It's enough to make you crazy.
I thought I'd consolidate some pics and musings in several threads into one thread. In case I want to easily find this again or maybe it will come up in someone's search looking for the same thing.

As I found out a few days ago, there apparently aren't any published manuals on the 300 Blackout for both subsonic and bolt action (or single shot) loads. AR shooters like heavy bullets to help them cycle their action. In a bolt action, even at subsonic speeds, the powders used to push those heavier bullets are a little louder than they ought to be, IMO. And accuracy is so-so. At least I wasn't real happy with it. Although after viewing a boatload of threads including pictures of groups, maybe I wasn't doing that bad after all. At any rate, the majority of threads on the 300 Blackout are semi auto shooters looking for a reasonably accurate load to cycle their ARs. But once in a while you come across some for a bolt action rifle. Of course trying to get a charge weight and OAL can be a bit frustrating. So many threads guys either don't divulge all the info, or say they gave up on it.

So after burning through 80 bucks worth of 220gr SMKs, basically the best I could do and still be reasonably quiet was this. This was at 50yds BTW. And the odd thing was, I was more accurate with a slightly shorter length, 2.212 compared to 2.260.

And these ten shot groups using two powders.

At $0.40 a crack just for the bullet, not the consistent accuracy I was hoping for at 50yds. And really not all that quiet.
I'd have to give the edge to Accurate 1680 at an average of 1013fps. The Hodgdons had to be pushed at 1100fps to get decent accuracy. If I wanted to push them both just a little over 1200fps, accuracy greatly improved. As did decibels.

So after scouring more threads, this time focusing on lighter weight bullets, I was able to come up with some folks saying that the lighter bullets require pistol powders. I tried Unique, Hi-Skor 700X and TiteGroup. This time I was using some 150gr Sierra GameKings I happened to have.
I wish I had a decibel meter. At similar speeds all the pistol powders were MUCH quieter. Stupid quiet.

So far I have found the 700X to not only be the most accurate, without playing around a lot with charges and seating depths yet, but the ES is very good, too. TG was the worst here, but I sort of expected that for such a small charge. Unique was worse as far as accuracy. The same setting that was giving me 2.212 OAL with the 220gr bullets gave me 2.114 with the 150gr, so that's what I went with.

The cold bore shot is the low shot. The next three in one hole.

So the search continues, but at least I'm not burning through expensive bullets anymore.