Why is caliber the size of the bore for a rifle, and length of the barrel divided by the bore in naval guns?
“Barrel length”, third paragraph, I hope, helps explain why:
A rail gun works by passing a lot of current through the projectile as it goes down the rails. There is a lot of wear on the rails due to the high current from the rail through the projectile. The rails are basically being melted at the point it is contacting the projectile much like arc welding.
Sure, but why use a term that means something else with rifles and pistols?
Probably begun by the Royal Navy, where a sloop is not a sloop if commanded by a post Captain…or however it goes. 
No idea. And I only mentioned the French navy because that was among the first results of a quik’n’dirty search for large-caliber auto cannon.
P.S.: such guns chamber rounds and extract spent casings automatically but are still dependent on human loaders to keep the feed racks full. So a qualification on “automatic”.
Just to note, the 30mm GAU-8/A Avenger was selected by the Dutch as the weapon to be used in the Goalkeeper CIWS. It’s never been fired in anger, but it kills missiles too.
Youtube of HNLMS Johan de Witt (L801) Goalkeeper CIWS firing.
Cheesy promotional video for the Goalkeeper, but it does have footage of a Goalkeeper making a hard kill on first a single Exocet, then two Exocets launched 5 seconds apart.
Kinda how it works. A rail gun has a series of electromagnets that are basically pulsed in sequence to accelerate the projectile. The heat is caused by both the energy that has to be dissipated by the electromagnets and the friction of the projectile along the rail. The projectile is obviously interacting with the magnetic field but it in itself does not create the field to which propels it.
Just a little nitpick…
You’re thinking of a coil gun. As the link you yourself provided shows, in a rail gun the projectile itself does form part of the circuit and helps create the magnetic field.
Greenpeace ships, I imagine.
Christ!
Because the F-35 could only carry enough GAU-8 ammo for a couple short bursts they’re going to mount a smaller Euro designed gun that actually packs a bigger punch, ammo design now is toward high tech and small size, it’s pretty amazing stuff.
The US Army has dropped plans for an MRSI artillery system that was pretty awesome.
Hotchkiss made revolving barrel cannon similar in concept to the Gatling, in the late 19th century, mainly for naval applications. The largest common caliber was 3 pounder, ie. 47mm. I’m not sure if any larger prototype was ever produced but the 1 pdr (37mm) especially and 3 pdr revolver cannons were common on warships of various navies for anti-torpedo boat defense (meaning the steam torpedo boats of that era). Those were relatively low power cartridges though for their caliber compared to for example modern GAU-8 30mm. The Russians have also used a 30mm Gatling type gun, the GSh-6-30, in a/c and anti-ship missile defense applications since the 1970’s.