There are actually four M7-series rifle grenade launchers:
1. M7
2. M7A1
3. M7A2
4. M7A3
The latter three have additional range rings and different retaining springs.
The M7 launcher had a fixed stud that kept the gas cylnder lock screw open all the time when it was installed. This meant that the Garand was effectively a straight-pull, manually operated rifle if loaded with an 8-round clip of .30 ammunition.
The M7A1 through M7A3 added a return spring mechanism ahead of the bayonet stud latch and shortened the plunger that opened the gas cylinder lock screw valve. Rifles that with these grenade launchers could fire semi-automatically with M1 or M2 .30 Ball ammunition. The gas cylinder lock screw valve was opened when a grenade was fired, as only its recoil was sufficient to bleed off the excess gas.
M7A3 launchers were designed to be used with the "hump back" gas cylinder lock that began being fitted to rifles during the Korean War. The extra metal acted as a guide and stop for the recoiling launcher.
The .30 grenade cartridge M3 was loaded one round at a time and not from the 8-round clip. All M7-series launchers were designed to be used with an auxiliary grenade cartridge called the M7 (also known as "the vitamin pill"). The cartridge was installed in the end of the launcher and tail boom of the grenade slid over it. The combination of M3 grenade cartridge and M7 auxiliary cartridge extended the range of the grenade out to 300 to 400 yards.
Direct and indirect fire sighting was done by the M15 grenade sight. This stamped metal device attached to an elevation ring screwed to the side of the rifle stock.
