Some examples of 6.5x54 M-Sch velocities vs. barrel length from the 1940 RWS handbook:
Load #8, the standard load, 10.3g = 159gr rn, 2.75g = 42gr R1 powder, pressure 2550 atm = 36200 psi :
45cm = 18" = 668m/s = 2192 fps, 50cm = 20" = 685 m/s = 2247fps, 60cm = 24" = 712 m/s = 2336 fps, 68 cm = 26.8" = 729 m/s = 2392 fps
"Hot" load #9, same 159gr rn bullet in front of 2.9g = 45gr R1, pressure 3100atm:
45cm = 710 m/s = 2329 fps, 50cm = 727 m/s = 2385 fps, 60 cm = 756 m/s = 2480 fps, 68 cm = 774 m/s = 2539fps.
load #126, 10.3g = 159 H-jacket bullet in front of 2.65 g R2, 3200 atm = 45450 psi:
45 cm = 711 m/s = 2333 fps, 50 cm = 732 m/s = 2402 fps, 60 cm = 760 m/s = 2493 fps, 68cm = 774 m/s = 2539 fps.
This table may explain why the 6.5x54 M-Sch standard load, listed by RWS at 2192 fps, is listed in the 1925 Kynoch catalog at 2395 fps: As rifles were more popular than carbines in Britain, Kynoch simply used a longer test barrel, 24" instead of 18".