What constitutes an assault weapon?

[Kozmik]

For self-referential completeness:

On Internet forums there is perhaps no more fiercely discussed topic than the question of what constitutes an assault weapon.

NYT article

[SanDiegoTim]

In California a semi-automatic rifle with a detachable (without a tool) magazine and a pistol grip is an assault weapon. This defines an AR15 and is patently absurd as the ubiquitous Ruger Mini 14, for example, is chambered for the same round (.223), comes with a detachable magazine and is capable of the same rate of fire. The only difference is that the AR15 has a pistol grip.

Other states use similar definitions. Theaforementioned "without a tool" loophole will probably be closed.

[appleciders]

I'll throw one thing that's been bothering me out here-- bayonet lugs should not be a criterion, in spite of the fact that they really have no good hunting or home defense use. My reasoning is this: no one is getting killed by bayonets in this country. Ergo, I don't think that we ought to regulate bayonet lugs. They're not a problem.

[SanDiegoTim]

The definition is 100% poilitical and subjective. Just perfect for politicos.

[MsRobyn]

NM

[Airman Doors, USAF]

As defined by the Assault Weapons Ban of 1994, here is what constitutes an "assault weapon":

Semi-automatic rifles able to accept detachable magazines and two or more of the following:

Folding or telescoping stock
Pistol grip
Bayonet mount
Flash suppressor, or threaded barrel designed to accommodate one
Grenade launcher (more precisely, a muzzle device that enables launching or firing rifle grenades, though this applies only to muzzle mounted grenade launchers and not those mounted externally)

Semi-automatic pistols with detachable magazines and two or more of the following:

Magazine that attaches outside the pistol grip
Threaded barrel to attach barrel extender, flash suppressor, handgrip, or suppressor
Barrel shroud that can be used as a hand-hold
Unloaded weight of 50 oz (1.4 kg) or more
A semi-automatic version of a fully automatic firearm

Semi-automatic shotguns with two or more of the following:

Folding or telescoping stock
Pistol grip
Fixed capacity of more than 5 rounds
Detachable magazine

Keep in mind that these definitions were created with the express intention of banning weapons with these characteristics. There is nothing inherently more dangerous about these weapons in, say, 5.56 NATO/.223 Remington, than in any other weapon of that caliber. A weapon with a flash hider but lacking any other feature on the ban list is not an assault weapon, but add a bayonet lug and it is.

There are people that want to argue that the definition is fluid, and in the sense that they can add to the ban list it is, but this is the only definition codified into Federal law at any point and as such this is the answer.