It's an M114.
Remember - they're not steel-hulled, but aluminum, and a 7.62 NATO round would penetrate one side of any of the PCs.
They've been on the range a long, long time, and they're holed like Swiss Cheese.
As to 19th Century Artillery versus Modern Armor - modern gun systems would've knocked out that position as soon as the smoke was seen, and they'd be moving while they did it.
Scouts Out!