I agree with you on the rifling. If their goal was to make a replica (even a loose replica) and the originals were s/b why make them rifled. Even stranger than the Mt. Howitzer being rifled is the fact that they manufactured a small naval gun in the style of a late 18th century cannon that was 7 1/4'' long with a .45 cal bore and this was also rifled. The .69 Nap. and this naval gun are the only CVA cannons that I own. The only answer I can think of is that they must have thought that the rifling would be a good selling point. Their decision to scrap the .75 Howitzer and go with a smaller cal. is probably due to the fact that the ball ammo in .58 would always be readily available (.58 cal. Fayettville, Cook & Bros., etc., Confederate muskets) and they may have also chose .58 because they already had rifling machinery set up in this cal. for a firearm barrel they produced.
The more I think about that barrel the madder I get at myself for not trying to win it. If it actually would fit well on this carriage it would make a respectable looking replica. To be honest about it, if the .69 cal. Nap. were entered in a scale replica artillery contest it would never walk away with any awards but it is well made and I've had fun with it over the years.
I've been studying the photo of that barrel and it seems to have a lot of steel around the bore. It could be opened up some and and a 3/4'' powder chamber left in the tube, then with some brass plating to finish it off, it would look pretty good.
Well, as some say, for an optimist there's always tomorrow, maybe one of those five barrels will turn up somewhere.