The Project 401 is a German experimental concept firearm that was produced by Diehl AG as a proof-of-concept.
Little is known about the weapon's history, although patents for this weapon were filed in March 1973. The sole prototype is now on display at the Bundeswehr Military History Museum.
The Project 401 never had a physical body and used a 120-round helical magazine. The weapon used gas operation and was select fire. The helical magazine has an internal "cassette" which rotates the rounds around a spindle and down into the weapon. The weapon has a two-piece bolt.
The weapon uses a caseless 5.56mm round.