Your OP is very vague...best for what purpose? Help us help you...details as to what you want the subsonic load to do would be most useful.
There are folks that want subsonic loads just for shooting through their suppressor to make sure it cycles.
And there are people like me that need an expanding subsonic load because I hunt.
I'm in SEPA and I hunt with my Axis chambered in 300BO. I use a suppressor, so I load with Lehigh CF bullets. Two years ago, I shot a doe with a 208gr Amax and it was a solid hit. I never got a blood trail...my friend found the doe months later. My beloved 208gr Amax bullet did not expand due to subsonic load. I made a bad choice and I won't allow that to happen.
If you need an expanding subsonic load, you are looking at Lehigh and Outlaw State bullets if you roll your own. There is a company on the 300BLKTalk forum that sells loaded Outlaw State ammo.