Hi, I've done a little work with subsonic ammo in my .308 here in the uk and settled on a load of 7 grains of unique with a 180g round nose flat base bullet I didn't bother drilling out the flash hole or anything like that . what I would suggest is if your using a moderator develop your load with it off and check your targets for bullet yaw before you try the loads with your mod fitted as if the bullet is very unstable it may foul the baffles!! you don't need the super high b.c. of match bullets because realistically you will only be shooting to 200 yards max with any real sucess ,I was getting 1'' to 1-1/2'' groups at 100yds and settled with that. my rifle has got a 20'' factory remington barrel which shoots 1/2'' groups at 100yds with tap ammo,federal gmm and my amax handload (168g) although I can only manage a real 3/4-1'' generally I've really got to be on my game to shoot 1/2'' regular but it is nice to know it's you and not your kit thats at fault!
If you get an old lyman reloading manual check out the cast bullet loads with pistol powders it's a good place to start.
good luck its great fun,,,,Russ