Figure of Merit (FoM) seems to be misunderstood by many - you'll find 19thC results for assessing precision of arms and providing figure of merit, being parroted today as group size. The Whitworth rifle trials results are widely miss-quoted leading to some wildly unrealistic claims today of its accuracy. I have information on line explaining determination of FoM in the UK, together with an image of a 19thC sheet for recording the shooting results. Group size can't be determined today from FoM results, but
an indication of the likely overall size of the group can be estimated. I've used my own shooting results with muzzle loading rifle at distances out to 1000 yards to calculate FoM and work out an average relationship to the actual group size - it's statistical analysis and any relationship varies depending on the pattern of the group. See the following, which also includes further discussion of the 1857 Whitworth rifle trials results -
Measuring Accuracy.
David