You get into some problems with flash suppressors and other muzzle devices with sabot rounds, one reason our M48A5 tanks that had a big honkin' muzzle brake on the end of the main gun tube didn't have a 90mm Sabot round available, and our M60 tanks with 105mm barrels without a muzzle device did get the Magic Bullets. There are other considerations with tank guns, of course, but a lot of the basics still very much apply.
Also, very fast burning powder, polygonal rifling, increased head space, less twist, much higher chamber pressures, aluminum cases or steel if necessary and a hammerforged stainless barrel. Heck, make it out of monel or nikasil or whatever stands up better than stainless. Invent a new metal if necessary. Whatever it takes to increase pressure and velocity.
But without corresponding increases in recoil or muzzle rise in full-chat, muzzle blast, or heat signature of the barrel in thermal viewers. You know that they're tinkering with a liquid-cooled M16A2 barrel in hopes of keeping the heat sig down?
But not at the expense of accuracy. Some of the ideas I just threw out may hurt accuracy.
And weight. Even the weight of one additional magazine is critical in a world where guys toss the Joker out of the card deck and saw the handles down on their toothbrushes and spoons.