Wow! Some interesting (and wrong) answers.
"Lock and load" comes from the rifle range, not really from combat.
To begin with, "lock" has nothing to do with the locking the bolt, or with flintlock muskets. It means to engage the safety, that is to put it in the "ON" position. "Load" means to load a round into the chamber.
In the days of the Model 1903 Springfield, the command was "Load and lock", since the M1903 could not be made safe with the bolt open (the mid-position of the safety was used only for disassembly). The clip was inserted in the clip slot, the rounds stripped into the magazine, and the bolt closed. Then the shooter engaged the safety.
The M1 rifle could be loaded with the safety on, so when it was adopted, the command was reversed. "Lock" meant to engage the safety, and "load" meant (usually) to insert an 8-round clip and load the first round into the chamber.
Note that it is not necessary that "load" mean a full clip or a full magazine. The initial part of the command tells what to load. For example, it can be "With one round, lock and load", or with an M14, "With a 20 round magazine, lock and load." The command would also specify special ammunition if applicable, such as "With eight rounds of tracer ammunition, lock and load."
Jim