There is a clear demonstration of what the student was asked to do as part of the Rapid Fire exercise in the 1942 U.S. War Dept. Training Film (T.F. 7 1094) “Rifle Marksmanship with the M1 Rifle –Preparatory Training.” Going from standing to prone, on the command “LOCK, SIMULATE LOAD,” the student retracts the M1’s bolt, depresses the follower to simulate the loading of an en-bloc clip, the bolt travels forward to simulate the loading of a round in the chamber and then the shooter’s right index finger presses the safety rearward to the “on” position. Both acts are done in one smooth motion, but clearly the rifle was loaded before the safety was engaged. The commands “READY ON THE RIGHT, READY ON THE LEFT, READY ON THE FIRING LINE” ring out, at which point the shooter disengages the safety. The range officer then says “TARGETS UP,” and the shooter drops from standing to prone and begins to engage the targets with dry-firing.
The procedure seems consistent through the M1 and into the M14 era. In United States Army Marksmanship Training Unit Service Rifle Instructions And Coaches Guide 1967 Edition, the “LOCK “ is clearly intended as a command to manipulate the safety: “On command LOCK AND LOAD, he engages his safety and loads, making sure that the magazine is latched, a round in the chamber, and the bolt fully seated. On the command READY ON THE FIRING LINE, he unlocks the weapon.” Seems pretty clear, right? In FM No. 23-16 Automatic Rifle Marksmanship 23 June 1965, the commands for a firing exercise include “LOCK, ONE MAGAZINE OF THREE ROUNDS, LOAD.”
Once moving into the detachable box magazine era of the M16/M4 (remember the M14 could be loaded by stripper clips, too), it gets even more complicated. The Dept. of the Army’s FM 3-22.9 Rifle Marksmanship, M16-/M4-Series Weapons, August 2008, gives “LOCK ONE ROUND. LOAD.” and then “LOCK ONE OF THREE SINGLE ROUNDS. LOAD.” With the M16, the term “lock” is used to both direct manipulation of the safety, “LOCK AND CLEAR ALL WEAPONS,” as well as to “locking” of a magazine into the rifle, “FIRERS, LOCK YOUR FIRST 20-ROUND MAGAZINE. LOAD.”
So it appears individual rounds are locked, the safety is locked and the magazine is locked as well. Confused yet?