Do you HAVE to have a flash hider to attach a bayonet? It won't work with a plain muzzle won't it?

It will flop around with a plain muzzle.

You could use a muzzle brake if it's got the right O.D. to accept the bayonet.  Either way you'll need a threaded muzzle to go with that bayo. lug and as has been pointed out many times at this point in time it's "illegal" to have on a "post-ban" AR.   After the AWB sunsets you can legally have a Flash hider and bayo. lug so I'd just wait and get both "evil" features then.  

it will work with a plain barrel provided the OD is the same size as a flash hider. A bayonet will flop around on a standard barrel as stated.

I know that it is illegal to install a bayonet on a post-ban. I have no plans of doing that unless the ban sunsets next year.
The reason was asking is, because I was curious why you can't mount a M9 bayonet on a 16" carbine. But now i know, and thanks for all the responses!

There are three solutions that will allow you to use a standard AR-15 bayonet on a 16-inch barrel that I can come up with quickly...

1.  A strap on bayonet lug that can be moved forward of the Front sight.

2.  A slip-over flash hider that comes over the barrel but MOST of these are too large in diameter to use with a bayonet so buy with care!

3.  Buy or build a mid-length grip upper that moves the front sight base forward.  Note:  I think Armakite and RRA's mid-length upper moves the bayonet mounting ring to about the slots in the flash supressor which isn't ideal but would work!

If you don't like those, there were a few guys making LONG handled bayonets that will work but they did look a little funny IMHO.