A 155 is a pretty big gun (6+ inches). It has a kill radius of 50 yards (50% of everyone within 50 yards of the blast center dies) and a range of about 15 miles. I like 8 inch guns better but I don't think the USA uses 8 inch guns any more. 155 is our biggest artillery piece.
http://news.antiwar.com/2014/07/15/isis-captured-52-us-made-howitzers-could-shell-iraq-cities/
IS Caliphate got 52 of our howitzers with a lot more equipment already mentioned by earlier posts on this thread.
We should have put GPS tracking devices on every piece of major military hardware we left over there for just such a contingency.