant gas temperatures may be higher, but total propellant energy down the tube is about the same - pretty simple thermodynamics when specifications dictate a maximum pressure. 

Very high rate of fire without spreading it out over multiple barrels isn't common with 20-30 mm weapons and might require some active cooling. 

.338 caliber might be problematic beyond its thermodynamics. High rate of fire means high temperatures. 4,700 fps would require an awfully light projectile and/or high pressures. The unit cost of ammunition with such light projectiles (read as poor flight and terminal ballistics)  kind of works against the idea. .50 caliber attempts have not worked well on a cost/benefit basis.