For one thing, you didn't tell us how you loaded your brass in regards to OAL. Where did you seat the bullets off the lands? Another thing... until you get those cases fireformed to your chamber, I wouldn't expect a lot in the accuracy dept. I would suspect, however, if you were off the shoulder to amount to much, you would be getting early case separation at the least, and quite possibly some gas leakage and denting of the case shoulder and body. Any signs of this?
My manuals don't list RL 19 for the 280 with 150 gr. slugs. I personally have had my best luck with IMR 4350 in the .280 Rem.