Load manuals list Varget as being fine for M1A's as well, I know it worked great in my Garand as well.

I load Varget for my bolt gun and its the best I have found, but in the M1A the pressure is still too high when the action begins to unlock. The brass gets strecthed, the rims can get ripped off, and the op rods can get stressed and bent. I know its in the manuals, and it does cycle, and it does shoot accurately, but why risk it. The AR platform rifles work across a wider range of port pressures....Varget should work well.
The M1 Garand is built to last....

IMR 4895 is best in all 5 of my 308s, all get .66" groups or smaller with 168 gr bullets. Other good powders are the new CFE, Varget and RL-15, H4895 works almost the same as IMR 4895.

For 150s, I've had good luck with RL15 & Varget. Great luck with VV N-540. It's nearly identical in burn rate to RL15 & Varget, but gave me better accuracy/lower extreme spread. It's worth a try if you can find it.

I can say that if/when available I use a lot of Varget..but today
I reloaded some mil brass w/168 gr Sierra's and 42.0 grain of RL15 and took my rifle to the range for my buddy to shoot..he shot 3 different handloads and projectiles and 1 factory..all pretty much same point of impact and touching hole groups. Rifle is setup L/H and he is Right..and a knob twister..couldn't
leave scope alone..but he still got touching or same hole groups..and the RL 15 shot 4 shots in about a nickle..I never even fired my weapon..this was in the rain..Don

I'm suprised nobody mentioned IMR's XBR8208, I've been using it with 150 grain noslers with great results. Its very close to 4895's burn rate.

I load 43.8 varget with a 165 harnady btsp and get great results. Good hunting load. It is also out of a 20 inch 1:10 barrel.

Also 4320 aint too bad, but its no varget.