Many artillery pieces were converted from older smooth bore weapons once technical problems in strengthening the original cast iron body had been overcome. The widely adopted solution, invented in 1863 by William Palliser, consisted of enlarging the bore to accept a wrought iron tube (called the "A tube") into which the rifling had been cut. The A tube was closed at the breech end by a wrought iron cup screwed into it. Iron was removed from the outside of the original gun barrel near to the muzzle so that a cast iron collar could be screwed over it and provide a shoulder at the muzzle to hold the A tube in place. The A tube was also held by a plug screwed into the gun underneath its trunnions. The outside of the breech portion was turned on a lathe so that another wrought iron tube, called the "B tube", could be slid over it to strengthen it.