This is my rifle/boat, this is my gun/ship.

I don't remember what got me thinking about this, but yesterday I remembered to look something up that I'd been meaning to for a while: when is a boat properly called a ship? That immediately reminded me to look up the definition of gun, because people love to correct you when you misuse any of the above words.

If you look beyond Wikipedia's boat page to the discussion page about the Ships category, you can get an idea of how seriously naval enthusiasts take the definitions of boat and ship. (In general, the discussion pages are cheap entertainment for those who enjoy watching nerds argue.) Some say that a ship is a sea-going vessel, whereas boats stick to inland waters. Others say that if you can hoist it out of the water onto a trailer, it's a boat. According to the current Wikipedia page:

A boat is a watercraft designed to float on, and provide transport over, water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were historically designed to be operated from a ship in an offshore environment. In Naval terms, a boat is something small enough to be carried aboard another vessel (a ship). Boats that are notable exceptions to this concept due to their large size are the Great Lakes freighter, riverboat, and ferryboat. These examples do, however, generally operate on inland and protected coastal waters. Modern submarines may also be referred to as boats (in spite of underwater capabilities and size), but this is possibly due to the fact that the first submarines could be carried by a ship and were certainly not capable of making offshore passages on their own. Boats may have military, other government, research, or commercial usage; but a vessel, regardless of size, that is in private, non-commercial usage is almost certainly a boat.

Um... OK. About as easy as Russian grammar. If you're not planning on carrying a cheat sheet around with you the next time you go near the water, you may prefer this succinct definition from the National Maritime Museum in Greenwich, England, linked to by someone on the aforementioned discussion page:

The Historic Ships Committee have designated a vessel below 40 tons and 40 feet in length as a boat. However, submarines and fishing vessels are always known as boats whatever their size.

Easy enough. Now how about guns? Why is a rifle not a gun, a distinction so important to the late Gny. Sgt. Hartman of Full Metal Jacket? Wikipedia say:

The term gun is often used synonymously with firearm, but this is common only for civilian usage. In military usage, the term refers only to artillery that fires projectiles at high velocity, such as naval guns (which are never referred to as cannon) or tank guns. A gunner is a member of the team charged with the task of operating and firing a gun. By military terms, mortars and all hand-held firearms are excluded from the definition of guns. The exception to this is the shotgun, which is hand-held, has a smooth bore and fires a load of shot or a single projectile known as a slug.

I didn't know before reading this entry that you could refer to a firearm's barrel as being rifled, meaning that it has "a series of grooves spiraling along the barrel."

In other news, my Australian diving teacher would like to remind you never to refer to diving masks as "goggles," or to fins as "flippers," or you owe him a dollar.