I have 2-280AI one was build before spec 280AI and I fire form brass for that other I use Nosler 280AI brass. The spec I've been using IMR-7878ssc with Fed 215m primer,160gr AB and it's load in Nosler manual, they used IMR-7828. It has 27" Lilja barrel. I've also shoot 162gr ELD-X,168gr Berger and haven't had any luck with R-26.
The other 280AI I've been using 150gr ABLR with IMR-7828,Rem 280 case fire formed,Fed 210m primer and it's mid load from nosler manual. It has 26" Kreiger barrel. I did try Barnes 145gr LRX no luck and when I first build it, I was shooting Barnes 150gr TSX using H-4831sc,