of its chambered cartridge, once regulated a double rifle cannot satisfactorily shoot bullets of different weights or velocities, as it usually affects the accuracy. The process of regulating a double rifle’s barrels is complex and can be time consuming, significantly adding to the cost of the rifle. Sights Whilst many modern double rifles are fitted with telescopic sights, the need for fast target acquisition in the face of dangerous game led to the development of the "Express" iron sights. Express sights feature a very wide and shallow rear V, usually with a white line marking the bottom of the V. The purpose of express sights is to allow for well aimed shots over moderate ranges when there was time for careful sighting, and to allow the rifle to be pointed like a shotgun to stop the charge of a dangerous animal at very short range when there is little time for aiming. Calibres Double rifles have been produced in all calibres from to . Traditional British double rifle calibres include the Rook rifle, Black Powder Express and rimmed Nitro Express families of cartridges and many of these can still be obtained today. European makers tend to chamber their double rifles in metric rimmed calibres developed by European firms, although British and American cartridges are offered. History The development of the double rifle has always followed the development of the double-barrelled shotgun, the two are generally very similar but the stresses of firing a solid projectile are far greater than shot. The first double-barrelled muskets were created in the 1830s when deer stalking became popular in Scotland. Previously single barrelled weapons had been used but, recognising the need for a rapid second shot to dispatch a wounded animal, double-barrelled muskets were built along the same format as double-barrelled shotguns already in common use. These first double-barrelled weapons were black powder, smoothbore muzzleloaders built with either flintlock or percussion cap ignition systems. Whilst true rifling dates from the mid-16th century, the invention of the express rifle by James Purdey "the Younger" in 1856 allowed for far greater muzzle velocities to be achieved through a rifled longarm, significantly improving the trajectory and as such greatly improving the range of these rifles. These express rifles had two deep opposing grooves which were wide and deep enough to prevent the lead bullets from stripping the rifling if fired at high velocities, a significant problem previously.