Why is it that whenever I see one, it's always cut down to 14 inches?  Vary rarely do you find them shorter, or longer.  Is that length the best split between round performance and handling?

Also, why two different federal length limits between shotgun, (18") and rifle (16")?  WTF?

Have no idea as to why the different Federal approved minimum barrel lengths, but in 1968, when these went into effect, there were few "assault rifles" etc. to deal with...mostly cut down sporting weapons of various types. I don't know the barrel lengths of some of the little .22 rifles such as the Winchester "Gallery Guns" (pump .22's used at shooting galleries, etc. but if they fell under the 18" minimum for the shotguns, that may have been the reason...still some around today.

The 14" shotgun barrels allow for the muzzle to be in front of the tube magazine...which helps a lot if your hand should slip and go forward on a hard forward stroke using a pump gun. They also allow a bit more velocity than shorter barrels such as 12 3/4 in. which, IIRC, is the shortest length an 870 barrel can be cut and fit flush with the front of the mag tube. The FBI and Secret Service placed orders some years ago with Remington for 14in. barreled 870's using modified chokes because they handled much better in the confines of a vehicle or when trying to exit quickly. While most don't know this, the FBI is...or at least was at one time...also responsible for procurement and maintenance on all firearms used by Foreign Service personnel / Embassies around the world...many, many thousands of them. This initial order certainly also had some influence on other manufacturers and agencies, etc. and I am certain that there was some decent level of research by these two agencies that resulted in the 14in. spec.