I have wondered about double lumps and round bodies for quite some time myself. Oh wait, you were referring to guns?
Sorry, couldn't resist.
As far as I know, round body generally refers to a type of double gun action, sxs or o/u, that has rounded edges and a generally slimmer profile. These type of guns, i.e., Holland & Holland, etc., are usually pretty expensive, but beautiful. I'm not sure, but I believe double lump refers to the lumps on the underside of sxs barrels that engage the receiver when closed. Some are soldered on, and some are integral to the barrel, which are referred to as "chopper lump" barrels if I'm not mistaken. Somebody please correct me on this if I am wrong. Hope this helps!