I am a fan of subsonic loads and find more pleasure in reloading my own which is cheaper and more rewarding. I don't have any information on the .308 but do on the 5.56mm.
The load data comes from Accurate Smokeless Powders Volume 2 on page 187.
Data:
Powder: Accurate Solo 1250
Primer: REM 7 1/2
4.2 Grains with the HDY 60 gr SP
4.5 Grains with the HDY 75 HPBT
I've reloaded both these loads and fired out of my Colt M16A1, 11.5" barrel, and AWC Raider Suppressor. The results were as follows:
60 GR SP - at 25 yds, .64 group and avg vel of 975fps. At 50 yds, the group was .90.
75 GR HPBT - at 25 yds, .45 group and avg vel of 920fps. At 50 yds, the group was 1.25. The bullet also started to tumble at the 50yds based on the impact print on the target. I did not have a chance to load another batch with more powder.
Conditions on the range at the time was 85 deg and 46% humidity. Calm wind.
I have tested the Engle Balistic Research ammo and found the accuracy on my configuration to provide about 1.5 inch groups at 25 yds and 3.0 inch groups at 50 yds. At 100 yds with the EBR, the groups were more like 7 inches and the vel was between 870fps and 1020fps.
That is the extent of my testing with my own loads.