These muzzle loading rifles came in a large variety of calibres, the most common calibres for medium thin skinned plains and deer sized game were , , , , and 12 bore (). For larger thick skinned type game 10 bore (), 8 bore (), 6 bore (), and 4 bore () were used, the last being a specialist elephant hunting calibre. Whilst most of these calibres could be purchased with rifling, the giant 4 bore and the less common 6 bore were rarely rifled, as the increased friction from the rifling caused excessive recoil. Various experimental breech loaders had been in existence since the 16th century, however developments such as the Ferguson rifle in the 1770s and early pinfire cartridges in the 1830s had little impact on sporting rifles due to their experimental nature, expense and the extraordinary strength and reliability of the percussion muzzleloader. In 1858, Westley Richards patented the break open, top leaver breech loading action, whilst a useful development these early break open designs had a great deal of elasticity in the action and upon firing they sprung open slightly, a problem that gradually worsened with repeated firing and with more powerful cartridges. Many gunmakers tried various methods to rectify this problem, all to little avail until Westley Richards invented the "Dolls head" lock in 1862 which greatly improved rigidity, this was followed by James Purdey's under-locking mechanism in 1863 and W.W. Greener's "Wedge fast" system in 1873, finally the basic break open action known to this day had the strength required to meet the stresses of large-bore projectiles. By 1914, triple, quadruple and even quintuple locking designs could be found in various proprietary actions. In 1861 the centrefire rifle cartridge was invented, and quickly a large number of blackpowder centrefire cartridges were developed in the same calibres as their muzzle loading forebears, including the .450/400 Black Powder Express, the extremely popular .450 Black Powder Express, the .500 Black Powder Express, and the .577 Black Powder Express, all with various case lengths. The first brass 10 bore, 8 bore and 4 bore cartridges were not developed until 1870, their arrival spelling the end of the muzzle loading era. Until this time all double rifles had external hammers whose fall struck the mechanism which fired the cartridge, however attempts to remove them were well under way. The first "hammerless" action of significance was invented by Thomas Murcott