US ww2 issue 20mm HE VS: 55gal. empty oil drum

[Wotan]

You are right its not "flat" but "flatter" then say the mgff or the MG151/20.

Even the 50 and .303s arent "flat".

[Crumpp]

The ballistics of the projectile can stand much improvement. It is believed that ballistics similar to that of the Caliber .60 projectile can be closely approximated.

http://www.quarry.nildram.co.uk/US404.htm

Ours, without a doubt. The ShVAK was twice as reliable. The Hispano simply required an unbelievable amount of maintenance. The smallest exposure to dust, congealed lubricant, or any other kind of little thing, and the gun would not fire. Very unreliable.

The ballistics of our cannon were better. Our cannon had a flatter trajectory, which is significant for applying lead. When you talk about the Yaks, then one didn’t even need a sight. The tracers were almost straight, take aim and fire, and where the nose is pointing is where the rounds struck.

http://airforce.users.ru/lend-lease/english/articles/golodnikov/part3.htm

If you scroll down you will see the ShVAK and MG 151 were very similar weapons.

http://www.geocities.com/CapeCanaveral/Hangar/8217/fgun/fgun-pe.html

So the higher muzzle velocity does not necessarily mean better ballistics.  

http://www.quarry.nildram.co.uk/highvel.htm

Crown of the barrel, mounting, rifling, Barrel harmony, cartridge, all come into play and influence ballistics as you probably already know.  
It's very similar to wing loading when examining plane turning performance.  It is useful for "guestimation" but is not the whole picture which can change when other factors are considered.

Sum it all up.  The difference between the MG151 and the Hispano I am willing to bet will be measured in a few cm's.

Crumpp

[Tony Williams]

A few comments: as has already been quoted in extracts from my site, the information I have indicates that there wasn't much difference in the overall lethality of the Hispano and the MG 151/20. One would be more effective in some circumstances, less in others.

The trajectory of a projectile is a function of two variables; the muzzle velocity and the ballistic coefficient of the projectile (which measures the air resistance, and thereby determines the rate at which the projectile slows down).  The ballistic coefficient of a 20mm shell is also determined by by two variables; the weight (the heavier, the better) and the shape (the more streamlined, the better).

The Hisso had a higher muzzle velocity than either the MG 151/20 or the ShVAK. It also fired heavier projectiles than either. There wasn't much difference in the shape. So the Hisso shells would have a shorter flight time, and a flatter trajectory, than the other two, which helped in improving the hit probability.

Tony Williams: Military gun and ammunition website and Discussion forum

[Crumpp]

The Hisso had a higher muzzle velocity than either the MG 151/20 or the ShVAK. It also fired heavier projectiles than either. There wasn't much difference in the shape. So the Hisso shells would have a shorter flight time, and a flatter trajectory, than the other two, which helped in improving the hit probability.

I agree.  But what exactly is that difference?


Crumpp

[Tony Williams]

Originally posted by Crumpp
I agree.  But what exactly is that difference?

I don't have specific BC information about the projectiles, but you can get a pretty good impression just by looking at the shape of the nose (there are cartridge illustrations linked to the 'WW2 Aircraft Gun Effectiveness article here: http://www.quarry.nildram.co.uk/WW2guneffect.htm ). The more tapered the nose, the better. This is of course complicated by the fact that different types of projectile for the same gun had different shapes, with AP generally been more pointed than HE.

Tony Williams

[Vulcan]

FWIW that video is more aimed at CRS, WW2OL and their nerfed 20mm model (right Ring?).

[Crumpp]

The more tapered the nose, the better. This is of course complicated by the fact that different types of projectile for the same gun had different shapes, with AP generally been more pointed than HE.

Thanks Tony for taking the time to post!

That one needs to get explained, Please.  You may be familiar with SOTIC.  In that course they teach it is the BACK of the bullet that determines the ballistics.  I have cut the nose completely off bullets on a KD range and shot a 1/2 MOA group at 400 meters on a clean target.  The cut bullet landed right in the group within normal bullet dispersion.  

Nick the back of the bullet now and the ballistics completely change.

Different weights, manufacturing tolerances. humidity, tempature, blah blah blah (You know the factors...)
Will all effect your despersion.  I have never heard that the nose had any real effects until impact.

Obviously the different weights of the different tapers will move your pattern.

It would be great to get a BC on the Hispano.

Crumpp

[Tony Williams]

The base of the bullet becomes more important at subsonic speeds, but at supersonic it's the profile of the nose which counts for most. Why do you suppose that long-range rifle bullets, rocket nose-cones and fighter noses taper towards a point at the front?

On my web article on 'Basic Ballistics', I have a 'rule of thumb' (derived from P O Ackley data - a famous ballistician) for calculating the BC of a bullet from the nose shape. You multiply the sectional density (which basically measures the length of the projectile) by a 'form factor' as follows:

Flat-nose lead: 0.8
Round-nose lead: 0.9
Round-nose jacketed: 1.0
Semi-pointed soft point: 0.9-1.1
Pointed soft point: 1.2-1.6 (depending on sharpness of point)
Pointed full jacket: 1.5-1.8
Pointed full-jacket boat-tailed: 1.9-2.0

To put it another way, the most streamlined kind of rifle bullet will lose velocity at only half the rate of a jacketed round-nosed bullet of the same calibre and weight.

Tony Williams: Military gun and ammunition website and Discussion forum

[Crumpp]

Interesting.

I shoot high powered rifles at work quite a bit.  In fact it was a .308 round we did this too:

I have cut the nose completely off bullets on a KD range and shot a 1/2 MOA group at 400 meters on a clean target.

In order to demonstrate that damaging the nose has no effect on the flight characteristics.  Now changing the weight will have a dramatic effect.  I was taught to file the nose to eliminate weight differences when reloading using a hand press in the field.

On my web article on 'Basic Ballistics', I have a 'rule of thumb' (derived from P O Ackley data - a famous ballistician) for calculating the BC of a bullet from the nose shape. You multiply the sectional density (which basically measures the length of the projectile) by a 'form factor' as follows:

I did not know this.  Thanks for sharing.

I still would like too see the BC on the Hispano.  I think it will be flatter but not nearly as dramatic as most people think.  I will bet the differences will only be measured in a few centimeters.

Crumpp

[Karnak]

Crump,

A few centimeters at what range?

And can we say that you mean about 3 centimeters when you say the nonspecific "few"?

[GODO]

Crumpp, more flatter, less flatter, well, not really important unless you are firing from a rigid platform that absorbs most of the gun vibrations, that may apply only to the P38 and Mosquito, the rest of hispano users have the guns at the wings and I doubt these "so powerful" wonders are also recoiless. I expect much more noticeable dispersion of these guns compared to the typical 151/20 mounting at the hub or wing roots. Outer 151/20 are a different story.

[Crumpp]

A few centimeters at what range?

Max ord

I expect much more noticeable dispersion of these guns compared to the typical 151/20 mounting at the hub or wing roots. Outer 151/20 are a different story.

We will never know GODO.  All we have is speculation since we have no BC for the Hispano.

In that the modelers definition of "flatter" becomes important.  

Looking at the MG 151's ballistics I don't think the difference will be very dramatic.  The Hispano will be flatter but not far off the MG 151.

Crumpp

[GODO]

Originally posted by Crumpp
We will never know GODO.  All we have is speculation since we have no BC for the Hispano.

BC? Ballistic coeficient? If so, it is related to the rounds only, totally independent of the gun or gun mounting. Lets say due ballistics alone we have a dispersion depending on muzzle velocity and BC, but there is other dispersion inducted by the gun itself and also affected by the gun mountings, the more rigid the mountings, the better, unless you have a gun that is able to absorb all the energy of every shot. The former dispersion is more evident as the round loses energy while the latter is applied since the very beginning of the shot. Hub, nose or wing root mountings are far more rigid than wing ones, and so, the gun inducted dispersion should be less noticeable.

[Karnak]

I recall one British document that noted that the Hispano mounting on the Mosquito was much more rigid than the Hispano mounting on the Typhoon.

[GODO]

Originally posted by Karnak
I recall one British document that noted that the Hispano mounting on the Mosquito was much more rigid than the Hispano mounting on the Typhoon.

That is pretty obvious.