I would have been much disappointed by myself, had I not recognized your barrel (remember it's not a complete haquebut) at first sight though the images on photobucket are tiny.
For all who wish to learn more on that item and its coat-of-arms, I post all the (unfortunately very small) images originally posted by the Springfield Arsenal because they were searching for information. I have to admit that was astonished to see the term of a the Late Gothic ornament quatrefoil misspelled as quatra
foil by a musem staff member ...
Your barrel with its unusually fine and profusely rich chiseled Early Renaissance decoration can be dated to ca. 1525-35. The originally swiveling bronze pan cover is now missing, just as on my Doppelhaken
In order to be able and tell you more I would definitely need good-resolution images of the whole piece, as well as of all the details you questioned.
As a Medieval and Renaissance arms historian I can affirm the information given to you on how to interpret the three initials on your barrel. I am quite sure though they mark some later kind of arsenal inventory signature, and not that of the original owner.
In any case, this is a very fine - apart from obviously being preserved in excavated or salvaged condition - and espacially historically high-ranking item in early firearms history, so congratulations!
Do you have other similar items to match? I posted many items and overviews from my highly specialized collection here over the years, just see my threads.