Light machine guns of the world

Modern infantry tactics are based on the squad, a dozen men with one light machine gun.  In action the squad moves forward until resistance is encountered.  At which time the machine gun is set up, and under  cover of its fire, the riflemen advanced to the next likely piece of cover.  Then the riflemen provided covering fire while the machine gun is moved up to the new position.  By WWII, the old close order tactics, which go back as far as the Greeks at Marathon,  had given way in all armies to the modern tactic.
   The light machine guns in question varied from army to army.  But they all fired the standard rifle round of the period, which was 30 caliber (7.62 mm) and a lot more powerful than modern military rounds such used by weapons like the US M16. The weapons all fired from the open bolt, a machine gun design feature that leaves the breech open after firing stops, allowing air to circulate thru the hot gun barrel for cooling.  It also avoids leaving a live round in a red hot chamber where it might cook off from the heat.  The down side to the open bolt design is the slight jar when the bolt goes forward and chambers a  round which can throw the gun slightly off target, a minor concern, only of importance when firing single rounds, sniper fashion. 
  Since the LMG was back packed into action, light weight was very important.  The American Browning Automatic Rifle (BAR) was the lightest at 15.5 pounds (unloaded).  Figure another pound and a half for a loaded 20 round magazine.  The heaviest was the Russian DPM at 26.9 pounds., with the German MG42 right behind at 25 pounds. 
    Most of them (BAR, BREN, and DPM)  fired at 500-600 rounds per minute, which was considered the optimum rate of fire by authorities of the period.  Those authorities felt higher rates of fire merely wasted ammunition.  The exception was the German MG42 which fired at double that, 1200 rounds per minute, which gave the German gun a unique and scary sound. 
   The BAR with a 20 round magazine, held the least ammunition.  The BAR magazine was located on the bottom of the weapon which made swapping magazines somewhat awkward.  The British BREN gun had a 30 round magazine on top of the gun, making magazine swaps easier.  The Russian DPM had a 47 round drum magazine on top.  The German MG42 was belt fed,  allowing long sustained bursts of automatic fire.