I haven't used Retumbo in my 280 Ackley... But extensive research tells me it is a little "too" slow.
Reloder 26 is going to get you the best velocity and have just under 100% load density, I've put RL26 through its paces, it is very good stuff.
If I had to pick a next best for this bullet weight range, it'd be IMR7828 SSC.
RL26 is very stable in temp swings, I get .3 fps per degree using 160 and 168 grain bullets, I haven't worked with 175-180 gain bullets, don't plan to either.
7828 just might have the edge with 175-180 grain bullets.
This a
calibrated QuickLoad rundown of my load: 61 grains of RL 26 under a 160 Accubond, seated to ~3.34"....I'm thinking about dropping back to 60.5 grains to ease up on the brass a bit, but I'm gonna wait and see how the primer pockets last before I make any changes....its a very stable load, I've tested it to 800 yards in a temps ranging from the teens up to 94 degrees, it does slow down a very little in colder weather, but it is very linear and predictable.
This is calibrated for 94 degrees, a hot day.
Code:
Cartridge : .280 Ack Imp
Bullet : .284, 160, Nosler AccuBond 54932
Useable Case Capaci: 64.438 grain H2O = 4.184 cm³
Cartridge O.A.L. L6: 3.340 inch = 84.84 mm
Barrel Length : 24.0 inch = 609.6 mm
Powder : Alliant Reloder-26 ?
Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.821% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !
Step Fill. Charge Vel. Energy Pmax Pmuz Prop.Burnt B_Time
% % Grains fps ft.lbs psi psi % ms
-08.2 88 55.99 2789 2763 47679 11328 100.0 1.334
-07.4 88 56.49 2812 2810 49080 11375 100.0 1.316
-06.6 89 57.00 2836 2857 50520 11422 100.0 1.299
-05.7 90 57.50 2859 2904 52002 11468 100.0 1.282
-04.9 91 58.00 2882 2952 53526 11514 100.0 1.265
-04.1 92 58.50 2906 2999 55094 11559 100.0 1.249
-03.3 92 59.00 2929 3047 56707 11604 100.0 1.233 ! Near Maximum !
-02.5 93 59.50 2952 3095 58366 11648 100.0 1.217 ! Near Maximum !
-01.6 94 60.00 2975 3144 60074 11691 100.0 1.202 ! Near Maximum !
-00.8 95 60.50 2998 3192 61832 11733 100.0 1.186 ! Near Maximum !
+00.0 96 61.00 3020 3241 63642 11775 100.0 1.171 ! Near Maximum !
+00.8 96 61.50 3043 3290 65505 11816 100.0 1.157 !DANGEROUS LOAD-DO NOT USE!