In my video on an 1858 Whitworth Prototype rifle, my statements on accuracy were incorrect. Thanks to David Minshall and Rob (BritishMuzzleLoaders) for spotting and correcting this mistake!

The system used to measure precision of rifled arms was the "Figure of Merit" (FoM). It is a measure of mean radial distance of shots from the centre of a group. This is not group size. The actual size of the original group cannot be determined from the Figure of Merit, however for a 20 shot group multiplying the Figure of Merit by about three and half will give an indication of the likely overall size of the group. So in trials in 1857 the Whitworth was recorded with a FoM of .37 feet (or 4.44 inch) - based on the above this would be a group size in the order of 15.5 inch. At 1100 yards the FoM of 2.41 feet was recorded - so a group size in the order of 8.5 feet. At 1880 yards the FoM was 11.62 feet, or a group size in the order of 40 feet.

British Muzzleloader has a great video on the “Figure of Merit”, linked below, I encourage you to watch if you’d like to learn more about this system.

My original video on the Whitworth can be found here:

For even more on the Whitworth, check out these videos: