[WernherVonTrapp]

That depends on what version/mod you're using. In stock, I could sink whole merchants with one. The mods bring it down to a more realistic level. The 40mm Bofors was a devastating round, being about 1.57 inches in diameter, HE and fired at high velocity. Anyone who has worked in plumbing can comparatively appreciate the girth of the round with a 1.5" piece of pipe.
My experience in SHIV has been that it works well against very small merchants (Fishing Boats, Sampans, Junks, etc.) and when combined with the DG, can do some supplementary justice to larger targets. I wouldn't try going solo with it in sinking anything large.