Me having suppressor experience, and loving the 45/70 here are my thoughts. Whatever powder you use, don't go below 30% case fill. Use a stout crimp to encourage complete powder burning. I have some data if you need it, don't use cast bullets with a suppressor, a lot of centerfire suppressors aren't user serviceable or cleanable. At your target range, say 50 yards, put your cellphone in the area of the target, and record a video of the shot, see if the sounds are acceptable to you.
Here is 308, Sierra 180gr roundnose that I moly coated. About 950 fps at 50 yards.
.308 Subsonic
