Presenting the Black Widow from the Marvel Universe Avengers Assemble Series - AKA the Inferno Cannon Black Widow.
No the Avengers Assemble 3.5" Black Widow does NOT come with that rifle. That's courtesy of a GI Joe Retaliation Crimson Guard figure that's waiting for its photoshoot - it just looks cool in her arms.
Natalia "Natasha" Alianovna Romanova is the soviet super-spy turned Avenger, Agent of S.H.I.E.L.D, lover of Daredevil, Winter Soldier and, get this, Hercules (never really saw that coming), the Black Widow.
This version of the Black Widow is off the Avengers Assemble cartoon that features a team consisting of the Hulk, Iron Man, Hawkeye, Captain America, the Falcon, & the Black Widow.