Anyone out there loading subsonics with the Hornady 255gr Sub-X in a 7.62x39 yet?

I picked up a cheap 16” Ruger ranch rifle in 7.62x39 just for the purpose of shooting subsonics for Rockchucks in the cherry and pear orchards. The bullets just arrived, I have a pile of Starline brass. I have no clue where to start. Hornady told me they currently had no load data for the combo at the time. Plan on running it with my 36M suppressor.

I’d be looking closely at 300 blackout data and selecting similar powders and extrapolating loads if I couldn’t find specific data. The subsonic stuff is normally low pressure so less risk than at other end of the spectrum.

I loaded up a ladder test using CFE BLK. In .2 increments. Started at 10.2gr and ended ar 11.8grs. I’m going to shoot them with the Labradar here shortly. Without the suppressor attached. To confirm velocities and make sure they are stable. The last thing I want is a baffle strike on my 36M.

Well that didn’t go as planned. 11.8gr of CFE BLK got me a whole 376 fps. I’m going to up the charge in 1/2gr increments until I hit 900fps. Then I will attach the suppressor and fine tune from there.

I'd look at pistol powders, since you don't need to run a gas system.

QuickLOAD predicts 6.0 gr Unique will give 1080 fps, at 31K psi max pressure, and 52% case fill.

As long as that bullet is, I wonder how much room there is for the powder.
And will the twist on that Ruger stabilize that bullet? What's that rifle's twist?

I just looked at Ruger's website and it says that the 7.63x39 Ranch has a 1-9.5 twist.
Edit... Berger's bullet stability calculator says that the 255gr will be marginally stable at 1100fps but I guessed at he length of the bullet.

FWIW, I've had a couple different loads that were "good enough" using CFE BLK, for subsonic 7.62x39. Granted none were as heavy as your sub-x. Hunters supply 238 gr was one.

Cast bullets are more effective for CF subsonic loads. Cast is also shorter than jacketed for a given weight.

I use Lil Gun with 208 Amaxes for subs. Hodgdon shows up to 230 Gr bullet in their load data, but gives you an idea. Most data discussed in forums is for AR’s and trying to get them to cycle a round. But you don’t have that obstacle.

300 BKT Subsonic Load Data