Yes, in fact I do, have a Lahti model 39. When we were kids, my dad actually had client come to the area, and they went and tested it out. My bros. and I were not with them, but the sub-sonic rounds did a job on the target( which was a 74 Chevy) and were heard for a long way. My bros. and I went there a few days later on our four wheelers, and observed the destruction. I've fire Gatelins, but never saw the Lahti fire in person. I've got a nice example of my own, but have no real place to try it out anymore with all of these post 9/11 things going on, and the world going wild. Unless you want to stop a tank, or take down a small aircraft, they are not very practical, and definately not easy to conceal.
The rounds alone are almost seven inches long, and the diameter of, well, say almost a golf ball... 20mm rounds are serious.