I had some problems in simplify3d with the files dilc1,2,3,4, they were not manifold resulting in transparent surfaces and support creation failure. I repaired them in Blender. I also cut the cannon in two halves. You have to reorient the parts in slicer and glue the parts after printing.
Thanks to makerslabcz for this awesome design. I will post my make also when it's all printed.