Accurately differentiating ranges on sidewheel parallaxing scopes is common problem. The usual cause of this problem being that the diameter of the sidewheel is too small to allow the ranges to be clearly marked. Why scope manufacturers do not supply their scopes with larger sidewheels is a mystery - it's a bit like buying a jacket and then finding that you have to buy the sleeves from elsewhere!

For example, on our 10 - 50 x 60 BSA Platinum scope, the range marking for distances in excess of 40 yards are very close together and so difficult to differentiate. It's worth noting that, with our non-FAC Air Arms Pro Target, the pellet's point of impact drops by around 3" (76 millimetres) between the ranges of 40 and 55 yards.

To eliminate possible errors in range finding, we fitted a prototype enlarged sidewheel (left) onto our existing sidewheel. This version has an outer diameter of 140 millimetres (5 1/2 inches) enabling us to mark ranges at 2 yard intervals from 20 yards up to 56 yards and at 5 yard intervals up to 75 yards. The reason for marking ranges up to 75 yards being that most Scottish Field Target competitions include a long-range target. The ranges were marked on using fine indelible markers. Should you choose this option, be aware that prolonged exposure to rain will result in fading. We protected our markings using a brush-on quick drying varnish.

Manufacture of this sidewheel was done using a conventional vertical milling machine, rotary table and lots of coffee. We plan to replace this sidewheel with a conventional style sidewheel. This will be fitted to the parallax focussing knob, rather than the sidewheel, and will have a wider rim.

As a result of these revisions, the new sidewheel will be simpler to manufacture and will weigh less than the current version. In addition, the wider rim will enable us to mark on our ranges and their associated clicks. A CAD drawing of the revised sidewheel is shown below.