The mystical sabot ... we had heard rumors of a sabot 338 outside, something smaller inside (not saying what at this time). We ran numbers on it and it suggested the diminutive 338 Spectre would push the light load at 2400+ fps from a 16" carbine, giving it 500+ yard reach ... No word on the availability of those sabots, however.
I do have some 6.8 basic brass and formed a 338. The original version from the 10mm Mag brass holds 28.8 gr water. The 6.8 based version holds 25.6 gr water. So more that 10% reduction in volume. I used the higher pressure (50000 v 37500) and lower volume and compared performance in QL for a 16" carbine
160 Barnes X - old 1800, new 1920
180 BT - old 1800, new 1900
200 BST - old 1700, new 1750
225 SST - old 1550, new 1600
250 SMK - old 1450, new 1500
So with exception of the 180 gr and lighter bullets, all it gets you is 50 FPS and again, the intent was the big heavy ones at subsonic speed ...