If you are going to build a custom rifle
for deer/antelope and want a smaller caliber then the cartridges I have used are:
240 Wby with 30 inch 1-8 twist barrel and 115 gr Bergers at about 3250 fps. You can match that with a 6-284, 6-06 or a 240 Gibbs etc so the exact flavor is up to you. The combination is easily capable to ranges beyond 1000 yards for antelope. I have not shot any of the eastern whitetails with it beyond 300 to 400 yards because of terrain limitations not because of cartridge limitations.
I shoot a 257 Wby with a 28 inch 1-8 twist barrel and it stabilizes both the 130 gr and the 142 gr Wildcat HP RBBT bullets. Accuracy is excellent. It will easily take elk out to the range you want to kill antelope. It is also capable of antelope beyond 1000 yards. You should contact Paul to see when he is going to make some bullets before you build the rifle and preorder enough for the rifle - maybe 500 - 800 bullets. I prefer the 130 grain bullets with the Wby case capacity.