Gentlemen,
I am attempting to load for a friend of mines .308 internally suppressed bolt gun. Its 18.5" Bbl 1:11.25 twist as far my measurements reveal. I know very little about the suppressed world. He left me with the ammo that was sold with the gun. At this point, I have not broke one down to weigh the bullet. The bullet is a round nose with a deep concave with lead showing. My guess is its in the 200 grain range. Where do start with this combo? I do not have a chronograph.
OK, you have a couple issues, first the twist rate is a bit slow for the heavy bullet you list , the heavy long 200 to 240 gr projos dont respond well in short barrel slow twist. 1 in 10 is better, 1 in 9 is gooder yet. since you are stuck with the Palma twist, go with a 150 gr, or 170 gr 30/30 bullet, and a charge of TrailBoss. I use a Berrys 150 gr plated that I moly coated ,, with 9.5 gr TB up to 10.5 depending on your barrel length. You can probably go to a 180gr, but that would be it in my opinion, you dont want an unstable projo in the Can. Be careful when working up the slow loads that they actually make it out of the barrel, that is why I moly'd mine, slicks them up a bit. Since you have no chrono, you'll have to listen for the sonic crack to determine if the load is to fast. Not optimum, but its doable. I would pull the rounds you have and see what bullet weight you have, and what powder charge, might be able to tell what kind of powder, maybe.....
Again, if the twist is that slow beware of the heavy bullets. Just because they might be great in some one elses suppressed rifle doesnt mean they wont be unstable in the Can in yours, I had to rebuild the baffles in the Under-Sheriffs can just because He trusted untested ammo. Seeing as how your rifle is integrally suppressed, it is even more important ....
Hope this helps
Trail Boss and 220grn bullets really shine in a suppressed 308.
I use Trail Boss with a 170 Nosler Partition made for a 30-30. I think 11 grains. It’s quiet, I use it to kill trapped pigs.
The Hodgdon Annual Reloading Manual for 2019 has a good section on the 300 AAC Blackout, including subsonic loads with 200+ grain bullets.
Treat unknown reloaded ammo as if it was loaded to 250,000 PSI!!!! I was at a gun show and saw a guy that was ready to buy a box of 257 Weatherby reloaded ammo that had 6 different bullets, 4 different cases and 5 different primers. He was getting it for his new $1400 Mark 5. When I asked him about it, he said the seller said it was safe. I asked him what kind of drugs he was on.
Treat unknown reloaded ammo as if it was loaded to 250,000 PSI!!!! I was at a gun show and saw a guy that was ready to buy a box of 257 Weatherby reloaded ammo that had 6 different bullets, 4 different cases and 5 different primers. He was getting it for his new $1400 Mark 5. When I asked him about it, he said the seller said it was safe. I asked him what kind of drugs he was on.
That guy was stupid. I will not shoot another’s reloads.
Treat unknown reloaded ammo as if it was loaded to 250,000 PSI!!!! I was at a gun show and saw a guy that was ready to buy a box of 257 Weatherby reloaded ammo that had 6 different bullets, 4 different cases and 5 different primers. He was getting it for his new $1400 Mark 5. When I asked him about it, he said the seller said it was safe. I asked him what kind of drugs he was on.
What kind of drugs are YOU on?
If the bullet and barrel twist that you are using will not stabilize subsonic, why try? Here is a program that I use to make comparisons.
http://mcgowenbarrel.com/twist2/