There were other "strange" guns connected with Arkansas river defense.
Excerpts from NOR reports about June 17, 1862, action at Saint Charles:
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[nor23_202] C. S. GUNBOAT PONTCHARTRAIN, Little Rock, Ark., June 23, 1862.
Hon. S. R. MALLORY, Secretary of the Navy, Richmond, Va.
[...] At the request of Major-General Hindman I had placed in battery at St. Charles (which is 90 miles above the mouth of the river) two rifle 32-pound guns belonging to this vessel and two rifle 3-inch fieldpieces that I found at the arsenal here. [...]
I am, sir, with great respect, your obedient servant, JNO. W. DUNNINGTON, Lieutenant, Commanding [C.S.S. Pontchartrain].
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[nor23_165] U. S. GUNBOAT ST. LOUIS, St. Charles, White River, Ark., June, 18, 1862.
Flag-Officer C. H. DAVIS, Comdg. Western Flotilla, Mississippi River. Memphis, Tenn.
[...] Their batteries consisted of two 12-pounder brass pieces, [disembarked from C.S.S. Maurepas], two 9-pounder Parrott rifles [3-in rifles above], and two 42-pounder rifled seacoast howitzers. [...]
I am, very respectfully, your obedient servant, W. McGUNNEGLE, Lieutenant, Commanding [U.S.S. St. Louis].
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[nor23_188] U. S. GUNBOAT LEXINGTON, Clarendon, Ark., July 2, 1862.
Flag-Officer C. H. DAVIS, Commanding U. S. Naval Forces on Western Waters, Memphis.
[...] We reached St. Charles [...]. I took occasion of this delay to drag farther into the river the two rifled 42-pounder guns which had lately formed the upper battery of the enemy's defenses, and which, thrown into the water, had shown themselves by reason of the falling of the river. [...]
I have the honor to be, sir, your obedient servant, JAMES W. STORK, Lieutenant, Commanding [U.S.S. Lexington].
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Now, Dunnington had personally directed (if not loaded and fired) his guns, and so we can take for granted they threw 32-pdr nature [#1] projectiles.
OTOH the Federal men which phisically lifted the guns and dropped them into the river identified them as 42-pdr.
Supposing the latter people knew their trade, and were not passing-by laymen of the "What a large gun, Wow!" sort, how can the difference be reconciled?
I venture to suggest a possible explanation: one of the various New Orleans foundries, whose records - as described above - was sketchy at best, cast at least two 42-pdr nature (7-in) bodies and then bored them as 32-pdr nature (6.40-in) rifled pieces. Why? Because at the time it was well known a rifled gun stressed its metal much more than a smoothbore one, and so it needed more metal around its breech. See the exterior band around a Parrott.
I cannot find any reference about the existence of such Southern guns; but West Point Foundry made one in 1857 ("42-pdr seacoast gun block, bored as 32-pdr & rifled") in 1857 for experiments. South of Mason-Dixon line, the first Brooke guns produced by Tredegar in 1861-62 were cast as IX-inch Dalghren shell guns bodies and then bored as 7-in rifles, so we can say the idea was around at the time.
So the Federal officers recognized 42-pdr by exterior size & dimensions, having no reason to measure the bore.
Just a suggestion.
Source is the usual Olmstead, Stark & Tucker, The Big Guns.
[#1] Artillerymen of XVIII century (and earlier) identified guns by their "nature", meaning the weight of the corresponding iron ball. This nominal weight had nothing to do with the actual weight of the projectile - an exploding shell of 12-pdr nature, not being made of full iron, was obviously much lighter than that - but was useful for classification. By ACW time the term had disappeared from parlance (I cannot find it in contemporary manuals) but I think it simplifies discussion and avoids confusion.
In our case "rifled gun, 32-pdr nature" means 6.40-in calibre. About actual weight of projectile, the rule of thumb of the era was, if you lacks a precise number, double the round ball one (for an obvious example, see the James rifles). So the elongated projectiles (called "bolt" if full metal) it fired weighted nominally 64 pdr. Now someone could call this gun a "64-pdr rifle". But the same term could also mean "64-pdr smoothbore gun [i.e 8-in], afterwards rifled" (throwing a nominal 128-pdr bolt). See the confusion?