Neither the .408 Cheytac, nor the .416 Barrett are supersonic at 3000 yards.
.408 about 2100 yards.
.416 about 2500 yards.
I don't know about how stable the bullets remain at transonic, and then subsonic speeds, but clearly 3000 yards is outta the realm of realism.
Elevation can be compensated for by means of a substantial down-angle base.
You give up on your 30mm cannon?
If you're really interested in ELR shooting- go over to Snipers Hide, there's an entire section on the subject.
07 FFL /Mosin-Nagant Custom Shop/Bent Bolts
Genuine Cerakote Applicator- ALL FIREARMS
General Gunsmithing and Re-Barreling, muzzle threading and brake installations