I'd have to assume that the shorter barrel could somehow mean less of the initial velocity is lost due to less friction as the bullet passes down the barrel. Obviously any given bullet when fired dow 2 barrels of different legnths is going to spend more of its energy going down the longer barrel as any contact with the barrel, rifled or not, is going to rob it of some of the initial force, a shorter barrel and the journey is that much shorter meaning that any contact that does happen, happens less, if you see what i mean?
Most of the time we're all, oh look, rifle, longer rifle and higher muzzle velocity, but we often forget to take into account that different rifles often fire a different round, even if it's the same calibre, i mean the countries all have their own methods of manufacturing, the starkest example being the 7.62 i think it is, which has the russian made (Somewhat "original") variant and the "NATO" variant, which are completely incompatable with eachother (Last i heard anyway)
The differences between the KAR98k and the G41-43 are negligable anyway, a matter of what was it? 15 feet p/sec? I've got a funny feeling if you get behind either rifle your shoulder will start to hurt after a while, and if you get infront of either rifle you're gonna drop like a sack of spuds.