For me, in a .303 i would use a gas check.
With a water quenched bullet, a gas check and the proper lube and number/size/shape of lube grooves, you could easily reach jacketed velocity with minimal leading if the bullet fit is correct.
The really important and often mis-understood function of a gas check is to prevent the grooves formed in the bullet from widening when it slams into the rifling at high speed, which forms a void on the leading edge of the grooves in the bullet on the far side of the lands allowing the gas to escape around the bullet... if that makes sense. the copper base will grip the lands better than lead alloy and it won't widen at the rifling marks, preventing the gas from slipping up around the boolit. This is the same logic behind why you should use water quenched hardcast bullets that are sized .001"-.002" over groove diameter for high speed loads.
in a nutshell, over 1400-1500fps... generally speaking, you want to use a gascheck. much over 2000fps and you need to use an appropriate high speed lube which is usually a little harder.
Does your bullet design (in mind) use tuble lube or stick?