http://www.midwayusa.com/product/165542/hornady-bullets-303-caliber-and-77mm-japanese-3105-diameter-174-grain-full-metal-jacket-boat-tail-box-of-100

I want to make some subsonic 7.62x39 using a 10.5" barrel

Does anyone have any load data to work off of?

Seeing that 168gn ammo out of 300 Blackout is going about 1800fps out of a 16" barrel, would it be fine to just drop the initial powder of the rifle powder down to subsonic speeds?

I would be using the .303 .3105 174 FMJ BT bullets from Hornady.

The bigger bullet would take up more volume in the case so that would help with preventing a double charge like would happen in a .308 subsonic where there is too much case volume.

Or should I just stick with pistol powder? I have Winchester .231, but have been getting a lot of deviation (like 200fps) when I was using standard pulled 123gn's.

Thanks!

I've never tried it, but you should use a pistol powder.  

There is a good bit of data on the internet for subsonic x39 ammo.

I'd be worried about secondary explosion effect with fast pistol powder and a a mostly empty rifle case. Be careful with this idea.

Not that this can't be done (I've tried red dot and clays in 300 blkout),  it would just add to the pucker factor for me.