I shoot a lot of sub 308.
One rifle has a 1/11 twist and does great with trailboss and 150 flat noses.
The other has a 1/10 twist and likes the 180 round nose better.
Both loads and rifles will shoot about 1.5 inches at 100. A little better may be possible, but that has worked for everything I have needed it to. Consistency is much more difficult than with supersonic loads.
If I were going to have a dedicated .30 sub gun, it would definitely have a 16 inch barrel and fast twist. 1/7 or 1/8. For subs only, id go with the 1/7 and use the 240 smk (think the 1/8 can handle it too depending on conditions...)
I shoot a lot of 308 supersonic as well, so I have been in a similar boat. No reason sub 308 will not work for you, but you have to understand that the case fill will not be right to give you very very good consistency. Even with trail boss it is difficult to get anything to shoot lights out. Just start at probably 12 grains and work your way down, preferably with a chronograph, till you get velocity and accuracy you want. 14 grains will just about fill the case, but you will probably end up needing 10.x grains. The powder company has some decent info on their site about the powder.
Be mindful of seating depth as that can have a big effect on the load. For me, I had the best luck with federal match primers. Some people say to widen flash hole and use mag primers (probably helps with less fluffy powders), but using trail boss i have never had an issue.
What twist are you running in your rifle? The short stubby bullets are easier to stabilize. You can always load a regular pointy bullet backwards in a pinch and it should do fine. the 180 rn is about 1.1 inches I think, and stabilized from my 10 and 11.25 twists. Be sure to test them for stability before a suppressor or anything ends up on the muzzle. Shoot paper or better yet paper behind cardboard at various ranges, especially up close. If you get clean round holes you are good to go. Keyholes/ oval is problematic lol.
Definitely neck size when you can. The brass will last a long time. I would like to swap a 300 xxx subsonic barrel on to my savage (mcgowen usually stocks them) but there are often ejection issues due to the super short case and rem 700 or savage ejectors. If you are running a T/C then those issues would not be there...
Maybe try the subs and once you get a can see how you like it? If the 308 subs don't live up to expectations then I would get a 300 whisper or blackout barrel on the TC. The tiny case gives MUCH better consistency. You have to get some additional tooling, but for a subsonic platform it is tough to beat it.
