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[FIXED] M-61 Vulcan and Gau-8 Avenger dispersion values


nighthawk2174

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This thread is all about all the information I’ve been able to gather that supports that the dispersion of the Gau8 and M61 in DCS are WAY WAY WAY too high.

 

Current values in DCS (Da0):

 

DRxKkjp.jpg - 22 milliradians

C5Z9s6m.jpg - 17 milliradians

 

M61

 

Video 1:

 

https://www.military.com/video/guns/machine-guns/m61-vulcan-20mm-cannon-on-f-106a/3732679073001

In this video a firing test in an aircraft resulted in dispersion of 95-99% falling just over 4milliradians

 

Video 2:

 

This video is of a test of an M61 vulcan

 

9GXE10y.png

 

 

As you can see the grouping is quite small but in order to get usable data out of this, we’ll have to do some math. Now from this still image we see that a bullet is about ~5x5px. Meaning that assuming (since this is cardboard) that the bullet hole is pretty much the size of the round we get 4mm/px or 0.1575 in/ px. Making the grouping 65x80px or 10.24x12.6in across.

 

From here we can now find the range, knowing that the grouping is just about 12.6 inches or 1.005ft across we can do some basic geometry and trig to get the range. Using the barrel diameter .6343ft and the approximation that if our camera was right behind the gun it would be 7.5ft away we can find the range to the target. This ends up resulting in a range around 100-115ft, which is quite reasonable here. This would mean a dispersion of 4.35-5.00 milliradians. Now this number makes the 5 milliradian values from the documentation bellow more than likely to be representative of active duty guns.

 

sYlNfnu.png

We get the range to be ~100ft leading to a dispersion value of ~4.35 milliradians.

 

 

 

Documentation:

 

 

From paper “Project Vulcan” tests of active duty M61 Vulcans

 

zYh5cyF.png

ANBHp6d.jpg

C4LnQbp.png

0tzuaH8.png

CEn6gu7.png

Mw9tt9j.png

C1k2nGF.png

 

Of note Ship based Vulcan's have additional clamping to significantly reduce their base dispersion values. This is done as various studies had identified as 1 milliradian or less of dispersion to be the required value for gun based CIWS.

 

AKrKxoH.png

HPXBwjQ.png

 

-These are from a paper disusing how to improve dispersion of the gun, and as far as I can tell the recommendations of the paper were put into place.

 

--

Breifly the

GAU-8]

l9mkQNg.png

Now knowing the approximate distance to the target from the M61 (100-115ft) we can give this gun a go. The grouping is x px. With a single bullet being 11x11 px meaning our scale is .1074 px/in. Taking this scale, we get the main grouping to be 115x82px or 12.3x8.8inches. Now that we know the size of the grouping and using some trig we get the dispersion to be: 4.45-5.12millirad for the largest dimension of the grouping.

 

Documentation:

 

https://ia802700.us.archive.org/6/items/combatdamageasse80007stol/combatdamageasse80007stol.pdf

 

In this doc we get several runs on M48 tanks, looking at the hits on the tanks, the firing ranges, and grouping on the tank would support the above dispersion values.


Edited by nighthawk2174
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From my little understanding :

 

The values you provide here are for a gun, or a plane+gun, that is attached to a Bench, locked.

The values in DCS (22 milliradians IF "Gau-8 : 0.0022" = 22 milliradians ?) are for an airborne gun, which can't be the same.

 

The Bench absorbs recoil and vibrations, not what an airborne plane can do. More obvious, if the stabilization system on A10c doesn't work the nose goes up when firing, giving a catastrophic dispersion.

 

For a better comparison you should provide airborne dispersion values from real tests in first place, which seems to me not possible ? (it's not tested this way for real ?) Or the comparison results would be aberrant ?

 

grouping.jpg.14ef11337fbeaf06d5a928c209ebebbb.jpg

 

Nice grouping !? no ?

 

 

EDIT : Ok Ramsay - your post and the link you provide are clear, I was thinking wrong


Edited by toutenglisse
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From my little understanding :

 

The values you provide here are for a gun, or a plane+gun, that is attached to a Bench, locked.

The values in DCS (22 milliradians IF "Gau-8 : 0.0022" = 22 milliradians ?) are for an airborne gun, which can't be the same.

 

Planes in DCS are in motion, there is no need to add additional dispersion to account for effects that are modelled in DCS.

 

Also ED's DCS values for "some" ammunition match published specs., while others do not.

 

IF "Gau-8 : 0.0022" = 22 milliradians ?

 

AFAIK,

 

• Da0 = 0.0022 does not equal 22 mil.

 

Dispersion in mils = Da0 (from shell table.lua) * 8 * 1000, so

 

• Da0 = 0.0022 = 0.0022 * 8 * 1000 mil = 17.6 mil

 

Source: https://forums.eagle.ru/showthread.php?p=3956553#post3956553

 

IRL 80% of the GAU-8's bullets should fall within a 5 mil circle.

 

For Info:

 

The 1962 M-61 test paper (page 24, December/January) lists 80%/5 mil => 100%/10 mil

 

• Da0 = 10 mil = (10 / 8 ) / 1000 = 0.00125


Edited by Ramsay

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Planes in DCS are in motion, there is no need to add additional dispersion to account for effects that are modelled in DCS.

 

Also ED's DCS values for "some" ammunition match published specs., while others do not.

 

 

 

AFAIK,

 

• Da0 = 0.0022 does not equal 22 mil.

 

Dispersion in mils = Da0 (from shell table.lua) * 8 * 1000, so

 

• Da0 = 0.0022 = 0.0022 * 8 * 1000 mil = 17.6 mil

 

Source: https://forums.eagle.ru/showthread.php?p=3956553#post3956553

 

IRL 80% of the GAU-8's bullets should fall within a 5 mil circle.

 

For Info:

 

The 1962 M-61 test paper (page 24, December/January) lists 80%/5 mil => 100%/10 mil

 

• Da0 = 10 mil = (10 / 8 ) / 1000 = 0.00125

 

First, there are no additional dispersion effects for planes in motion in DCS there is a person in another thread on this that compared the dispersion of a jet on the ground and in the air and they were identical. Additionally I highly doubt this has any impact at all on actual dispersion values. The amount of time an actual shell is in the barrel while accelerating and then the amount of time it would spend in the air flowing over the jet is just so unimaginably small that it has practically no impact. The reason for dispersion, as pointed out by the paper I linked, is the actual warping of the barrel due to the torque being applied to it. It causes the barell to both twist and warp up/down a very very very small amount but enough to imapct dispersion.

Me quoting these values as the actual miliradian values comes from in game tests:

 

https://www.dropbox.com/s/d1w8d8lsvqhafw6/Tacview-20190520-012651-DCS-Gau8test.zip.acmi?dl=0

 

In this ACMI you can see that I got my A-10 right at 2007 ft from the target. The resulting dispersion circle has a radius of 1.5 to 2 btr lenghs (25.7ft = length of a btr). It is harder to tell in the ACMI so I will see if I can either A) replicate the scenario and take screenshots or B) see if I took any and update this post latter. Additionally the GSH301 and GSH23 have a value set of .0005 and .0007 respectively. There is a whole fourm thread where testing was done to prove this I just haven't found it yet.

 

LkkCOn2.png

eBd0LHm.png


Edited by nighthawk2174
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  • ED Team
From my little understanding :

 

The values you provide here are for a gun, or a plane+gun, that is attached to a Bench, locked.

The values in DCS (22 milliradians IF "Gau-8 : 0.0022" = 22 milliradians ?) are for an airborne gun, which can't be the same.

 

The Bench absorbs recoil and vibrations, not what an airborne plane can do. More obvious, if the stabilization system on A10c doesn't work the nose goes up when firing, giving a catastrophic dispersion.

 

For a better comparison you should provide airborne dispersion values from real tests in first place, which seems to me not possible ? (it's not tested this way for real ?) Or the comparison results would be aberrant ?

 

[ATTACH]216735[/ATTACH]

 

Nice grouping !? no ?

 

 

EDIT : Ok Ramsay - your post and the link you provide are clear, I was thinking wrong

 

 

Before starting the discussion one needs to have an agreement about dispersion unit - what kind of units you are operating: 100% circle, 80% circle, 50% circle, standard deviation, median deviation and recall the ratios between them.

 

You are absolute right: the documents that were mentioned, as far as I can see, is for factory test bed or naval heavy duty mounts. Airframe mount can not be so rigid, so the overall dispersion indeed will be noticably more.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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Currently in DCS the value is for a 100% circle but all the rounds are evenly spread within that cricle. Its not like rlf where the 100% circle is determined by a few outlier rounds. Now it is just a pure guess that an aircraft mount can't be as rigid as a test bed. In the first video the tests were conducted with the gun in the actual aircraft. Just to also fully disprove this point:

 

2UIM0O7.png

 

here you can see the mounting mechanism for the M61 in the F15. It is every bit as extensive as the test bed if not even more so as it actually has a clamp near the muzzle where the test bench does not.


Edited by nighthawk2174
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  • ED Team
Currently in DCS the value is for a 100% circle but all the rounds are evenly spread within that cricle.

 

Absolutely wrong statement. Absolutely. In both parts.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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You are absolute right: the documents that were mentioned, as far as I can see, is for factory test bed or naval heavy duty mounts. Airframe mount can not be so rigid, so the overall dispersion indeed will be noticably more.

But that's something that should come out of the flight and model, not be inherent to the gun or the bullets, surely?

 

Sure, it doesn't matter specifically what the dispersion unit measure — what matters is where it sits in relation to other guns+bullets under the exact same circumstances. That means the only sensible thing to compare are those locked-down bench numbers.

 

If airframe mounts make a difference, that should go into the modelling of the airframe not the bullets. If aircraft motion makes a difference, then it should go into the motion modelling, not the bullet. Etc. Every time this discussion comes up, that same argument is trotted out: “in the air, in the A-10, we see number x, so it's accurate” but that argument only works if the whole setup is so ill-conceived that bullet parameters are tainted by aircraft and motion parameters, when they should be completely separate.

 

So yes, before this discussion descends into the usual back-and-forth between different collections of tabulated data, that's really the question that needs to be answered: what does the hard-coded deviation measure? Are airframe and motion deviations included, or are the kept properly separated?

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The big problem here is that people are comparing componet accuracy (the gun, either bolted to a test bench or in an aircraft tied down during the test) and system accuracy, where the gun is fired from an airborne aircraft, where the recoil forces actually cause the aircraft to move by a tiny ammount. In very simple terms: the gun fires, which causes the aircraft to move, which causes the gun to point in a slightly different direction.

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If anyone has any questions about GAU-8 accuracy or rated mil dispersion, I've fired it. 80% of rounds within 5 mils, 100% of rounds within 13 mils, can vouch for this published dispersion. Questions?

 

 

 

Habu


Edited by Habu23

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Concerning mounted on the ground or on a plane I don't really see a problem as showcased in this video of the GAU-12/U used in a gun pod on an AV-8A.

 

 

From the video:

 

- on a fixed mount on the ground: design specification 5 mil, achieved 3 mil

- on the airplane: design specification 7 mil, achieved 5 mil

 

This shows that

a) the ground and plane mounted accuracy doesn't deviate by much even when fired from a moving aircraft (and this is pod mounted, not mounted inside the fuselage with better center of gravity concerning recoil)

b) a gatling gun is very well capable of being *more* accurate than design specification

 

So I would expect the same (i.e. not a lot of difference) for the M-61, too.

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Agreed in all the stuff I've read on this topic the amount of dispersion difference between airborne and ground based was never discussed leading be to the conclusion that compared to the barrel deformation the amount of inaccuracy that could be caused by putting it into an aircraft is negligible.


Edited by nighthawk2174
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this has been reported before, and marked as a bug in the 18 forums, but it's such an easy fix... low hanging fruit for ED on this one.

 

The F-14 has a much lower dispersion, yet it's the same gun as the 18, 15, and soon to be 16... it's so easy to fix this it's insane.


Edited by Banzaiib
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The F-14 has a much lower dispersion, yet it's the same gun as the 18, 15, and soon to be 16...

 

According to Wikipedia (yes I know), the M61A2 has thinner barrels compared to the M61A1, to reduce weight. Am I right in thinking the material the barrels are made of is therefor different/sturdier? What would the difference of barrel thickness be when it comes to dispersion?

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That would just open another can of worms of speculation.

 

More interesting is that the design specification hasn't changed so it won't make a difference.

 

https://www.gd-ots.com/armaments/aircraft-guns-gun-systems/m61a1/

 

The formatting on that site is a bit weird, the M61A1 weighs 248 pounds, light and heavy barrel both mean the M61A2.

"[...] because, basically, in this day and age, if you get to the merge and no one's died - it's not good for anybody." - Keith 'Okie' Nance
"Nun siegt mal schön!" - Theodor Heuss, September 1958

"Nobody has any intention of building a wall." - Walter Ulbricht, June 1961
"Russia has no plans to invade either Ukraine or any other country.
" - Vladimir Chizhov, Russia's ambassador to the EU, January 2022

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I found some extra footage that may be able to hep visulize the amount of dispersion these guns have:

 

 

-For this one in particular are there any F4E fans out there that know the size of the F4 gun reticule shown here? If so we get an estimation on the dispersion from that and from the size of the mig21 being a reference point.

 


Edited by nighthawk2174
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Fair enough. I was under the impression our lot 20 Hornet had the A2 gun. Hence my question...

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