The issue I ran into by trying to use my 308 for subs was the twist rate. The trail boss powder you mention is perfect for both 300wsm or 308win as it fills the case up to a decent amount. Obviously the 308 win is the better choice as it will have more case fill that the 300 short mag. The powder column seamed to be fine and ignite great every time. Even after I intentionally tried to flatten the powder columns. However, doing a paper test similar to an archery paper test I found that the bullets were wobbling or not stabilized very well. This wasn’t an issue right out of the muzzle but showed up around 30 yards for me. It’s going to be different in every rifle barrel so maybe do your own tests. The 1:10 twist seamed to stabilize the 165’s ok out to 100 yards. I wanted more energy so I tried 180 and 220 rnsp. Those were the issues for my rifle. The 220’s you could visually watch them fly down range. Fun to see but by the time I got the velocities fast enough for a nice bullet flight I was beyond subs speed. If you want a .30 bolt gun for subs, I’d order your barrel with a 1:9 or 1:8 even better. Which makes it less effective for other stuff but kinda bridges the gap and could help the heavies with lower velocities fly better. This is why 300 BLK has faster twists which you may know already. You need more speed to spin the heavies at slower velocity’s. As you test these, I’d recommend doing it without the can attached until you are certain your bullet isn’t producing a key hole out the muzzle. Save yourself the baffle strike and attach your can after you’ve established velocity and paper tests.
