I have been shooting mostly subsonics for the past few years.
I have used 300 Whisper, 308 Win and 45-70 and the target choices are goats and pigs.
A few things came to mind on subsonics loads.
1. they are not quiet unless you use a silencer.
The only thing you will get is the fact that there is no sonic boom associated with your projectile's flight.
The muzzle blast is always there, but less noisy.
2. they have banana trajectory.
you would need to be close, really close otherwise you would need a very accurate range finding method.
3. the heavier and wider meplat the projectile, the better it guarantee kills. 30-06 is better than 270
heavy bullets usually retains their energy better than lighter ones.
4. If you hit something solid on the way or pass the target, you would need to ensure that the ricochet is not going to harm others.
in Supersonics loads, the projectile often either disintegrated or embedding itself into whatever.
in subsonics, the projectile WILL ricochet!!
5. best way to find your load is to start at the lowest supersonics and reduce slowly to subsonic.
Temperature will affect the speed of sound, and trans-sonics speed affect accuracy.
6. use FAST burning powder!! they are usually more bulky (Red dot and trailboss is great for 30-06)
and they ensure that the bulk of the detonation is contained within the heaviest (stronger) part of your barrel.
7. Soft cast bullets are often better to use than hard jacketed ones.
cast bullets are naturally lubricated and is easier to dislodge if you made a boo boo
8. Check your barrels after you shoot to ensure that the projectile did indeed get out off the barrel
always listen to the sound, if you hear bloompfth isntead of booom.. then you may have problem.
9. The length of your projectile vs barrel twist have a GREAT impact to your accuracy.
The longer the projectile, the faster the twist got to be.
Having said the above, subsonics are FUN, they are almost recoil free, economical to shoot and safe to your ears.
For your starting load, try the trailboss on 30-06, using IMR load data.
Have fun, and tell us how you do.