I would prefer the 243 Win (awesome field cartridge), but would rather have 20" length barrel. All cartridges will perform on 18", 16.5" will work but may not feel as balanced as 18" or 20". 6.5 CM will suffer most velocity loss due to longer projectiles and greater bearing surface, which yields greater friction (there's a reason Hornady introduced the cartridge with a 26-inch barrel in their marketing), but all 6.5mm have this problem regardless of barrel length. It is one of a few reasons the 264 Win with a 24" barrel barely yields 270 Win velocities. This is not anything worth losing sleep over however. While the cartridges you mentioned will suffer a lower expansion ratio than typical due to shorter barrel length their powder capacities are not substantial enough to be of any real concern. Regarding expansion ratio (ratio of volume of powder in chambers [case volume plus bore volume in grains of water] to the volume of case powder in grains of water). As most already know its the reason most 3.4" cartridges (6mm-06, 240 Wby, 25-06 and magnums are on 24" barrels minimum, but several should be using 26" barrels and greater if looking for maximum expansion ratios and velocity/performance.
