The fastest I've heard of (wildcat) is a .22-243, 30 grains, approaching 5,300 fps. Years ago a friend and I built a pair of .22-'06 rifles, but with bullet construction being the limiting factor, we only got around 5,100 fps from them. Not practical for anything, but fun - at least until the barrels burned out.
I've heard that stainless barrels were a bit more resistant to wearing out with high velocity rounds. True or not? Also, would moly coating the bullets and bore help out much?
I wonder what sort of RPM those 5,300 fps projectiles would be turning at? Probably would be very difficult to keep them from coming apart before reaching the target. Probably would need to use solid bronze or copper projectiles, I would think.
i hear the the DU sabot rounds for the M1A's 120mm gun hit like 5300 fps! as for practivle bullet velocity i dont think any lead alloy bullet can handle better that 5000 to 5300 fps, most rarely excede 4000 to 4300 fps. i have read of tests using special polycarbonate blocks in magnetic linear accelerators, the old railgun, that could hit nearly 20000 fps. but it was just tests under controled conditions and required enough power to make a satellite based laser practical.
fps from .22-284 wildcat, using the sintered iron bullet from .22 short Gallery loads. 5" groups at 100 yds, and the bullets left a visible 'trail" in the air, because they were disintegrating in flight.