HiTech, you may check real MG-FF dispersion here

[GODO]

These images come from luftarchiv:






Even with low ROF and low muzzle velocity, MG-FF was a very accurate weapon. MG151/20 mounted in hub or wing roots probably were as accurate if not more.

With dispersion like that, a gunsight is a very usefull tool, but not with current AH2 dispersion.

[Karnak]

What is the range on that?

[Kweassa]

Wondering the same. My guess is no more than 120 yards.

[ra]

The wings probably don't vibrate as much as when in flight.

[GODO]

Originally posted by ra
The wings probably don't vibrate as much as when in flight.

Why, are the wings attached to some fixed structure? Or just the opposite due the "bouncing", if any, of landing gear?

[Karnak]

Another thing to note is that ballistics are different depending on the type of round.  HE and AP travel different paths.  I'm not sure what those rounds in the photo are.

Also the two guns had distinctly different dispersions.

[HoHun]

Hi Godo,

>Even with low ROF and low muzzle velocity, MG-FF was a very accurate weapon. MG151/20 mounted in hub or wing roots probably were as accurate if not more.

There are several parameters for accuracy. The MG FF/M was very good with regard to dispersion. Mounted in a fuselage mount, dispersion was 1.0 mil, compared to 1.5 mil for the MK108, 1.9 mil for the MG151/20 and 2.0 mil for the MK103.

As a general rule, long-barreled high-velocity weapons had a higher dispersion due to the greater effect of barrel oscillations (and the greater recoil). Their benefit obviously was a flat trajectory, but as with everything in aviation, you've got to pick your compromise :-)

In a wing mount, dispersion was much higher. It was 5 mil for the MG151/20, for example. That's a dramatic difference - it means that the pattern is 7 times larger for the wing mounted guns.

For the US 12.7 mm MGs in a wing mounting, I've seen a 6 mil dispersion figure. (Mounted in B-17 turrets, dispersion ranged from 8.3 mil in the Ball turret to 25 mil in the manual tail turret.)

Additionally, wing guns had to be harmonized, adding on offset of around 10 mil to the dispersion. This was systematical so it isn't directly comparable to random dispersion, but beyond convergence range, the projectiles diverged pretty badly.

Likewise, you can measure trajectory curvature in mil - for example, for the Fw 190A-7/A-8, the MG151/20 trajectory peaks at about +2 mil above the sight line, and drops to -1 mil below at 600 m. For the P-47D, it's maybe +0.5 mil at both distances.

The low-velocity MK108, I don't have the type-specific graph so I've got to make an estimate, but it seems it could peak at +1.5 mil and descend to -1.5 mil at 540 m (-3.5 mil at 600 m though - from there on, the drop becomes sharp).

(With decreased air density at higher altitudes, the trajectory tended to become even flatter for all weapons. Shooting from a moving platform also made the trajectory flatter, though shooting at a moving platform made matters worse again :-)

That's all pretty difficult to visualize, but oversimplifying it for a moment, I'd say centreline guns beat wing guns for accurate long range shooting any day, regardless of which guns are mounted.

Regards,

Henning (HoHun)

[GODO]

HoHun, you hit the nail. And this is what should be included in AH. I would expect little dispersion from wing root MG151/20 compared to outer ones un 190A8 or any 109 with gondolas. But then it should be applied also to the currently hispanos or 50", and that should make a clear difference between dispersion in a Spit and dispersion in a P38 or mosquito for MGs and for guns.

As we have currently a "punch" power and trajectory particular for each gun, we should also have a dispersion depending on the gun type and gun mounting.

[Urchin]

Are you sure we don't?  Compare the dispersion from say a nose mounted Hispano or MG-151 vs the wing-mounted ones.  I'll go do that now actually, although I can't post the pics.

[GODO]

I did the test with 109 x3 20mm and all the guns got pretty similar dispersion patterns with cnv set to 300 yards. Now take into consideration the dispersions posted by HoHun and compare current dispersion from wing root or hub 151/20 with wing mounted 50", that is, 2 mil vs 6 mil.

[Crumpp]

What is the range on that?

About 250 meters off hand.

You can figure it out exactly by "mil' ing" a known portion of the target with a known object mil size at that range.  

Just eyeballing it and comparing the torso of a man (about 1 mil at 700 meters) and run it through the mil-dot formula.

I got a slide rule at work that will figure it out exactly.  

Height of target (meters) X 1,000/Height of target (mils)  = Range (meters)


It will give you good approximation.  Even better is to look in the POH and see what range they set the convergence too.

Crumpp

[GODO]

Tested also 109 1x20, 190A8 2x20 and SpitIX 2x20, all got the very same dispersion per weapon with target set to 200 and convergence to 300. SpitIX 50" dispersion was clearly inferior to any of the tested guns. I would post the target image, but while testing mk108 the hit marks of previous guns were reset and I had no patience to repeat all the tests.

[artik]

The main problem in hitting with MG/FF is its low velocity.

In real fight the target moves thus it is very hard to make correct aiming with low speed of canon's rounds.
I have mostly problems in deflection shoots - so you need to come closer.....

In labaratory conditions all is much better and simpler.....

[HoHun]

Hi again,

>That's all pretty difficult to visualize,

... but I'm working on it :-)

Have a look at this diagram, showing the view through a gun sight with 70 mil ring at a P-47-sized target, with the dispersion circles of 6 mil and a convergence of 300 m.

The top diagram is for 700 m distance, the following ones are 100 m closer each. The bottom diagram is 100 m.

Regards,

Henning (HoHun)

PS I've got to find a quick way to generate these diagrams, it's way too much work.

[HoHun]

Hi Artik,

>The main problem in hitting with MG/FF is its low velocity.

I'd agree, but that's a different problem :-) Firing at a static target is difficult enough for now ;-)

Regards,

Henning (HoHun)