No.... 18 inch radius = 36 inch diameter = 3 feet
Automatically Appended Next Post: To answer your question it depends. I play orks and I love the belly gun but sometimes it actually isn't that helpful. The lictor bonus to cover saves combined with actual cover and night fighting made my gun very useless with a 2+ cover.
On another note against marines it can be very awesome but after the first turn you are more than likely going to be shooting your own models off the board by shooting it. 20 inch blast will be your average and then it can still scatter.
So I wouldn't say it's game breaking or cheese but at the same time it's not as effective as some people might think.