The M1 adaptor was a device meant to turn a standard hand thrown Mk. 2 fragmentation grenade into a rifle grenade.
The grenade was seated into the cup of the adaptor, held in place by metal fingers. The spoon of the grenade was slotted inside of a retaining clip.
To fire, the entire assembly would slip onto an M1 Garand that had been fitted with an M7 grenade launcher. The M7 provided a surface for the M1 adaptor to sit and it cut off the gas system in the Garand. The pin on the Mk. 2 grenade would be pulled, and the rifle would fire a hand loaded blank cartridge.
Upon firing, the retaining clip in the M1 adaptor would move backwards, striking a specially designed brittle metal retaining bar which would break. The clip would fall away entirely, allowing the spoon of the Mk. 2 grenade to release. The grenade would detonate with its normal timed function.
The M7 grenade launcher was replaced by the M76 grenade launcher when the U.S. military adopted M14 rifles. The M76 provided the same ability to launch rifle grenades.
Rifle grenades were eventually replaced by 40x46mm dedicated projected grenades for standard applications.
how far downrange can i launch the Mk. 2 with it
Wikipedia chart says 160 meters without a booster inserted, and 206 meters with a booster. There is no immediate citation and my internet is spotty, so if you want full confirmation I’d recommend looking for a TM or FM with a chart in it.
The booster charges were made from .45 ACP brass with no primer and more propellant inside the brass. They were loaded into the M7 grenade launcher’s muzzle before loading the rifle grenade onto it.
200m according to the wiki page