The title of the thread is more or less self-explanatory. In the course of the war, many belligerents found themselves short of ground MGs and transferred surplus aircraft guns to the ground role. To take just one nation, the British used a considerable number of ex-aircraft Lewis guns (many US made in .30-06) and later on employed Vickers K guns on the ground, most as vehicular guns and some with the commandos. Ex-US Browning M2 .50 aircraft guns were used on jeeps by the SAS, and I have found cases where Browning Mk II guns in .303 were mounted on vehicles and others were employed as AA weapons. So far so clear. I have a question, though. It relates to cooling. Aircraft guns had pretty high rates of fire. Despite this, cooling the guns was not a problem because of the cold temperatures of the airstream at altitude and so the barrels of the guns could be relatively light. But what do you do about cooling if you take an aircraft gun and try to use it on the ground? Were gunners simply instructed to go very light on the trigger, short burst only to avoid overheating, or what?
That's a good question. I don't know the answer but this info might prove interesting. The cook-off problem would be worse with higher cycle rates. The gun's original design was as a water-cooled machine gun (see the M1917 Browning machine gun). When it was decided to try to lighten the gun and make it air-cooled, its design as a closed bolt weapon created a potentially dangerous situation. If the gun was very hot from prolonged firing, the cartridge ready to be fired could be resting in a red-hot barrel, causing the propellant in the cartridge to heat up to the point that it would ignite and fire the cartridge on its own (a cook-off). With each further shot heating the barrel even more, the gun would continue to fire uncontrollably until the ammunition ran out, since depressing the trigger was not what was causing the gun to fire (although rarely as full rate automatic fire; it takes time for heat to soak into a cartridge, so usually it would manifest as a series of unexpected random discharges, the frequency increasing with the temperature of the barrel). Gunners were taught to cock the gun with the palm facing up, so that in the event of a cook-off, their thumb would not be dislocated by the charging handle, and to seize the ammunition belt and pull to prevent it from feeding, if the gun ever started an uncontrollable cycle of cooking off. Gunners were trained to manage the barrel heat by firing in controlled bursts of three to five rounds, to delay heating. Most other air cooled machine gun designs were fired in the same way, even those featuring quick-change barrels and which fired from an open bolt, two features that make air-cooled machine guns capable of somewhat more sustained fire, both features that the M1919 design lacked. M1919 Browning machine gun - Wikipedia
As a matter of interest, the British .303 Browning aircraft gun was modified to fire from an open bolt, thereby avoiding any risk of cook-off. This was done because the cordite used in most British ammo was not only liable to cook off, it would even cause the cartridge to explode, wrecking the gun. This happened during flight testing of an early closed-bolt version, which nearly resulted in the aircraft crashing. Clearly, the open-bolt .303 Browning would have been much better as a vehicle-mounted gun than .the US .30 version (with the addition of a much heavier barrel and a halving of the rate of fire, of course!). That could have been adopted instead of the 7.92mm BESA, saving on acquisition costs and the need for a separate ammunition production line.
There are some photographs from a parade in a Polish town (I forget which one annoyingly, so I can't find the pictures) of the German Volkssturm pulling 15mm aircraft machine guns on wheeled carts. The story goes that the local factories produced ME-109 wheels, and another these machine guns. So a bit of tubing, and the contents of the two factories are combined to make the ground mount machine guns.
I quote from the Central Guideline Shooting with Handguns 1202 Short bursts of three to five rounds each are to be fired with the bipod machine gun. The aim must be to keep the sheaf of bullets as tightly together as possible around the first accurate shot during each burst and to learn to re-aim between bursts in as short a time as possible. 1203.With the machine gun on bipod, single targets or single target groups are engaged with one to two bursts of fire. An area target or closely spaced target groups shall be engaged with multiple bursts of fire. As far as I know, this was also applied to the aircraft MG 15 and 81 for ground combat. The MG 15 was rock solid and caused no problems with its 1000 bpm cadence and 75 round saddle magazine. The MG 81 suffered mostly from the lack of a case separator, the need for special oil and the very cumbersome barrel change. If you didn't follow the guideline and fired in "Hollywood mode", even the barrel of the MG 3 (ex MG 42) burned out. (I know what I am talking about...cough)
The British use of aircraft MGs on ground mounts was (except for the VGOs) usually a case of extemporisation. I suspect the user's choice was between having MGs or not, so barrel overheating would not be a concern.
Ahh, the Pacific widowmaker with 1350 rounds/min firing this beast must have looked very similar like this:
The MG 15 may have been OK from the functioning point of view, but it was really too long and heavy for a ground LMG. Or so John Weeks says. As for the MG42/MG3...yes. People (casual WWII students) talk about the gun's high ROF as if that was an advantage, but you sure couldn't keep it up for very long. In Saving Private Ryan, the US infantrymen make a point of drawing a German MG team's fire and then making their rush when the crew has to change the barrel.
Extemporisation...yes, usually, but the many air Lewis guns issued for ground and AA use were often heavily and even semi-officially modified to suit, with wooden butts and bipods added, etc. The VGO (Vickers K) was also officially modified for commando use with a butt and a bipod.
Errr.....really great movie, but IMHO perhaps not quite the optimal reference..... I´ve shot the Iron Hog often enough: A barrel change takes three seconds and is primarily to maintain service life. But when SHTF, it doesn't matter if the barrel is annealed to scrap afterward...and that takes quite a while, if you keep at least halfway the fire discipline
