I am plunging into the world of reloading. A member here is willing to teach my son and I some stuff (very thankful). I do have a questions that my sound "dumb". I was looking to start with 308 and 300 blackout. I see "load data" for subsonic 300 blackout on the bullets web site. 308 subsonic is not listed. Is the powder load the same in grains for both 308 rifle as 300 blackout given its the same bullet?

Both guns are bolt actions and same barrel length.

Yeah they'll both have there own separate load data.
If you Google hodgdon sub sonic data there's a pdf with several loads for both those cartridges

The powder load is not the same for a 308 as for a 300 blk. Best powder for 308 is Trail boss as it made to take up the volume in a 308 case at sub sonic load levels. Blackout needs a different powder for heavy bullets, such as a1680, H110, IMR 4227.

Trail boss is good for lightweight subsonic loads in 300 blackout for plinking (110 grain Armageddon’s for example) but it won’t function an ar.

I’ve been working on some 308 today, using 190 grain sub-x bullets. Started at 11.5 grains of TBoss, wound up at 10.7 grains for 1050 fps out of a 20” 10 twist barrel.

Same bullet in a 16”, 7 twist 300 blk was 1050 fps with a load of 11.2 grains of A1680.
With subsonic loads always start on the high side and work down to your target velocity.

Thank you guys. The case size is something I didnt even think about. Makes perfect sense.