I recall an article where one (of the very few) vendors in the 5.7 market said they tried to make a shorter barrel (than the standard 4.8 inches) and it simply wouldn't work (at least in the actual FiveSeveN design - the spring is as short as it can be).
I will have to see if I can dig it up...
Edit: not the article I was looking for, but an interesting thread:
http://fnforum.net/forums/fn-five-s...ct-fiveseven-pistol.html#/topics/11691?page=2
Of course you can make a gun with whatever length barrel you want, the bullet might just plop out in a large-ish fireball though...