I got to shoot my rounds I reloaded yesterday.
I shot the 4.0 gr of trail boss, and the 3.6 gr of trail boss.

I liked the 4.0 better. It held zero (I shot it at 10yds, 25 yds, and 50 yds). I didn't try it past 50 yds because I figuered I would not ever need it past 50 yds for varmints.

I shot about 5 rds of 3.6 but it just seemed "too slow".

I "tested" the 4.0 loads in a phone book, wet newspaper in milk carton test, and 1/8" mild steel plate test. The 3.6, 4.5, and 5.2 where just tested in a phone book.

Remember, these are just subsonic rounds, with sierra 55 gr. game king rounds. I shot these from a 14.5" SBR with a Ranger 2 suppressor. They are extremely quite. the only thing I can hear is the hammer.
The 3.6 gr load did not expand and I could literally see the round flying through the air. The zero was off (4" low) and it had too much wind drift (5" drift) at 50 yds.
The 4.0 had the same zero and was better on wind drift. Only 1" drift at 50yds with 25 mph winds. The bullet did not expand in the phone book nor in the wet news paper. It did not penetrate the 1/8" MS plate.

Next I tried 4.5 grains of trail boss on the phone book test. It penetrated maybe 1/2" more than the 4.0 and it tried to expand. The plastic tip broke off and the nose opened very little. But it still did not expand. This round was still subsonic and only a little louder than the 4.0 load, but still nearly silent.

Then, I did a load with 5.2 grains of trail boss. This load broke the sound barrier and it had a nice crack of the bullet. Not what I am looking for, but in the phone book, it had about the same penetration as the 4.5 grains, and this bullet expanded somewhat.

So the bullet goes trans sonic somewhere between 4.5 grs and 5.2 grains. I didnt do any more loads to see exactly at how many grains it breaks the sound barrier.