Another option is going back to HVAP, where there isn't actually a sabot at all, just a light metal shell around a hardened high density penetrator. The disadvantages are that because the overall round is lighter than a solid round of the same size, it loses velocity rather quickly.

 

However, this is likely to be less of an issue in the case of a rail gun, where maximum range determined by tactical considerations (200-400m is still going to be effective max for infantry combat, LOS limits heavy gun ranges) is going to be less than the effective ballistic range of the round.

 

Small arms rounds that are intended to be compliant with applicable conventions will have the problem that the light shell is going to deform in the target's tissue which is a no-no. It could be possible to design the shell to use a light, hard, hollow material that would break up on impact, which is even more effective at wreaking havoc on tissue than deformation is, and might get by on the letter of the law.

 

For havy rounds, using a variation on the old fashioned HVAP might be even more useful. The characteristics would be that the outer cylendrical shell would be made of a light, relatively soft material (perhaps plastic) designed to deform in the case of a frontal impact. Inside this would be a layer of pellets for fragmentation effect. In the front would be a HEAT-style shaped charge cone with an aluminum liner. The front of the round would have a streamlined nose cone designed for secondary frontal fragmentation effects. This round could be fired in several modes. It could act as an HVAP round against heavy armor with the inner long-rod penetrator doing most of the damage (the shell could also be detonated to provide secondary shock damage and possibly spalling inside the tank in HEP/HESH fashion). Against light armor it could be fused to explode on impact acting as a heat round with secondary fragmentation effects. Against infantry in the open or in defilade it could be fired as an airburst munition. Against airborne targets it could be detonated via a proximity fuze. Agaisnt bunkers the penetrator would punch through while the shell pancaked and then detonated as a HEP device.

 

Another option would be if rear portion of the penetrator was internally scored (I'm thinking a star-shaped hole down the center, possibly with horizontal serration as well) then filled with explosives. It could function as a long-rod version of the old-fashioned HEAP warhead. As long as the internal scoring didn't compromise the round's penetration significantly, a properly timed burst would dramatically increase behind armor effects.

 

Combined with the above shell design it would improve overall fragmentation, especially against frontal proximity targets in the anti-aircraft role.

Thinking about it a bit more, the shell should be hard and termperature resistant in the case of a rail-gun projectile. It could still be designed to fragment in the case of a direct impact allowing the casing to pancake.