I have a .280AI built very similar to yours...same barrel length and taper. My best load uses 58gr RL-22 w/ 168gr SMK. It averages an even 2900fps w/ a SD of 5 over my Oehler 35. At 300 yards I can consistently shoot sub 2" groups. I'm no BR shooter...so in the hands of someone better at reading conditions it could do better. I've gotten 139gr Hornady Interlocks to 3150fps using 62gr RL-22 and decent accuracy. For giggles I loaded 100gr Hornady HP's over IMR4350...got to 61gr at 3420fps with great accuracy.