While on the PD many years ago, I developed a subsonic load for a suppressed Model 700 Remington in 30/06. The 1 in 10" twist rate limited the maximum weight bullet choice to a 190 grain spitzer boat tail at 1,100 fps.
The above load was very accurate out to 200 yards, but that was it. After 200 yards it went wherever it wanted. Of course 200 yards is plenty of range for a subsonic load.
Another note is that from full power loads to the subsonic load, you added 12 MOA to the scope for a zero at 100 yards. With suppressor screwed on, you had to add another 12 MOA for a total of 24 MOA elevation from the standard configuration to the suppressed and subsonic configuration. This was with a hunting weight barrel.
I will also note that if you plan to reuse your cases over any length of time, the headspace length of the cases will shrink. This is not due to the firing pin strike as many shooters believe, but due to the pressure in the primer pocket being higher than the pressure in the case. This is what forces the case forward to shrink it. The solution is to enlarge your flash hole by at least 25 percent. Cases so modified will only need to be neck sized for much of their life as the HS will remain constant. Segregate these case for only low pressure loads. I have been doing this for over 40 years with perfect results.
Back when I developed the 190 grain MK load, there was no option of the 300 Blackout. To be honest, I wouldn't pursue the Ought-Six option as you already have a Blackout and its small case capacity and rapid rifling twist is ideal for your need of a subsonic load.
These days, my light loads use cast bullets in the Ought-Six.
