Apparently, blogger/reporter Michael Yon found this sucker under a barn in Mosul, Iraq circa 2005, and now everyone is wondering what it is.
To me the thing looks like a polish moose-gun. You make 'em by taping a bunch of tin-cans end to end after removing the tops and bottoms. You leave the base on the last can, and punch a couple of holes in it. Fill the thing with lighter fluid, stuff a potato down the barrel, and hold a match to the hole. Either the potato gets launched a good hundred feet at speeds in excess of 40 mph, or your arm catches fire from dripping lighter-fluid.
The scope on this one seems a bit excessive, but maybe the insurgents were using special anti-tank potatoes and didn't want to waste any.