It's probably me but I just shot 20 test rounds of sub-sonic .308 loaded as follows:
Winchester Brass
175 SMK
Fed 210 Primer
Trail Boss
My groups were good ~.5-1" at 50 yards with no tumblers but the speeds through my Chrony F1 were all over the place:
Grains | Speeds
10.2 | 1059 | 847 |...