Probably if you are going to be dealing with NSSA or some other group, would be best to use a "recognized" definition for howitzer that can be documented from Army pubs or something, otherwise someone with an agenda will always be able to pick you definition apart. I don't have it handy but I'm sure we can find something in the Ordnance Manual or other pub they'd respect, that defines a howitzer as a cannon having a barrel, let's say 5-8 calibers long, as opposed to a mortar which had less and a gun which had more. I've seen such "official" definitions, just can't find 'em at the moment. At least get agreement upfront on the technical definition of "howitzer" to be used, using some easily-measurable characteristics, or any other agreement will be unreachable.
I tend to think the US Army defined "howitzer" as a smoothbore cannon within certain caliber-length criteria, as well as being chambered, and only the Navy had its rifled boat howitzers.