Correction:
The Russian Federov Avtomat (1916) was the world’s 1st assult rifle, and was used in WWI. However, the Avtomat was not extremely reliable, and for this reason, the idea of the assult rifle was abandoned by governments for some time.
The MP44 was the world’s 2nd assult rifle, and a few months later the M2 Carbine entered service as the 3rd.
My criteria for an assult rifle comes from what I have learned from military people that have told me that these are the things that make a weapon an assult rifle. One of them is my father who is a Korean War veteran.
http://www.biography.ms/Assault_rifle.html
You may read that the 1st in design was the Italian Cei-Rigotti, but it did not have selective fire (it was semi-automatic only) and this disqualifies it from being a true assult rifle. This weapon was never used by military personell because it was unreliable.
The term “assult rifle” has been replaced today with the term “assult weapon” in most countries because the functions of these weapons have increased, such as adding a folding stock, grenade lauincher, bayonet mount, etc, and a more clear definition has been needed for lawmakers to regulate highly dangerous weapons owned by pricate citizens. Some of these features did not exist in most early “assult rifles”.
Definition of an “assult weapon” at Wikipedia:
(keep in mind that not necessarily all of these criteria must be met for a weapon to be an “assult weapon”. These are simply the features that a weapon may have if it is an assult weapon.
"An assault weapon is a semi-automatic rifle, shotgun, or pistol with a combination of the following characteristics:
Large capacity detachable magazine, usually defined as holding more than 10 rounds
Military-style appearance, including semi-automatic replicas of military fully-automatic assault rifles
Folding or telescoping stock
Ability to use a grenade launcher, either fixed or detachable
On rifles and shotguns, those with pistol-type grips
A bayonet mount
Threaded barrel capable of accepting a flash suppressor or sound suppressor (aka silencer)
Weapons that include a barrel shroud or other covering that permits the shooter to hold the firearm with the non-trigger hand without being burned
On pistols, those on which the magazine attaches outside of the pistol grip"