2. 1. 3. 1. 4.2-inch (30-pounder) Parrott rifle
The 4.2-inch (30-pounder) rifles were the most widely used of the Parrott siege guns. It was mounted on a conventional siege carriage. The early pattern guns had the elevating screw under the breech, while newer pattern gun had a long screw running through the cascabel. The long elevating screws of the newer models was subject to breaking (Abbot 1867, p. 90). The 4.2-inch Parrott rifles were preferred over the 4.5-inch siege rifles because of the superiority of Parrott shells over the various shells available for the 4.5-inch siege rifle. The 4.2-inch Parrott rifles did not have the same problems with bursting that were with found with the larger Parrott rifles. During the siege of Petersburg 44 4.2-inch Parrot rifles fired 12,209 rounds (Abbot 1867, pp. 160,170). Only one gun burst when a shell detonated before clearing the muzzle (Abbot 1867, p. 87). One 4.2-inch Parrott rifle also burst during the campaign against Charleston harbor, but only after it had fired 4,606 rounds (Abbot 1867, p. 87).
I may have misquoted the Paulsons about the strength of the smaller Parrott guns. I think they were referring to the information above. The smaller field guns had a reputation for failure, but I haven't come across any information on the number of failed tubes.