I've been loading 300 blackout rounds for a little while now. I measured the fps of the rounds when I shot them and they were just under the speed of sound. Once I saw how close they were to not being sub sonic, I lowered the powder by a few tenths of a grain to make sure I don't ever have any strays that go over the limit. How close do y'all ride that line or is it harder to break that sound barrier than I'm thinking?