Do we know of any rifled barrels being referred to as "tubes"?
Field artillery pieces are often refered to as "tubes", whether rifled or not, (especially by the O's, as in "How many tubes are you firing, Captain?")... But the actual "tube" is the part that comes into contact with both the breech AND the ammunition, rather than the part that is on the "outside", which is actually called a "Jacket".....
In the Howitzer Section Chief's Gun-Book, there's a form on which the Section-Chief records the remaining "Tube Life", DA Form 2408-4, Weapon Record Data. The "Tube Serial Number" is recorded and kept as a "living document".. The daily number of rounds fired from each Fire Mission (to include charge) are added up (taken from DA Form 4504) and total cumlative number of rounds fired through it are recorded...
The cannon's serial number is separate from the tube S/N, since all howitzers will undergo what's known as "Re-tubing" after the current tube's service-life is reached (I can't remember exactly how many total rounds for the M109A3 I owned, but it was about a thousand, give or take, with scheduled bore-scopings).. The 2408-4 also records how many times that particular cannon has been re-tubed, IIRC (Been a LONG time since I was a Section-Chief)...
As an aside, in general "Redleg-speak" a "gun" is smooth-bored, and a howitzer is rifled. The rifling in a howitzer is made up of "lands" and grooves"...
Probably way more info than you wanted, but I felt like typing something besides "model-stuff"..