My CZ 550 American is on the way. What are the best powders for these bullet weights? I've got RL-15 and IMR 4320 on hand. Thanks in advance, Okie John.

My favorite 270gr Speer load -
58.5grs Re-15
Norma cases
WLR primers
OAL-3.28"
This load gives me 2375fps out of a 24" barrel, and will shoot MOA all day long! My 286gr Norma Plastic Pointed Dual Core load uses N-201 exclusively, so I guess I can't help you there.
Good shooting!

Here's one for the 286-grain Nosler Partition:

Reloader 15 - 59.0 grains
Norma brass
WLR primer
Combined overall length - 3.345"
Chrono - 2,420 fps (warm ambient temp ~85 deg F)

In my fairly new CZ550 American this load consistenly shoots 3-shot groups sub-MOA. On a good day it has done a number of .5 inch groups at 100 yards off a stable bench.

These rifles are real shooters.

phil, are you getting pressure signs? Okie John.

These are starting loads using H4895 which I find a very good powder with this cartridge. Again, these are starting loads:

270 Speer Norma Case H4895 56 Grains WW LR primer

286 Varied Norm Case H4896 56 grs WW LR primer