These are the numbers I got shooting 175gr Sierra MatchKings in my AAC-SD(20"/1:10 twist) today. Should be pretty close to 178gr HPBT. Of course, velocities will vary depending on your barrel length. I saw no pressure signs all the way up to 49gr and may load some a bit higher looking for a new node. All were loaded at 2.860" OAL(magazine length). If you load to 2.80" I am pretty sure you will start compressing powder somewhere around 48-48.5gr. But, if you have a longer barrel you won't have a problem getting velocity without compressing powder.
44gr- 2300fps
44.5gr-2325fps
45gr- 2339fps
45.5gr-2385fps
46gr- 2408fps
46.5gr-2428fps
47gr- 2452fps
47.5gr-2565fps
48gr- 2584fps
48.5gr-2630fps
49gr- 2698fps