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I have an AR 15 upper receiver that I want to outfit with an M4 type barrel in 5.56. There are several lengths 16', 7.5' & 14.5' that I am interested in. Is there a law requiring the barrel to be a certain length in order to be legal? Will the 7.5' & 14.5 require a longer Flash Hider to make it legal?  Thanks for any input you may be able to provide.

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Dude - a violation of NFA rules without the tax stamp.....10 years in Federal prison

Don't screw around with this, its not worth it!!  

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Your best bet is check out the armory section under class 3, but anything under 16" without a permanently attached flash hider is considered an NFA item and would be illegal to have without a NFA tax stamp from the ATF.  14.5" with a perm flash hider is the most common, but ive never seen a 7" with something along those lines.  It might be because of baffle striking that could occur.  If you want smaller than a 14.5" go with 11" or 10.5" over the 7".  Much better ability to stablize the bullet and more accurate.

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A baffle strike with a flash hider?  Flash hiders don't have baffles, suppressors do.

OP, 16" is the legal limit without a tax stamp for rifles.  That 16" is measured from the bolt face and includes any permanently attached muzzle devices.  10.5" and 11.5" are not going to necessarily be any more accurate than a 7.5" barrel.  They will have more velocity and a little less blast, but inherent accuracy will depend on the barrel itself.  14.5" barrels are easy to bring up to the legal length as all you have to do is weld or solder a flashhider that adds at least 1.5" to the muzzle.  There is a 5.5" long flashhider that can be permanently attached to a 11.5" barrel to make it legal as well.  The 14.5" barrel with permanent muzzle device actually makes some sense as that is the shortest you can legally go without a tax stamp.  The 11.5" with permanent device is for looks as it replicates the old XM177.  Anything else is pretty pointless as you're loosing a lot of velocity, but you're still stuck with an overall length of 16" on the barrel.  If you want one of the shorter barrels, get a tax stamp and go have fun.  If you don't want that hassle but want the shortest barrel you can get, get the 14.5" and add a flash hider.

note: The A2 flash hider is not long enough to bring a 14.5" barrel up to 16", you'll need something like a phantom, vortex or extended A2.