Calculated "base" dispersion of guns based on energy delivered

[Krusty]

Perhaps using 100% is misleading? Perhaps you should focus on the 75%, because while one gun may have, for example, 1.9mil 100%, it may be 1mil 75% (hypothetically).

I'd like to see what the MAJORITY of the bullets are going to land in, not what all of them will land in. All guns have their errant bullets, especially guns with high ROF. I don't want to see the 100%, because the few bad rounds are throwing the entire efficiency of the gun off.


So when people use 100% in computations, can you please include the majority (which I assume is 75% ??) as well, so we, the non-mathematically inclined, can read it and say "Hrm... the 100% is rather high, but it looks like MOST of the rounds are fairly accurate" and thus make up our own minds? Please?

[GODO]

Hohun, aren't the M2 4 mils for 75%?

[Krusty]

Godo, that can't be right.

At normal ranges (est 300yds) the spread would be 12meters, wider than some planes' wingspans!!

M2, if I recall, was the US version of Hispano, yes?

[straffo]

You're right Krusty ,having the dispersion only is not enought  the distribution is needed too.

The M2 is the 0.50 I think

[HoHun]

Hi Krusty,

>Perhaps you should focus on the 75%, because while one gun may have, for example, 1.9mil 100%, it may be 1mil 75% (hypothetically).

I think the factor between the 75% and the 100% distribution should be fairly similar for all guns. I've simply used 2 for the calculations.

Thus, you can arrive at the 75% dispersion by simply dividing the above dispersion radii by 2.

Regards,

Henning (HoHun)

[phookat]

Furthermore, a statistical model can be applied in-game to include both the 75% and 100% figures.  As in, pick a random 0-100, if it is 75 or less place it randomly in the 75% ring, else place it randomly in the 75-100% ring.

[Tony Williams]

Originally posted by butch2k
Tony official data says :
4 mils for 75% and 8 mils for 100% for the M2

Thanks - that's slightly worse than I had expected.

It may be that the 100% figure is always about double the 75% one.

TW

[Tony Williams]

Originally posted by Krusty
M2, if I recall, was the US version of Hispano, yes?

The most common type of US 20mm Hispano was designated AN-M2 (to indicate that it was used by the army and the navy). So the term 'M2', in this context, refers to the .50. The next versions of both weapons were designated M3, but neither had any impact on WW2.

TW

[butch2k]

NOTE that they are several definition of the mil, the one used by me and in US manual is 1/1000th of a rad.

[Crumpp]

The military definition of a mil is 1 meter width at 1000 meters distance.  The same unit of measure we use today, they used back then.  Mils are primarily used in for laying indirect fire weapons as it is a more accurate measurement than a degree.

IE - 360 degrees make one revolution.  IIRC 1 degree = 13 something meters @ 1000 meters

6400 mils make one revolution.  1 mil = 1 meter @ 1000 meters.  

Those Buff gun dispersions are using this definition.

A 3 mil dispersion will land 100 percent of its rounds in a 3 meter circle at 1000 meters if no other factors effect it.

It is definition number 1 on this page.

http://www.hyperdictionary.com/dictionary/mil


Crumpp

[butch2k]

Crummp that's exactly what i'm saying :

1 mil = 1/1000 of a rad ("British angular mil"), you are speaking of the modern day Nato mil : 1/1600 of right angle, which are roughly equal.

There are at least a couple of definition like :
1 mil = 1/1000 of a rad (used in some us manuals)
1 mil = 1/1000 of right angle (used in Some wartime us Manuals)
1 mil = 1/1600 of right angle (Nato std)
the last two obviously being not equivalent.

[gripen]

Originally posted by GODO
Im convinced of the opposite.

Do you think that the dispersion caused by kinetic energy and rate of fire is more significant than dispersion caused by mounting?

Originally posted by HoHun

Actually it was possible to point the aircraft nose with very high precision. Lopez in "Fighter Pilot's Heaven" describes camera gun runs with a P-38 which shows that he was tracking the practice target with about 1 mil accuracy in the best recorded runs.

Hm... I don't know if this kind of accuracy was reachable in the practice, the P-38 was the heaviest fighter as well as one of the most stablest and the target was probably not maneuvering. And these tests might had been done with gyroscopic gun sight.

gripen

[Charge]

Some thoughts, correct me where I'm wrong:

Barrel length ->longer barrel flattens trajectory, no effect on accuracy by itself

Velocity-> has no effect on accuracy itself but combined with ROF causes decreasing accuracy

Caliber/bore-> the bigger the better when accuracy is concerned
(So firing 10 single rounds 30mm gives better result than firing 10 50Cal single rounds in 1000 meters??)

ROF-> the bigger the worse it gets when accuracy is concerned

Weapon weight->The heavier is better for accuracy

Rigidity of mounting-> Increases accuracy and helps to overcome the bad effects of high ROF.

Wing/engine mounting-> wing mounting increases dispersion and engine mounting decreases it.

Centerline/wing mounting-> in terms of ROF the center mounting is better as the countering force for recoil is directly behind the weapon and not to the side of it as in wing mounting (small effect in all...)

The uniformity of ammunition-> increases accuracy as all the ammo has same muzzle vel and weightand thus similar flight paths.



According to these assumptions the MK103 should be very accurate except when hung under a wing. But it is not? So what of my assumptions is wrong?

The MK108 has many good features but has bad trajectory and needs rigid centerline mounting to be accurate in rapid fire because of its light weight.

I'd imagine ME110 with MK108s and P38 with 50Cal and Hisso to be very accurate mountings and of course all the a/c with centerline mountings.

MG FF is still quite accurate despite its wing mount owing to its low ROF and moderate muzzle velocity. The hispano is also good despite its non-optimal mounting in Spit or Hurri's wing due to its weight and relatively slow ROF? I'd imagine the wingpod mounted MG151/20 to have quite high dispersion and very much different than if mounted centerline as in 109.

-C+

[rshubert]

Originally posted by Tony Williams
Agree with Henning. There is no necessary relationship between gun power and dispersion; it depends on the gun mechanism and ammunition.

The Browning was never a particularly accurate gun in any calibre - the short-recoil design meant that the barrel had to move around instead of being fixed. The Hispano was reckoned to be significantly more accurate.

 

For once, I disagree with you Tony.  Carlos Hathcock, a famous Vietnam era Marine Sniper, once made a one-shot 2500 yard (measured) kill with a scope mounted M2 HB machine gun using standard military issue ball ammo.  It's documented in the book, Marine Sniper.  I don't have the ISBN or author handy--I'm at work right now.  The browning short recoil system has been used for highly accurate premium grade hunting rifles, many of which are capable of 1 MOA or less accuracy.

The facts are really these:  Repeatability of shot placement is affected by many factors, only one of which is the "inherent" accuracy of a particular round.  One of the biggest factors is rigidity of the gun MOUNT.  Airplane wings vibrate when the guns go off, adding to the vibration of the gun and all the other factors.  To support your statement about the hispanos, their lower firing rate would help accuracy, since there would be more settling time for the entire mechanical system between shots.  Also, since the diameter of the hispano barrel is greater, it is more rigid--and thus the amplitude of the sinusoidal vibration will be less.  As has been said, there are MANY factors.

[HoHun]

Hi Charge,

>Barrel length ->longer barrel flattens trajectory, no effect on accuracy by itself

The barrel is subject to oscillations that negatively affect dispersion. The longer the barrel and the higher the velocity the greater the dispersion.

>The MK108 has many good features but has bad trajectory and needs rigid centerline mounting to be accurate in rapid fire because of its light weight.

Let me define "point blanc range" as the extreme distance at which you can aim the sight at the dead centre of the target and have the centre of your pattern on the target area despite the trajectory drop.

In short, point blanc is the longest range at which you can successfully pretend that you're shooting a laser gun.

Well, for fighter-sized targets, everything up to 550 m has to be considered point blanc range for the MK108.

With regard to the factors infuencing dispersion, here's what I came up with - without any physical background, but matching historical evidence:

dispersion% = mechanism_factor * (a1 * v0% ^ 3 + a2 * calibre% ^ 2 + a3 * barrel_length% ^ 4 + a4 * rate_of_fire% ^ 4)

Percentages are relative to MG FF/M.

mechanism_factor portrays the relative accuracies of the different mechanisms. I picked 0.9 for API blow back (MG FF/M, MK108) and long recoil (Hispano II), 1.0 for most other weapons, and 1.1 for short recoil (MG 151/20, Browning M2).

This is somewhat arbitrary, but a1, a2, a3, a4 change if you pick other mechanism_factors, and those I chose gave the best fit to historical data.

a1 = 0,685413726
a2 = 3,71420497   
a3 = 0,490312235
a4 = 0,275524736

Here are my results:

Weapon - RL Dispersion (mil) - Calculated Dispersion (mil)
MG FF/M:    1,0 - 1,00
MK 108:     1,5 - 1,52
MG 151/20:  1,9 - 1,86
MK 103:     2,0 - 2,04
Hispano II: 3,0 - 2,95
M2:         4,0 - 4,04
0.30":      5,0 - 4,24

As you can see, the fit is quite good (+/- 2% accuracy) except for the 0.30" Browning. I've to admit that I've only guessed the "RL" accuracy, so the 16% deviation there doesn't seem that bad either.

Regards,

Henning (HoHun)