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The British gun that is often touted as some sort of mid 19th century supergun is the 68 pounder 95 cwt smoothbore. It was not originally designed as an anti-armor gun but was used as such since the more modern rifled guns of the time were not very high velocity due to problems with the design of the rifling and powder used.

The British 68 pounder 95 cwt was designed as part of the standardization of guns on ship before the Civil war when it was decided to increase the size of guns above the 32 pounder and to make different versions of 8 inch/68 pounder guns of different lengths and weights. The shorter 68 pounders were basically shell guns while the longer 95 cwt gun was for a limited number of upper deck pivot guns.

So when armored ships were developed and the rifled guns proved to be not as powerful as hoped the British dusted off their 68 pounder 95 cwt gun and used it.

The US for example also had similar guns, it had 8 inch shell guns, (later 9 inch) and it had some 64 pounder 106 cwt guns as piviot guns on the upper deck. However by the Civil War these 64 pounders 106 cwt were considered to be obsolete and were not used much.

The British did sell some of their 68 pounder 95 cwt to the Confederates but they did not do much. Some were sold as converted to rifles. There was one on the Alabama when it fought the Kearsarge but it did not accomplish much against even a wooden ship.



Here is a link to a contemporary book from 1865 (A Treatise on Ordnance and Armor) by Holley which covers testing of US, British and French armor and guns. Its 900 pages. Its on google books so it has some problems with bad scanning of the pages.

books.google.com/books?id...QY#PPR3,M1

There were a lot of problems with early rifled cannons, no matter which country made them. None of them had particularly good rifling systems and some were quit bad.

They were also lower velocity then their smooth bore equivalents because of their heavier projectile weight/diameter ratio and tight fit which made chamber pressures much higher then the smooth bores which limited how much powder could be used. This is why most rifled cannon had massive built up breaches to contain the high pressure.

Their projectiles were more complicated then simple cannon balls and needed more care in manufacture and use and had a higher failure rate. Cannon powder residue built up in the bore and was harder to clean out then smooth bores, while the projectile rifling system especially ones which used lead would leave a metal residue in the rifling which was very hard to remove and both could cause the rifled projectile to get stuck in the bore causing the cannon to explode. Because of the difficulties in manufacturing a uniform product it was not uncommon for the projectile to have whatever rifling system fail to engage the rifling properly or to simply break off and even get stuck in the bore.

Because of the rifling, which absorbed heat more, the additional friction from the projectiles going down the bore, and the high pressure in the chamber rifles had a tendency to overheat more.

The big advantages of early rifles was that their longer higher weight projectiles went farther because they had less wind resistance compared to their weight even though they had a lower muzzle velocity then a similar smooth bore. And their spin made them more accurate especially at longer range.

However this long range was not very useful in ship vs ship engagements since 1,000 yards was considered to very long range firing with only a small chance of a hit. The USN and US Army both received 8-inch Parrott rifles, the Army called them 200 pounders while the Navy called them 150 pounders. This is because the Army used a longer heavier projectile which was lower velocity but went farther and with more accuracy but the Navy used a shorter lighter projectile so they could have higher velocity which would be better at penetrating armor at shorter ranges and was willing to give up longer range performance.