Good question Paul. Here's the information:
Rifle: 18" FN-FAL type, gas tuned so it would not eject. I did a short study and determined that the gas setting made basically no noticable difference to muzzle velocity, FWIW. The 20" was also a FN-FAL type.
Environment: 800' elevation, 18 degrees F.
Remington Factory 150gr: 2548, 2568 fps in 18"; 2686, 2697, 2718 fps in 20"
Radway Green mil-surplus, 150gr: 2534, 2519, 2548, 2540 fps in 18"; 2729 2720 2741 2729 fps in 20"
Load I was testing: 150gr SP over 46 .. 50 gr of WIN 748. OAL ~2.74".
46gr: 2518 2510 fps in 18"; 2650 2655 in 20"
47gr: 2471 2500(??) in 18"; 2711 2758 in 20"
48gr: 2545 2571 fps in 18"; 2772 2875 in 20"
49gr: 2599 2603 fps in 18"; 2826 2832 in 20"
50gr: 2692 ---- fps in 18"; 2881 2898 in 20"
For the 18", no primer "cratering" or other signs of overpressure were present in any of the fired cases. For the 20", there was slight cratering in one of the 49gr primters, and both of the 50gr primers.
-z