Thanks for trying Mr. G. For some unknown reason I really want the answer to this one, the first time I kinda shrugged it off and gave a quick answer but now I'm interested.
So, where to now? Well let's take a look at what was turned over by Spain to the US, in the treaty of 1821 on that subject. I copied both English and Spanish versions of the treaty right from the original documents at the Library of Congress a few years ago, and found the Spanish one on the 'puter just now, let's take a look. Here's the full-sized one in a link since the Graybd shrunken one may not be as legible as required:
http://i17.photobucket.com/albums/b62/cannonmn/miscforumsetc/Forums62/treatydocument19june1821artydetails.jpg Note that it lists two forts, Castillo de San Marcos with most of the guns, and a nearby fort called Mantanzas with only two guns. Iron guns were listed separately (on left) from bronze mortars and howitzers. Turned over were 10 each 24-pounder iron guns and 11 each 18-pounder iron guns.
Now looking at Al Manucy's classic "ARTILLERY THROUGH THE AGES" we see on pp. 48, in 1834 there were 5 ea., 24 pdr. iron and 4 ea., 18 pdr. iron present, however we don't know if those were old Spanish guns or new American guns, so we'll have to rely on the 1821 turnover inventory which were definitely Spanish guns.
This still does not prove the guns at UNI came from St. Augustine, but it does prove that this was possible since there were both sizes present, at least in 1821.
I got the size info from one of the articles linked earlier, which gave the fol specs for the UNI guns:
A. Bore 6 1/8 in., Length 140 in. B. Bore 5 3/8 in., Length 124 in.
I mentally equated "A" to a Spanish 24 pdr., and "B" to a Spanish 18 pdr., but didn't do any look-up, so somebody ought to check on me there, I hate to go on my faulty memory but just don't have time to look it up right now.
