score:7
So to answer the question as posed in the title.... 2016ish. Syrian rebel forces mounted what looks like a cannon of late 1800s vintage to a pickup truck and there is video of it firing in anger at government positions. Granted it doesn't look like the video after that of rounds detonating is from that cannon, as the cannon is on a pretty flat trajectory and those look like mortar rounds impacting. It was in action as of 2016 but who knows how long it actually served before the barrel burst or it was otherwise destroyed/abandoned.
As for European military use of such cannon, I would guess the Franco-Prussian war saw the last use of cannon that conceivably could have fired "roundshot" in the form of then-outdated "Canon obusier de 12" but I do not have my Franco-Prussian books to hand to see if any ended up employed by irregular/Republican forces. I have a fuzzy memory of some being employed by the Paris Commune but I'm not sure if that'd count for what you're asking.