Calculated "base" dispersion of guns based on energy delivered

[HoHun]

Hi Karnak,

>That was Tony William's data, not mine.  I have no data on this subject other than Mr. William's books.

Well, it wasn't data, which is the problem :-/

It was a "soft" statement, and it doesn't worry me when one or the other soft statement contradicts a calculation.

I get nervous when they *all* contradict the calculation, though :-)

Regards,

Henning (HoHun)

[Crumpp]

I think this is good thing!  Give the weapons the advantages they had and not some generic formula.

Good Job Godo and Hohun !

Crumpp

[phookat]

Originally posted by HoHun
For the most wildly dispersing bomber guns, we probably have to blame the mountings alone, so it's not an universal principle though :-)

Ah yes, that is sensible.  We also have Wotan's data for some of the turrets, and that is prolly a good starting point for those.

[Tony Williams]

Originally posted by GODO
Hispanos being more accurate than 50s?? If you are talking about the very first shot of a single gun in a pretty fixed mounting, may be, still very hard to believe. But probably just the opposite with substained fire.

A good example would be the P38, with a single hispano and several 50" in the nose. I would like to see any comment of P38 pilots about the relative accuracy of these weapons.

As I've said before, the major determinant of dispersion in RL would have been aircraft movement in flight, followed by the rigidity of the gun mountings, followed by the accuracy of the gun. The first two are so significant that I don't believe that anyone would notice any differences in the third.

The trouble is that I read so much stuff that remembering where I read it is usually a problem...and I can't now recall where I read the comment about the Hispano's accuracy. I do have hard data on the accuracy of the .303 Browning as installed in the wings of the Hurricane and Spitfire; these weapons achieved 10 mils dispersion, with 75% of the shots within 5 mils. However, I'm not sure to what extent that's down to the gun, or to the mounting. Which prompts the question - does it matter, since it's the installed accuracy which counts?

A slightly later period, but this quote from 'Flying Guns – the Modern Era: Development of Aircraft Guns, Ammunition and Installations since 1945' might be of interest:

"Accuracy of the guns varies depending on the weapon and the installation. It is measured in mils (one mil equals one metre dispersion at 1,000 m). The .50 inch M3 could manage about 5 mils. The four Mk.12 cannon in the F8U were regarded as inaccurate, reportedly achieving only 12 mils (or 3.6 m at 300 m). The F 100 with four M39 cannon could get all of the shots within 8 mils and 75% within 4 mils. The M61 is capable of about 3-4 mils when internally mounted, although the centreline gunpod used in the F 4 is less rigid and can manage only 8-10 mils. "

P.S. I've just checked the gun pattern diagram for the Spitfire V included in 'Flying Guns: WW2'. This indicates the shot pattern for the guns at different ranges. I don't know how accurately they drew the dispersion circles, but the .303's scales to 0.5m at 100m, the 20mm to 0.45m.

Tony Williams: Military gun and ammunition website and Discussion forum

[GODO]

Originally posted by Tony Williams
As I've said before, the major determinant of dispersion in RL would have been aircraft movement in flight, followed by the rigidity of the gun mountings, followed by the accuracy of the gun.

Tony, the aimer can follow and know accurately the movement of the aircraft inflight while looking through the sight, so, that dispersion is somewhat under control, in fact the pilot can generate that kind dispersion on purpose. Dispersion of the gun and its mounting is what the pilot cant really control.

This thread is about calculated dispersion on nose mounted guns based on some known real dispersions of nose mounted guns.

We can generalize the mountings as nose mounted / wing root mounted, outer wings mounted, manned, turret and grounded guns. A final factor would be gun batteries, where the movements of one gun can also affect the others. We can use the nose-mounted dispersion as a base to extrapolate the dispersion for the other cases multiplying it by predefined values.

IMO, we are in the right direction.

[butch2k]

I have 3mils value for the 75% dispersion for the nose mounted hispano (comapred to 4mils for the M2) on the P-38.

[Wotan]

Originally posted by phookat
Ah yes, that is sensible.  We also have Wotan's data for some of the turrets, and that is prolly a good starting point for those.

Actually the turret data is completely irrelevant in the current discussion.

Here it is anyway:





Take the tail turret on the b17, 45ft dispersion at 600yrds. You can't really believe that would help determine the dispersion of the p51.

What makes me think this is just a goose chase to keep Mando pre-occupied is that as others have stated how secure the gun is mounted has more an impact on dispersion then anything. Not just wing vs. nose but how the gun is secured in the wing.

Look at the bomber gun dispersion. The variety is due to how they are mounted.

I doubt if Pyro will take Mando’s guesses over his own. But have at it...

[HoHun]

Hi Tony,

>As I've said before, the major determinant of dispersion in RL would have been aircraft movement in flight, followed by the rigidity of the gun mountings, followed by the accuracy of the gun. The first two are so significant that I don't believe that anyone would notice any differences in the third.

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.

The Luftwaffe measured the accuracy of their guns, and they arrived at very different values for the different cannon, with the MG FF/M being twice as accurate as the MK103 when they were both mounted in the nose of an aircraft.

Of course, the position had an influence, too. Due to an ambiguity in the German records, I'm currently not sure if it increased from 1.9 mil to 2.5 mil for the MG151/20 or whether I have to read this as 0.95 mil to 2.5 mil, but it certainly made a difference.

(When you're mentioning dispersions, are these dispersion radii or diameters?)

>I do have hard data on the accuracy of the .303 Browning as installed in the wings of the Hurricane and Spitfire; these weapons achieved 10 mils dispersion, with 75% of the shots within 5 mils.

>Which prompts the question - does it matter, since it's the installed accuracy which counts?

For us, it matters, since the same Brownings mounted in the nose of the Mosquito will fire more accurately :-)

>P.S. I've just checked the gun pattern diagram for the Spitfire V included in 'Flying Guns: WW2'. This indicates the shot pattern for the guns at different ranges. I don't know how accurately they drew the dispersion circles, but the .303's scales to 0.5m at 100m, the 20mm to 0.45m.

Thanks! If these circles mean anything (for the Luftwaffe diagrams, they were not equal to dispersion radius, which was given as figure in the operating procedure), that would confirm that the Hispano II was slightly less accurate than the MG151/20 since the latter was expected to land all hits within 70 cm x 70 cm at that distance - slightly less than the Hispano's 90 cm circle. (Note that despite a specified dispersion of 2.5 mil, for passing the gun calibration test 3.5 mil were allowed.)

However, I'm not sure that this is a valid interpretation since the 7.7 mm guns with a 10 mil dispersion would have required a greater circle then.

Regards,

Henning (HoHun)

[HoHun]

Hi Butch,

>I have 3mils value for the 75% dispersion for the nose mounted hispano (comapred to 4mils for the M2) on the P-38.

Thanks! :-) Do you know whether this is the dispersion radius or the dispersion diameter?

Regards,

Henning (HoHun)

[butch2k]

dispersion diameter

[Tony Williams]

Originally posted by HoHun

(When you're mentioning dispersions, are these dispersion radii or diameters?)

If these circles mean anything (for the Luftwaffe diagrams, they were not equal to dispersion radius, which was given as figure in the operating procedure), that would confirm that the Hispano II was slightly less accurate than the MG151/20 since the latter was expected to land all hits within 70 cm x 70 cm at that distance - slightly less than the Hispano's 90 cm circle. (Note that despite a specified dispersion of 2.5 mil, for passing the gun calibration test 3.5 mil were allowed.)

However, I'm not sure that this is a valid interpretation since the 7.7 mm guns with a 10 mil dispersion would have required a greater circle then.

I measured the diameters, not the radius. You are right that the .303 figure is only half that mentioned previously, so possibly they took the 75% figure for the dispersion circles.

A figure of 4.5 mils for 75% hits for wing-mounted Hissos seems to match up reasonably well with 3 mils in the more rigid nose mounting of the P-38.

The 4 mils/75%  figure for the M2 also seems to match up with the 5 mils quoted for the M3 - so we can say with some confidence that the .50 was capable of 5 mils / 100% accuracy, somewhat worse than the Hisso as I said.

TW

[HoHun]

Hi Tony,

>I measured the diameters, not the radius. You are right that the .303 figure is only half that mentioned previously, so possibly they took the 75% figure for the dispersion circles.

Thanks for the data, the pieces start to fall into their places now.

What's the relation between the 75% and the 100% radii? I'd tend to say it must be somewhere between 1.5 and 2.0.

Regards,

Henning (HoHun)

[phookat]

Originally posted by Wotan
Take the tail turret on the b17, 45ft dispersion at 600yrds. You can't really believe that would help determine the dispersion of the p51.

I didn't say it could.  What I said was that HoHun's formulas can be the basis for airframe mounted guns, and your pics can be the basis for turret-mounted guns.

Originally posted by Wotan
What makes me think this is just a goose chase to keep Mando pre-occupied is that as others have stated how secure the gun is mounted has more an impact on dispersion then anything. Not just wing vs. nose but how the gun is secured in the wing.

Look at the bomber gun dispersion. The variety is due to how they are mounted.

I think it much more likely that there are some mounts for which you can apply a formula and a multiplicative factor (airframe mounts), and some mounts which are so bad that they completely dominate (i.e. turrets).  Consider Mand's picture from the beginning, with the MG-FF's.  That took into account mounting, and still had low dispersion.

[butch2k]

Tony official data says :
4 mils for 75% and 8 mils for 100% for the M2

[HoHun]

Hi Butch,

>4 mils for 75% and 8 mils for 100% for the M2

OK, then I'll use the following values for 100% dispersion for re-calibrating my function:

MG FF/M - 1.0 mil
MG 151/20 - 1.9 mil
MK 108 - 1.5 mil
MK 103 - 2.0 mil
Browning 12.7 mm - 4.0 mil
Hispano II - 3.0 mil

Does that look OK to everyone?

Regards,

Henning (HoHun)