Mackay Rifle Club
Target Shooting since 1885

For Target Rifle, F Class Standard, and the New F-TR class only .223 Remington and .308 Winchester from the above image are an option. The advantage to shooting in one of these classes is it limits the number of variables, it levels the competition to being more about shooting skill over equipment or manufacturing advantages. In general .308 is seen as a better all distances, and 223 very good up to 500yds.
For F Open Class the range of calibres available is huge, certainly in the hundreds if all the custom "wildcat" calibres are considered. The choice of calibre comes down to a number of considerations:-
  • Distance accuracy
  • Ability to manage wind drift
  • Velocity with the favoured heavier projectiles
  • Barrel life (some cartridges are known to have a short useful barrel life)
  • Recoil and the shooters ability to manage it
  • Reloading costs, bigger cases and projectiles costing more
In most Open Competition Shoots the competitor will be expected to shoot at a number of different distances but will be restricted to the same rifle/barrel throughout. For example, a cartridge like the 6mm PPC above, which normally is shot with a relatively light projectile,  will be superb at shorter ranges, however will start to have problems beyond 300 Yards. It would make a poor choice for long range. 
The majority of Open F Class shooters choose a calibre based upon a variant of the .308. This means the bolt will accept a larger range of calibres and only a barrel change is necessary to change calibre. All but 3 (1st left, 2nd, last) of the above cartridges in the image will fit a .308 bolt head. The 6XC above is based on the 250 Savage as parent case, it is very close to a .308 head and according to the Norma website  here will fit.  The 300 Winchester Magnum is the most powerful and has the highest recoil of the cartridges in the image.  
With all F open shooters the usual preference is to use a heavier projectile, the twist rate of the rifling need to be faster to provide the correct bullet spin and the barrel usually has to be longer "throated" as the longer projectile is seated too far forward for a standard chambering.