Rotating internal magazine, short recoil, which meant the barrel itself recoiled, basically like the MG42 but using a rotating bolt instead of rollers.
https://en.wikipedia.org/wiki/M1941_Johnson_rifle#Design
"The Johnson's short recoil reciprocating barrel mechanism resulted in excessive vertical shot dispersion that was never fully cured during its production life"
In the tests against the Garand, troops were more accurate with the Johnson
Initial trials were held at Quantico Marine Depot, Virginia, in May of 1940. This trial was supervised command of Captain George Van Orden and the firing party consisted of four distinguished Marine Corps. marksmen and a Chief Marine Gunner as the shooting coach.
The test took place on the afternoon of May 6 and the whole day on May 7, and consisted of instruction in the operation and firing methods of both types of rifles. Courses of fire at 300 and 1000 yards both slow and rapid fire were then shot. All shooters firing both M1 Rifle and Johnson Rifle at each stage.
Results concluded from the tests found that the Johnson Rifle was more accurate with a score of 81.2% total hits to the M1's 71.3%. There were no malfunctions with the Johnson Rifles or the M1Garands, however the M1's dropped their centre of impax during the test. The M1's were found to load and reload faster than the Johnsons but rate of fire in the rapid mode was identical with both rifles.
Capt. Van Orden's Summary Opinion stated "The Johnson Semi-Automatic Rifle, Rotary Magazine Type, is materially superior to the U.S. Rifle, cal. 30, M1 in accuracy and potential combat efficiency"