In England, they went entirely overboard in testing ironclad armor. They fired a 300 pound Armstrong breech loading rifle at 5.5" armor plate backed up by .9" of teak at 200 yard range. The first shot did not penetrate the teak backing. The second & third shots did. The fourth shot "malfunctioned," a euphemism for blowing the breach 40 yards to the rear.
Can you provide a citation as to the effect that that was an Armstrong breech loader? I believe the test you're talking about was a muzzle loading smoothbore firing 150 pound shot that would have been 300 pounds if rifled, if it's the one I'm thinking of.
https://api.parliament.uk/historic-hansard/commons/1862/apr/10/the-warrior-target-at-shoe-buryness
With regard to the question of his hon. Friend (Mr. H B. Sheridan) he thought he had better refer all who took an interest in the important experiment which took place last Tuesday, to an article which had appeared in The Times. Nothing could be fairer, or, he might add, more graphic, than the description which The Times gave of that experiment. He would only state shortly that there were four shots fired from what was called the 300-pounder Armstrong— that is, as a smooth-bored gun, it fired a shot of 150 lb.; but, if rifled, it would fire an elongated shot of 300 lb. On Tuesday last the first experiment was with a shot of 150 lb., and 40 lb. of powder. It was fired at the Warrior target, at the distance of 200 yards; but he was bound to say that the target had had
766a good deal of shaking by previous experiment. The first shot struck the target on a part which had been, he thought, rather more shaken than the other parts of it. The shot entered, made a hole in the armour plating, and passed through eighteen inches of backing, being stopped by the inner skin of the ship, but it crucked that skin. The next shot, with 40lb. of powder, hit nearly on the same spot. The fragments of the shot went through plating, backing, skin, and all; and there were proofs that it would have created considerable damage in the interior of a vessel. The third shot Sir William Armstrong requested might be fired with 10lb. of powder additional; but some doubts were entertained, not whether the gun would stand such a charge, but whether so large a charge would be burnt to give the shot greater velocity. The third shot was fired with 50lb. of powder, and fired at that particular part of the target which represented the strongest part of the ship. The fragments of the shot went through the plating and backing, and he believed were buried in the supports at the back of the target. It was not very easy to say whether the whole charge of powder had burnt, but evidently a good deal more powder had burnt, because the effect was much greater. The fourth shot took place under the same circumstances. It went through the plating and backing, and buried itself, like the previous shot, in the scaffolding at the back. In reply to the last question of his hon. Friend, whether the 300-pounder gun could be worked on board ship, it was hazardous for him to say decisively, but he believed they could carry a 300-pounder on board ship; but at the same time it was questionable whether a ship could carry plates sufficiently thick to resist it.
It also burst not on the fourth shot, but on the 162nd:
https://api.parliament.uk/historic-hansard/commons/1862/jul/10/question
136
§MR. BERNAL OSBORNE
said, he rose to ask the Secretary of State for War, Whether the 12-ton gun of Sir William Armstrong's construction has burst during the recent experiments at Shoeburyness; also, how many Guns of the same description and weight have been ordered, and if any of a still larger size are to be constructed on Sir William Armstrong's principle?
SIR GEORGE LEWIS
said, he had to state, in answer to the question of the hon. Gentleman, that on the occasion of the recent experiment at Shoeburyness the breech of the 12-ton gun was blown off, the cause of it being the unusual severity of the proof to which it was subjected, the charge of powder extending to 60, 70, 80, and 90 lbs. of powder. Not less than 162 rounds had been fired; and as the gun was merely an experimental one, it must be considered that it was exposed to a stronger trial than such guns were usually subjected to. The number of the guns of this size ordered was fourteen, and there were also a 600-pounder rifled gun and a 300-pounder smooth-bore in course of construction.
This was the prototype of the 9" 12-ton gun