Typically they would have one or two .30 calbre in the nose turret and one .50 calibre in each side blister. Also the Coast Guard did use the PBY for SAR and coastal patrol. An excellent reference resource would be PBY Walk Around by Squadron publications, here is the link, it is available for $12.95:
http://www.squadron.com/ItemDetails.asp?item=SS5505
I have used these Squadron reference books on several projects and they are quite helpful especially in showing difference in variants of the particular aircraft. I hope this is helpful to you