Bug? Weired gun/bullet behaviour

[Flagrum]

There were some reports in the past where one would fly through their own bullets (and hitting the own aircraft? Not sure ... but still). Afaik it was most noticeable when the bullets ricocheted(sp?) from the ground. In the posted videos and/or tracks it was visible that something was strange, but it was not really clear what was happening.

 

So I made a test today: P-51D sitting at the end of the runway, firing all guns until empty. I reviewed the whole thing in TacView:

 

It took about 30 seconds to expend all bullets.

 

The first ~10 seconds were constantly like this:

 

then, during the next ~5 seconds, the performance degraded quickly and rmained stable for the following ~5 seconds like this:

 

then, for the remaining ~1/3 of the total firing time (the last ~10 seconds), it looked like this:

 

Can this be the result of overheating guns? Somehow I doubt that - or at least, I doubt, that this is the whole reason for it. An overheated gun would produce more spread, right? But less V0?? And the fact, that the degradion is not constantly getting worse, but comes in 2 distinct steps is strange, isn't it?

 

An other (intersting?) observation: it looked to me as if the performance drop seen in the last pic was when only the inboard guns were still firing (as the outboard and middle guns on each wing were already empty). Could it be, that there is perhaps some miscalculation happening? Perhaps something like "One 'shot' of the weapon system equals energy X. One 'shot' consists of 6 bullets. Therefore bullet energy = X/6" - which then leads to this weired effect when "One 'shot'" only consists of 2 bullets in the end?

Tacview-20140613-221945.txt.acmi.zip

Edited June 13, 2014 by Flagrum

[SkateZilla]

overheated barrels can cause slower rounds that go less distance, and more occasions where rounds can run into each other.

 

Simple solution to find out if it was heat related would be to fire in groups. with cooling time in between.

 

even during extended bursts, cannon barrels would overheat and rifling deteriorates / erodes.

 

What does the overhead views look like?

 

2nd and 3rd stage look more spaced out than the initial first stage.

Edited June 13, 2014 by SkateZilla

[gavagai]

Can this be the result of overheating guns? Somehow I doubt that - or at least, I doubt, that this is the whole reason for it. An overheated gun would produce more spread, right? But less V0?? And the fact, that the degradion is not constantly getting worse, but comes in 2 distinct steps is strange, isn't it?

 

Two stages makes perfect sense if there are two separate things occurring at different times. Don't expect physics and chemistry to conform to your logical intuitions.

 

As for the lower initial velocity, think of how metal expands when heated...

[Flagrum]

overheated barrels can cause slower rounds that go less distance, and more occasions where rounds can run into each other.

 

Simple solution to find out if it was heat related would be to fire in groups. with cooling time in between.

If you look at the last pic, the bullets would not even make it to the middle of the runway (Mozdok, 3500 meters), they get to about 1/3 of it.

 

But anyhow, I will follow your suggestion. Hrm, 5 second bursts with a cool off time of, 1 minute? Sounds reasonable?

[Flagrum]

What does the overhead views look like?

 

2nd and 3rd stage look more spaced out than the initial first stage.

It seems to me, that the bullet flight time is cut off and the bullets just disappear in mid flight (the yellow bullet trace in TacView ends, but that is also observable in-game) after a certain time in flight. I would guess, that is some fix made by ED to reduce the lag (produced by thousands of objecty flying dozends of miles).

 

So it is hard to tell if or how much spread there is initially. During the first 10 seconds, the bullets disappear when they still having much of their energy. In the end, they travel slower and when they disappear, they have already lost, like 95% of their energy. So just looking at the yellow lines might not tell us much about the real spread.

[Flagrum]

Two stages makes perfect sense if there are two separate things occurring at different times. Don't expect physics and chemistry to conform to your logical intuitions.

 

As for the lower initial velocity, think of how metal expands when heated...

Well, what two seperate things shall there be happening? The inner guns are firing constantly during the whole time.

[SkateZilla]

like I said, what does it look like in TacView from the Top?

 

you already see in the side view,

Image 1, the Trajectories are tightly packed.

Image 2, the Trajectories start to vary more vertically immediately after leaving the barrel

Image 3, the Trajectories are even more vertically varied after leaving the barrel.

 

if they vary that much vertically, you can assume they vary horizontally as well. which would decrease travel distance from a side view.

 

If the barrels are overheating, the rounds aren't getting the spin they were getting when they were cooler,

 

Without rotation of the round, you might as well be firing a musket.

[Flagrum]

If you look at the last pic, the bullets would not even make it to the middle of the runway (Mozdok, 3500 meters), they get to about 1/3 of it.

 

But anyhow, I will follow your suggestion. Hrm, 5 second bursts with a cool off time of, 1 minute? Sounds reasonable?

See attached TacView file. To me, it looks the same as before - only with one minute gaps. The first bullets fly much much farther than the last.

(difficult to put that into screenshots ... sry.)

 

So far, I am not convinced that this is how it should be. :o)

Tacview-20140613-233503.txt.acmi.zip

[Flagrum]

like I said, what does it look like in TacView from the Top?

 

you already see in the side view,

Image 1, the Trajectories are tightly packed.

Image 2, the Trajectories start to vary more vertically immediately after leaving the barrel

Image 3, the Trajectories are even more vertically varied after leaving the barrel.

 

if they vary that much vertically, you can assume they vary horizontally as well. which would decrease travel distance from a side view.

 

If the barrels are overheating, the rounds aren't getting the spin they were getting when they were cooler,

 

Without rotation of the round, you might as well be firing a musket.

 

Allright, the same three situations from above:

first 10 seconds:

 

during the middle 10 seconds. Note: this is somewhere during the first transisiton. You can see the bullets that were fired by all guns at the end of the trajectory and closer to the plane the bullets that are fired by only the inboard guns.

 

the last phase:

[Flagrum]

I can understand that the max distance gets shorter when the guns are overheated. What I don't understand is, why this is not a continuous process but instead a 3 phase process.

Phase 1: 10 seconds: max distance/speed.

Phase 2: 10 seconds: transistion from max to min distance/speed - roughly linearily.

Phase 3: 10 seconds: min distance/speed

 

And a cool down period between bursts make no difference - as if it is not the heat, but the number of bullets fired that matter...

 

edit:

Just occured to me: another thing to consider when analysing the bullet spread: the guns boresights are not parallel but they converge - by design - somewhere in front of the plane. Therefore the "spread" is artificially getting more and more after that point when the trajectories have crossed.

Edited June 13, 2014 by Flagrum

[billeinstein]

http://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law

Stationary AC has less air cooling.

 

You may find the Specific Heat Capacity of the guns and the fly time of the bullets in configuration files.

[Flagrum]

http://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law

Stationary AC has less air cooling.

 

You may find the Specific Heat Capacity of the guns and the fly time of the bullets in configuration files.

You are saying, that my test with 1 minute cool down between the bursts was probably too short?

 

Well, I found the LUA where the heat is defined (aircraft_guns.lua). Indeed, it seems that the heat produced by every shot is accumulated in the 6 kg steel of the barrel and the 32 kg steel of the gun itself - quite some mass that can probably store a lot of heat ...

 

I ran a test where I modied the file so that a shot did not produce any heat at all. The result was interesting. No derivation of the bullet velocity/distance - all bullets, from the very first to the very last, flew a trajectory as depicted in the first screenshot that I posted.

 

That demonstrates that the detoriating performance of the guns is indeed caused by the guns overheating. If the negative effects are realistic that way they are now ... I can not say. To me it "feels" a bit overdone ... but I leave that to be judged by some more knowledgeable than me.

 

Lessons learned (for me): only shoot if you expect to actually hit something. :o)

 

Thanks guys!

[Merlin-27]

Interesting find, Flagrum. Thanks for sharing your findings. I had no idea that was simulated either. The gunfire lag has taught me to keep bursts fairly short along with hopes of ammo conservation but this is another good reason to choose wisely.

[Random]

Need to find out if tge guns ever cool down. I've noticed recently that I can't hit a barn door when low on ammo. Regardless of how recently I was shooting. Round seem to go low which would fit with your overheating theory, but even after several minutes without firing and 250mph airspeed?

[Flagrum]

Need to find out if tge guns ever cool down. I've noticed recently that I can't hit a barn door when low on ammo. Regardless of how recently I was shooting. Round seem to go low which would fit with your overheating theory, but even after several minutes without firing and 250mph airspeed?

That would be interesting to test, yes. But atm I have no idea of how to produce comparable test conditions if the plane would be flying...

 

How long were your bursts usually before, when you experience this? I mean, one thing to keep in mind with all this: a M2 is a ~ 40 kg chunk of steel, enclosed in a container where no flowing air can reach them (i.e. build into the wing). If that chunk of metal is now so hot that it is almost red glowing ... it will just take some time until it is cooled down again.

 

And maybe - an other thing worth testing perhaps - if you fired an overheated gun for too long, it probably could be permanently damaged. I am no gun expert, but what others here in this thread said, could indicate that (i.e. destroying the rifling of the barrel).

[KLR Rico]

Wow, that's awesome that this has been modeled (or at least attempted?)

 

How about testing the cooldown by a firing a relatively short burst, using time compression to skip forward an hour or so, and then repeating until empty?

[Random]

Could work!

 

Could test if it will completely jam with heat too by setting unlimited ammo.

 

Could test with various ambient temperatures too.

[hattrick]

really cool find Flagrum! This explains alot for me having recently had difficulty hitting bandits at the end of my ammunition belts. Please do share tests with airborne data. I would think 5 seconds bursts with 5 minute cooldowns at speeds if 250MPH kr more would be more than enough to keep the barrels cool. Heres to hoping that barrel cooldown is modeled in flight.:gun_sniper:

Edited June 26, 2014 by hattrick

[Flagrum]

really cool find Flagrum! This explains alot for me having recently had difficulty hitting bandits at the end of my ammunition belts. Please do share tests with airborne data. I would think 5 seconds bursts with 5 minute cooldowns at speeds if 250MPH kr more would be more than enough to keep the barrels cool. Heres to hoping that barrel cooldown is modeled in flight.:gun_sniper:

Well, sorry to disappoint you ... and that so late ... but no, no cooldown as it seems.

 

 

BUT! ... still there is hope: http://forums.eagle.ru/showpost.php?p=2690166&postcount=10 ;o)

[Solty]

Can someone check if 109 and 190 are also affected?

[Chief1942]

I sense there are some major variables between your tests and the response of the guns used in the air and at altitude. When you look at the placement of the .50's in the P-51's wing mounts, they are relatively close together and for all intents and purposes, fully enclosed within the wing. My point is, there is very little chance for heat dissipation if the guns are fired for too long a period. I have fired .50 cals and even one fully out in the open, at moderate surface temps, will heat significantly if one fires too long an extended burst. Unless the operator uses short, infrequent bursts, they WILL be replacing the barrels in short order. One thing that comes into play as well is that with heat buildup, there is a subsequent change in barrel harmonics (how the barrel reacts as the projectile proceeds from the breach to the muzzle). That causes changes in the trajectory of the round once it leaves the muzzle. Armorers are usually fairly knowledgeable about these issues relative to the ballistics and the weapon function, and durability. Not so much the pilots when in the heat of battle. When I worked in "Ordy" on A-4's that were equipped with 20mm cannons (2) we had to caution pilots about sustained bursts (above about 3-5 round bursts) on their strafing run training, but I can't speak to how much they could remember of our admonitions while trying to fly the aircraft and hit the target at the same time in a combat environment. The barrels still had fairly short lifespans and they were even more exposed to the air in the wing roots of the A-4 than those .50's enclosed in the wings of the Mustang.

Edited February 25, 2016 by Chief1942

[arglmauf]

Let's assume the behaviour is correct: Is there a burstlength/pauselength pattern that we can maintain that will give us the best retained accuracy from the the first to the last bullet (not perfect, just the best retained)? If it's not broken, let's find out how to do it properly then.

[Solty]

I sense there are some major variables between your tests and the response of the guns used in the air and at altitude. When you look at the placement of the .50's in the P-51's wing mounts, they are relatively close together and for all intents and purposes, fully enclosed within the wing. My point is, there is very little chance for heat dissipation if the guns are fired for too long a period. I have fired .50 cals and even one fully out in the open, at moderate surface temps, will heat significantly if one fires too long an extended burst. Unless the operator uses short, infrequent bursts, they WILL be replacing the barrels in short order. One thing that comes into play as well is that with heat buildup, there is a subsequent change in barrel harmonics (how the barrel reacts as the projectile proceeds from the breach to the muzzle). That causes changes in the trajectory of the round once it leaves the muzzle. Armorers are usually fairly knowledgeable about these issues relative to the ballistics and the weapon function, and durability. Not so much the pilots when in the heat of battle. When I worked in "Ordy" on A-4's that were equipped with 20mm cannons (2) we had to caution pilots about sustained bursts (above about 3-5 round bursts) on their strafing run training, but I can't speak to how much they could remember of our admonitions while trying to fly the aircraft and hit the target at the same time in a combat environment. The barrels still had fairly short lifespans and they were even more exposed to the air in the wing roots of the A-4 than those .50's enclosed in the wings of the Mustang.

You mention A4... which has a Colt MK12 20mm cannon that fire's 1000rpm. While P-51 is using AN/M2 Browning that is using 12,7mm rounds at 750rpm. There is a big difference between a cannon and a machine gun in terms of overheating. And placement in wings should help the case, as air has to hit the root of the wing and it is certain that air goes through those holes as the mustang pulls over 2G turns that create the distinct whine.

 

To me this seems overdone.

Edited February 25, 2016 by Solty

[Flagrum]

The thing is - and that was the only reason I updated this thread - the cool down seems to be buggy as it never ever cools down at all. See the linked thread for the bug report.

 

That would probably explain why the dispersion is so exagerated in the end - as the guns are then probably virtually glowing orange ...