Question for anyone that might know the answer:
I have a new Army bayonet that my wife bought me and was curious how much of the blade would protrude from under a 16" barrel with an A2 type flash hider. Or should I buy a 14.5" barrel instead?

Bayonets don't fit regular 16" barrels.  The distance from the bayo lug to the flash hider is too great.  They'll fit the 14.5" barrels fine.  The 16" midlength barrels might work as well.

Do I need a license to buy a 14.5" barrel? I have a preban receiver that is in compliance.

You can buy an M7 bayonet that has already been modified to fit the 16" if you really want.  But dude, I must ask, are you really going to base what length barrel to get based upon a bayonet that you happen to own?  There are more important issues to consider.

It is perfectly legal to purchase a 14.5" barrel.  However, unless you are registering as an SBR, you will have to permanantly attach some type of muzzle device(brake or FH) that is long enough to bring total barrel length to 16"

Great, thanks for the help guys. I'm not basing what barrel length to buy just because of a bayonet, I plan on taking this rifle with me when I get deployed to Afghanistan sometime next year and want it to be able to do almost everything that an M16 does, and that includes having the correct length barrel so if I do have to put a bayonet on it, it'll work correctly. Thanks for all the input!

Well, if it's an M16, then by all means go with a 14.5" like all the other M4's.  However, you will be giving up a lot of terminal performance in which case go with the 20" like the marines.  A bayonet is a poor implement for anything less than the 20".  When bayonets were widely used way back when, they were waaaay out there and were quite long making them quite effective in stand off attacks, but if you have it mounted on a short barreled weapon, the enemy is going to already be too close - I would rather have the knife in my hand at that distance.

Curiously though, how many times have you seen bayonets mounted, much less USED in such a manner over there?  I am not saying it doesn't happen, but I am curious because I have never personally seen or read of a bayonet attack with modern weapons but I am sure it happens from time to time.

Good Luck taking ANY personal weapons on a deployment unless your assigned to a special operations unit.