I have both a M1903 carbine with 18" barrel and a half-stocked rifle by Gibbs with a 22" barrel. The difference is in the ballistics: The 10.3g = 159grs RWS standard factory load is listed with a MV of 670 m/sec = 2198 fps from a 45cm = 18" barrel. This velocity makes it barely illegal for use on any hoofed game except roe deer, as a minimum energy of 2000 joule at 100 m is required. From my 18" barrel it does slightly better, averaging 680 = 2230 fps over my chrony, but still illegal. From the 22" barrel of my Gibbs it clocks at 705 m/s = 2312 fps, making it legal. BTW my handloads with the 160gr Hornady do 735 m/s = 2400fps from the 22" barrel, duplicating the prewar #9 RWS load.
These results conform quite well to the old table "velocity vs. barrel length, 6.5mm", page 225 of the 1940 RWS handbook.