I am building a new rifle to shoot at long range. I want to match the trajectory and drift of my hunting rifle, with a cartridge I can comfrotably shoot all day. I want a cartrdigde (.257 or smaller) that will shoot a bullet with a BC of .4 or better at 3200 fps. There don't seem to be many options in the bullet department. The best compromise I have found would be a 22-250 with a 1:8" twist and a 75 grain Hornady A Max. I already have a 25-06 and don't need another one. Any suggestions for a different caliber or bullet?

how about a 260 Remington with 107 gr. MatchKings (.409) @ 3100 fps

Check out Lost River bullets.

If it's just for LR varmints or target work, a .260 w/ 24"-26" bbl. would get you close w/ 107gr SMK or 108gr Lapua. The 95gr Hordy. Vmax is .369 & the 100grNBT is .350, both can easily reach 3200fps from a 24"-26"bbl.

.25-284 on a standard length action. Plan for 130gr VLDs. I may try this myself. Nate here's what I'm looking at

Go for the .260 Rem if you want those high ballistic coefficients, or try another 6.5mm chambering, up to and including 6.5-284 and 6.5-06.

LLS