machine guns... what happens when you hold down the trigger?

[Citabria]

I found this very interesting... you guys might like it.
its from that cool groundpounder sim..
Combat Mission  
 http://www.battlefront.com/cmdemo.html

 

Hi,
When designing a machine gun there are a number of problems that have to be overcome. One of the biggest problems is how to deal with heat, particularly heating of the barrel and the chamber the round sits in before it's fired. When a round is fired there are two sources of heat, the propellant gases and friction between the bullet and the bore. Friction accounts for a very small amount of the heat transferred to the barrel; it's the propellant gases that are the problem. For a fraction of a second after a round is fired the gases reach a temperature of 2000degrees C and some of this heat is transferred to the barrel.
Barrels can be regarded as heat "sinks", they heat up quickly but if air cooled they cool down more slowly. When a barrel reaches a temperature of arround 500degrees C three things start to happen.You start to get "cook off" , which is when a chambered round ignites before it's struck be the firing pin due to residual heat; the rifling is stripped off by the friction of the bullet and you get barrel droop. All these factors make the machine gun unusable and the critical temperature is normally taken to be 475degrees C. The big question is how long does it take an air-cooled barrel to heat up to 475degrees C at a given rate of fire? Some examples follow that attempt to anwer this question.
1)A Stoner 63A LMG firing at a rate of 50 rounds a minute will take fifteen minutes to reach 475degrees C.
2)A SA80 assault rifle firing at 60 rounds a minute will take six minutes to reach the temperature at which "cook off" starts.
3)A Bren gun LMG firing at a rate of 120rds/min. will require it's barrel changed every two and a half minutes.
In summary one can make a small table of the above examples.
Rate of fire. Time weapon can be fired.
50rds/min. 15 minutes
60rds/min. 6 minutes
120rds/min. 2 and a half minutes.
Sadly I am not really computer literarate or I could enter the equations governing the heating of barrels and you could all work out for yourselves the possible duration of fire at any given rate of fire before jamming. The important piont to emerge is that a machine guns use-able rate of fire is below the "cyclic" rate of fire that is so often quoted. The German MG42 had a "cyclic" rate of fire of 1200rds/min. the US.30 M1919A4 a cyclic rate of 450rds/min. Yet even the M1919's cyclic rate is far above that which either weapon could achieve in the real world, assuming no extra barrels for the MG42.
Of course the MG42 and most modern machine guns had a quick change barrel so it would be interesting to see what happens when a crew have more than one barrel.If I once again use the Stoner63A LMG firing at a rate of 50rds/min.but this time with one spare barrel this is the result.
2000 rounds fired in forty minutes with each barrel being changed every five minutes results in barrel one being at 395degrees C, barrel two being at 410degrees C and the trigger being at 62degrees C. Once the barrels have been changed the first time they tend to increase from 300degrees C to 400degrees C during their five minutes of firing while the cooling barrel decreases in temperture from 400degrees C to 300degrees C, and so on. However over all the temperture of the barrels increase as the barrels cool slightly less in five minutes than they heat when being fired over the same period at this rate.
At this point its important to remember the exponential nature of the rate at which barrels heat up given even a small increase in the rate of fire above 50rds/min. If you increase the rate of fire to 60rds/min. the barrel heats from 300degrees c to 400degrees C in about two minutes.This in turn would mean that the cooling barrel would only have two minutes to cool, thus falling from 400degrees to 350degrees before it was its turn to be used again. Both barrels would reach a temperture of 475degrees C in about 17 minutes, from a cold start. To maintain a rate of fire of 75rds/min. for 15 minutes you would require two spare barrels and four spare barrels to maintaina rate of fire of 100rds/min. for 15 minutes.
Given the above what are the implications for CM if any? The major implication is that in my opinion, others will differ, the firepower of the MG42 in both LMG and HMG roles is overstated.In the LMG role it will have had a use-able rate of fire no greater than that of the Bren gun, or in CM2 the Soviet 7.62mm Degtyarev DP LMG.All three guns will have been restricted in their rate of fire by the problem of over heating at rates of fire far below the cyclic rate of even the Bren, the slowest of the three.(Assuming no spare barrels for the MG42 in its LMG role.) Indeed in the fifties when Britain was about to change to the 7.62 NATO round a report concluded that although the MG42 was a better all round weapon, in the LMG role the Bren was superior. As a result Britain kept the Bren until the 1980s in 7.62mm.
If you assume two spare barrels for the MG42 in its HMG role then its firepower should only be 50% higher than the M1919A4. Two spare barrels would give a rate of fire, over fifteen minutes, of 75rds/min. for the MG42 against 50rds/min. for the non-barrel-changing M1919. However I have no idea how many spare barrels German HMG teams had in reality.
It's worth remember that the three most famous machine guns developed as a result of the experience of WW2, the M60,FN MAG and Soviet 7.62mm PK, all have cyclic rates half that of the MG42. If the MG42's high cyclic rate had been a positive, rather than negative characteristic, it would have been copied post war. Non of this detracts from the fact that the MG42 was the "gold standard" machine gun to come out of WW2. Its just that the characteristic it's most famous for, its high cyclic rate, was not a positive feature.
Sources.
Small Arms, DF Allop and MA Toomey. Brassey's 1999. A textbook written by lecturers at the Royal Military College of Science, Shrivenham, UK. ISBN 1 85753 250 3.

Jane's Infantry Weapons 1993-4, and 1999-2000.
All the best,
Kip.
Stunning game, am still in shock!

[mason22]

heh, when i hold down the trigger, usually run out of bullets...  

[Pongo]

I didnt read it right through but he has to consider the weight of the barrel when deciding how one gun will heat up vs another. Most modern SAW size weapons have a much lighter barrel then a ww2 era MMG like the Browning or the Mg42. The extra heft of the barrel really helps it to withstand longer bursts.

[Toad]

Well, I can supply some anecdotal information for you.

My father flew a B-25C with the 345th in New Guinea <he has posted here as "Panther">.

When they hit a field, he told me they held the trigger down all the way across the field, resulting in a 15-30 second burst from the 8 forward firing .50's in the nose. They flew across the field line abreast and he said you didn't want anyone in front of you for up to 15-20 minutes after that firing run.

He said the the guns often kept firing <"cooked off"> when you first released the trigger and might fire 5-10 rounds, stop for a while, fire a few more, etc. With 8 guns doing this, it kept you on the edge of your seat.

This generally quit within a few minutes, but he said sometimes long after the run and all firing had stopped, a gun would "cook off" a few rounds many minutes later.

He also mentioned that by the end of the firing run, tracers were going up, down and sideways due to barrel heating. The first part of the run was pretty accurate but by the end it was more like a shotgun. Didn't matter, as the strafing was more to keep the AA gunners heads down than to actually strafe an object to destruction.

The parafrags did that!

[funked]

Heheh nice, both of you.  

Interesting that one could attack an airfield at low level in WW2.  I guess they didn't have computer-guided lasers killing them a mile out like we do.  

[ra]

In 'Report of Joint Fighter Conference' a Navy ordnance specialist discusses how long  fighter pilots could hold down the trigger before the rifling in the .50s would wear down.  He said the first burst was limited to 75 rounds, every additional burst 25 rounds, or damage would occur.  He also mentions that one of the advantages of the 20mm was that the lower rate of fire allowed longer bursts without causing rifling problems.  

ra

[Gadfly]

This would be an interesting and useful "first" for AH to model.  I think it would really make a difference in the arenas and the plane choices.  Go for it Pyro!

Lizking


 

[Citabria]

 

[This message has been edited by Citabria (edited 05-26-2000).]

[Hans]

One area you guys should remember is that the .50 cal on an aircraft is VERY much air cooled.  Your own foward motion will run fresh, cool air around and down the barrel to help cool the barrel, though it won't stop it from heating up.

So, a fighter could fire his M2-Browning longer than a tank or jeep could.  The 300mph wind blowing down the muzzle aids him.

Hanz.

[CavemanJ]

 

Originally posted by Gadfly:
This would be an interesting and useful "first" for AH to model.  I think it would really make a difference in the arenas and the plane choices.  Go for it Pyro!

Lizking


 

Just what we need, another reason for dweebs to climb into the DweebC uber-cannon hawg.
I'm really starting to hate the Corsair, which is a sad thing

[Rendar]

Look at the WWII gun camera footage.  You can see that even when the enemy pilots have a very good bead on their targets, they only fire in short bursts.  This was also true of planes attacking bombers.

------------------
Rendar

[ra]

75 rounds from a Browning .50 cal was about a 9 second burst.  Rifling problems mostly occured on strafing runs.  It wasn't a problem in air to air.

ra

[Toad]

BTW, Panther got his wings at Brooks field 57 YEARS and 2 days ago.  We had a small party  

He actually got two sets, Pilot Wings and Observer Wings. Got the Observer wings because the powers that be thought a B-25 Recon squadron would be cool. That idea never made it to combat, but he did graduate from the school.

Now, on topic, he also said they replaced an awful lot of .50 barrels. It was expected and it was no big deal; they had lots of them. The armorers inspected and repaired/replaced after a mission.