Details

The Johnson Light Machine Gun, Model of 1941 is a short-recoil, select fire, magazine fed automatic weapon, and an attempt to provide the US Army an alternative to the BAR.
As mentioned, this was designed to supplement (or replace), the M1918/1918A2 BAR. Melvin Johnson was a Marine with firsthand experience with the BAR, and knew it’s flaws from the user end, and with the US teetering towards entering WW2, he was desperately looking for ways to give the US what he could to keep them in the fight. The Rifle was his first shot, this being his second.
So the heart of the system, and the heart of all the Johnson weapons, is a Short-recoil action. Short recoil works by having the bolt and barrel locked for a short amount of time under recoil, and after a point the barrel stops, and the bolt is unlocked allowing it to eject and chamber a new round.
Johnson achieved locking and unlocking the system with a set of rollers and angled wedges inside a cam track.
One of the benefits of this system is that the barrel can be removed quickly and replaced, whether for storage (as was promoted to Paramarines) or for sustained fire positions.

On top of it’s uncommon cycling mechanism, the M9141 does a few interesting and fairly rare things.
First: it fires from a closed bolt while in semi-auto, and from an open bolt in full-auto
Second: Both rifle and magazine can load from M1903 Springfield clips
Third: Using this system, you can load an additional 5 rounds over magazine capacity into the weapon.

The Magazine is odd too in a few ways – it’s single stack (it only holds 1 row of bullets), it has no feedlips, and the way it attaches to the gun is a big spring clip, that also incidentally holds rounds in while it’s not in the gun.

One of the things Johnson wanted to do to make the LMG more range-viable, was to give them sighted good enough to fire out to extended ranges.
In service of that- Lyman was contracted to make this dual aperture system.

Ultimately, the bid to become a secondary standard for the Infantry Automatic Weapon kind of fell through – The US really wasn’t interested in the idea. Johnson would go on update the pattern based on feedback from Aberdeen Proving Ground, and Johnson even had in the works a belt-feed option patented, and Johnson sold machining and technical package to the Fledgling Israeli army in the 50s, but the story basically ends there.

Technically fascinating, but ultimately a flop.