Slang, US Mil, adjective
The term generally refers to switching a selective fire weapon over to fully automatic fire.
The old M-16A1, the US military's Main Battle Rifle for the last 30 years or so (now replaced by the M-16A2), is what the pro's refer to as a selective fire weapon. Most small arms only have one mode of fire - bolt action rifles and pump shotguns cycle manually. Semiautomatic pistols fire one round for every pull of the trigger. The M-16A1, being a selective fire weapon has what is predictably called a "selector lever". On the 16, if this level is set to "Semi", the auto sear is disengaged from the action, and the weapon cycles one round per pull of the trigger. With the selector flipped to "Auto" the auto sear is engaged, cutting the disconnector out of the action, and your M-16 will now cycle until the magazine runs dry, just like a machine gun. This is where the rock and roll comes in.
In the Vietnam War US soldiers were issued these wonder weapons. In a fire team, only one man, usually senior, was supposed to be an "Automatic Rifleman", that is to say, have his rifle set to "auto." Everybody else was supposed to be a "Rifleman" - rifle set to "semi" and cranking out sustained aimed fire.
In wars past, like WW II the Korean War, these riflemen would have been issued a MBR like the M1 Garand, a weapon only capable of producing semi-auto fire. In sunny Vietnam, military planners soon learned a new rule of infantry combat: without superior training and rigidly enforced fire discipline, soldiers in combat will fire their weapons at the highest available ROF (rate of fire). So it was not uncommon to hear people screaming in a firefight "Rock and Roll, rock and roll!" Meaning, "everyone flip your rifles over to full-auto!" This could go a long way towards explaining why over 50,000 rounds of ammunition were fired for every confirmed kill in Vietnam, and why the new M-16A2 no longer features full-auto fire. It instead has a mode called "Burst", with fires bursts of 3 rounds with each pull of the trigger. It's a mechanical enforcement of good fire discipline. Like the Cadre in ROTC used to teach us, "Aimed fire wins battles".
"Rock and roll" is also a great example of how an age and it's technology create slang. If selective fire assault rifles had been available in the American Civil War, they everyone might have been screaming "Gin it up!" or "Dance with the Devil's Fiddler, Boys!"