Bug Report: F-15 gun accuracy does not match sources

ETA:

Testing has revealed that the Extreme Spread of the F-15's M61A1 cannon current in DCS is over 20 mil, with 80% of rounds passing within a circle 8 mil in diameter. The contention is that this may be due to a misinterpretation of the confusing figures used in documentation.

The video below shows an M61A1 mounted to an aircraft being test fired on the ground at a target with an 8 mil circle. 99% of rounds from a 100 round burst fall within an 8 mil circle. The speaker states that observed performance was between 99-95% within the 8 mil circle during all tests. The current in-game dispersion is larger than that seen in the video, and may be due to the difficulty of interpreting the rather vague published text on the matter. The linked video below appears to be conclusive, however.

This US Airforce video at 4:15 shows an M61A1 20mm cannon accuracy test.

I wish to state that, in the course of investigating in-game performance, I found that the cannons in-game perform almost exactly as the most common sense interpretation of the accuracy figures would suggest (i.e. 80% of rounds passing through an 8 mil diameter circle for M39 and M61A1 cannons and 80% through a 5 mil circle for the Gau-8 ). ED has done excellent work in making the current system conform to the real performance of the guns to what I believe was the best of their knowledge. I mean only to present this video as evidence because it seems to indicate that the current interpretation of the data is incorrect.

M61 super shotgun device, puts the Doom Slayer to shame :D

IIRC the M61 should have an 8 mil dispersion at 1000 feet. So 90% of the bullets should pass through an 8 foot diameter circle at 1,000 feet.

It's the same with the F-5E's guns, the Huey's miniguns, the GAU-8/A cannon...

IIRC the M61 should have an 8 mil dispersion at 1000 feet. So 90% of the bullets should pass through an 8 foot diameter circle at 1,000 feet.

8 mil is an angular measurement, so "8 mil" is 80cm in diameter at 100 meters, 4 meters in diameter at 500 meters, 8 meters in diameter at 1000 meters, etc. 8 mil is always 8 mil, no matter what distance you measure it at.

It has a relationship with meters similar to how MOA relates to inches.

The M61 Vulcan cannon has an accuracy requirement of 8 mil, 80%. If more than 80% of fired rounds land outside of an 8 mil diameter circle during maintenance tests, repair is required.

8 mil is an angular measurement, so "8 mil" is 80cm in diameter at 100 meters, 4 meters in diameter at 500 meters, 8 meters in diameter at 1000 meters, etc. 8 mil is always 8 mil, no matter what distance you measure it at.

 

It has a relationship with meters similar to how MOA relates to inches.

 

The M61 Vulcan cannon has an accuracy requirement of 8 mil, 80%. If more than 80% of fired rounds land outside of an 8 mil diameter circle during maintenance tests, repair is required.

Yeah, I worded that poorly. Regardless it should be 8 mil dispersion 80% circle. A quick and dirty way is milliradians will equal the diameter of the circle (in feet) at 1000 feet almost exactly: Tan(.008 rad)*1000

How are you testing it in DCS? I’ve wondered how accurately it’s modeled but never tested.

M-61 has free-floating barrels to increase the spread. Whether they're used in the F-15C and how they're trimmed is anyone's guess. I recall some study about a specific 14mil pattern being set up for the F-16 with the same gun, but I can't find it now :/

A bit off topic but just my opinion if they were to fix something it would be the lag faced in MP. I would be willing to bet the missile and gun fixing of realism threads would go down 90%. I see probably 60-80% of peoples missiles missing due to lag.

https://clips.twitch.tv/CloudyStupidCarabeefRuleFive

Yes, you're way off topic, and it has nothing to do with actual gun dispersion.

In any case, I'm also thinking that we're pretty much never testing the gun from a stationary aircraft and so, it is possible that the dispersion appears to be larger than it ought to be (ie. it doesn't fit correctly inside an imaginary 8 mil circle in the HuD).

Edit: According to what OP found in the LUA files, this would be an easy thing to test and fix.

Second Edit: Are there too many zeros in the OP's post for everything non-M61 in fact or is it a typo?

I completely missed the bit about the lua file. My bad.

Milliradians should be .00X radians, right?

That's what I'm thinking, in which case while the M61 has too much dispersion, a bunch of other guns have too little.

:gun_rifle:Waaay too much public math in this thread.

I tried to follow, but [in English] are you talking about the spread at a certain distance...

... or the boresight/ pipper correlation?

The dispersion (spread at distance), not the parallax nor sight line calibration.

The dispersion (spread at distance), not the parallax nor sight line calibration.

Thank You.

M-61 has free-floating barrels to increase the spread. Whether they're used in the F-15C and how they're trimmed is anyone's guess. I recall some study about a specific 14mil pattern being set up for the F-16 with the same gun, but I can't find it now :/

Interestingly, although not exactly "free-floating barrels," the Museum at Hill Air Force Base has an M61a1 20mm cannon display which mentions that "dispersion patterns could be modified with the use of barrel muzzle clamps."

It looks like purposefully changing dispersion was designed to be an option! The next questions are: Was it ever used operationally? If so, was it a pilot's preference sort of thing? A unit level decision? Etc.

Until we have specific information, I think the "barrel muzzle clamp" has to be set aside in favor of what facts we have access to. The clear information we do have is that the unmodified gun's accuracy requirement is 8 mil, 80%. This means 80% of shots fired in a test must be within an 8 mil diameter circle. This implies that a larger dispersion would be a failure which, under normal circumstances, would require repair.

Waaay too much public math in this thread.

Public math?

It was 100% studied, but whether the results were used, I don't know. Sorry, I can't provide a source. I saw a study for this a long time ago and I don't have it nor do I recall the name :(

In any case, I did the best I could with testing. Whatever the numbers in the LUA are, they're not radians. 0.0022 feels like too big of a dispersion, 0.0008 feels way too tight (closer to 3 mil or less). My testing wasn't terribly scientific since I can't park the thing and shoot it at a wall. A bunch of sacrificial commandos placed at intervals were the targets, then analyzed more in-depth with tacview to eliminate subjective distance measurement by eye.

The value I came up with is around 0.0015 for the M61, and I suspect values for other guns would need to be raised or lowered to suit.

Again, not terribly scientific but there you have it.

Interestingly, although not exactly "free-floating barrels," the Museum at Hill Air Force Base has an M61a1 20mm cannon display which mentions that "dispersion patterns could be modified with the use of barrel muzzle clamps."

 

It looks like purposefully changing dispersion was designed to be an option! The next questions are: Was it ever used operationally? If so, was it a pilot's preference sort of thing? A unit level decision? Etc.

 

Until we have specific information, I think the "barrel muzzle clamp" has to be set aside in favor of what facts we have access to. The clear information we do have is that the unmodified gun's accuracy requirement is 8 mil, 80%. This means 80% of shots fired in a test must be within an 8 mil diameter circle. This implies that a larger dispersion would be a failure which, under normal circumstances, would require repair.

None of my lua hackery worked. Can’t get the cannon to fire on the ground.

An alternative would be to test the A-10C cannon on the ground to derive whatever units the LUA file is using from the GAU-8. Possibly some other aircraft have the capability too. Maybe a custom object with a target texture.

It seems odd how much larger the values are for the M61 and GAU-8 compared to nearly every cannon defined weapon in the shell table lua.

Aries, you seem to be well versed in the testing procedure. Any ideas as to how we could simulate it in DCS?

the more correct phrase is 'math in public'

 

as in one should not do it if one is weak in math or fearful of looking silly in front of others that might be far better than you are at doing math where someone other than you and/or your instructor can see the results.

Meh. I’m not a proud man. I’d rather look like a fool and learn why I’m wrong than not try at all. That’s the only way learn.

Edit: Some test footage, skip to 4ish minutes

None of my lua hackery worked. Can’t get the cannon to fire on the ground.

 

An alternative would be to test the A-10C cannon on the ground to derive whatever units the LUA file is using from the GAU-8. Possibly some other aircraft have the capability too. Maybe a custom object with a target texture.

 

It seems odd how much larger the values are for the M61 and GAU-8 compared to nearly every cannon defined weapon in the shell table lua.

 

Aries, you seem to be well versed in the testing procedure. Any ideas as to how we could simulate it in DCS?

 

 

 

Meh. I’m not a proud man. I’d rather look like a fool and learn why I’m wrong than not try at all. That’s the only way learn.

 

Edit: Some test footage, skip to 4ish minutes

First off, the angular relationship between 8 feet at 1000 feet vs 8 meters at 1000 meters is, of course, the same. You were exactly correct SinusoidDelta. I'm tired. :)

Second, thank you very much for finding that video! You've just given us the parameters of the test and a valid original source to present alongside our findings!

For a Test

First off, if we can somehow get bullet hole textures to show up on a target model, this will be much easier.

If we can't somehow get bullet holes, we're going to have to use a target that cannon shells will pass through without bursting, place the game camera next to the target, slow down game time as much as possible, make every canon round a tracer (or otherwise visible), elongate the tracer objects to keep them visible long enough as they pass through the target to record, record the "impacts" on/through our target to a video, and use video editing software to advance through the video frame-by-frame to mark each impact point on the target.

People we need:

1. Someone who can make and texture models for DCS and adjust them as needed
   
2. Someone who knows how to hack the game to modify the A-10C to make it a more stable shooting platform on the ground
   
3. (If we can't have bullet holes) Someone who can make every cannon round a tracer, slow the game time down more than currently possible, and possibly elongate the tracer models/effects to allow more time to mark them
   
4. Someone familiar with video editing software and willing to go frame-by-frame through a few minutes worth of slow motion impact footage to mark all of the outermost hits
   

Things we need:

1. A way to accurately measure distance in game.

• The most desired range would be 25.4 meters, as that is the shorter of the two distances used in the test video above. Other ranges could be used, we'll just have to convert the results so they can be compared with the ranges used in the video. Closer ranges should make for a more accurate test and are preferred.
  
• If the TGP is used for ranging, we'll have to adjust for the range error caused by the difference in range between the TGP mounted on a wing station and the gun's muzzle. This can be figured close enough using known dimensions and/or a little measuring with photoshop.
  

2. We need 2 indestructible, immovable, target models.

• If we can't have bullet holes, we need a target a bullet can pass through instead of exploding on.
  
• One 8 mil x 8 mil target with a "circle" texture that touches all four sides. (8 mil square with circle on it that touches each side = 8 mil circle.)
  
• One target about 16 mil x 16 mil. The bigger target would be placed behind the smaller target to record shots falling outsize the 8 x 8 mil target.
  

Once we know the range we're going to shoot the target from, we can then make the target dimensions match 8 mils at that range.

Since we can look up its dimensions, the tail of an aircraft parked next to our target in-game or in a model editor should be good to establish scale.

3. We may need to experiment with ways to immobilize the A-10C testbed. I don't know what's possible here, so I'll suggest hacks to increase mass and stiffen suspension or perhaps to over-weight the aircraft with ordnance and/or place a blocking object behind it to limit rolling and vertical stringing due to recoil.

4. If there is no way to get bullet holes, we'll need to record the firing with the in-game camera at a distance and angle that makes the points at which individual cannon rounds pass through the target as clear as possible. (No need to mark hits falling in the center 75% of the target circle.)

Or, someone with a connection to ED can simply tell us what real world values the numbers for the Da0 in the shell table lua equate to before I go stark raving mad, burst out of my apartment making airplane noises, and doing this in front of my neighbors: :pilotfly:

I think you can put chocks on an F-5 ... and there might be a breaker to release the WoW restrictions.

So the dispersion is wrong, but how wrong.

If it's not 80% inside the specified radius and 20 % outside, I haven't seen anyone say what the proportions are ... (as 20% are allowed to be outside, and some can be quite a long way outside)

Dispersion in game appears to be (but is not necessarily) 100% in. GAU-8, M61 and M39 all have patterns which are larger than they ought to be by quite a bit.

So the dispersion is wrong, but how wrong.

If it's not 80% inside the specified radius and 20 % outside, I haven't seen anyone say what the proportions are ... (as 20% are allowed to be outside, and some can be quite a long way outside)

Im not sure what you’re asking. We don’t have any accurate way to test the spread in DCS yet. We don’t know what units are being used in the lua but they certainly aren’t milliradians.

We don’t have any accurate way to test the spread in DCS yet.

That was my point.

That was my point.

Which is why I asked two posts before yours if anyone has an idea how we can test dispersion in DCS...