Yakovlev Yak-9T + NS37 Cannon

[Urchin]

Charge,
You are absolutely right. I should have been more clear and said there is no reason a shorter barrel would cause more dispersion.

[Yeager]

"As far as I'm aware, there is no inherent characteristic of barrel length that affects dispersion."

If dispersion is to be considered the impact spread of multiple projectiles fired separately at the same target over a given period of time then the above statement would seem incorrect.  When it comes to firearms.  The shorter the barrel the less accurate the weapon.  The longer the barrel (provided it is a rifled barrel) the more stability that is imparted upon the projectile.  I haven't done enough smooth bore shooting of different length barrels to say its the same deal for smooth bores but I suspect it is.  If the commenter was referring to multiple projectiles fired simultaneously like a shotgun firing birdshot then the term "dispersion" takes on added dynamics.

[bozon]

The barrel must be long enough to enable the bullet to acquire its max rotation velocity. Beyond that longer barrel does not help. I am sure there is some rule of thumb for this, something like 1.5 rotations or so. For higher muzzle velocity rifles, the barrel needs to be longer if the desired rotation rate is the same (the bullet moves more in the time it takes to complete the 1.5 rotations or whatever number),

[Krusty]

Physics-wise, longer barrels impart more force on the round as the gasses push the round down the barrel.

This means faster velocity and better ballistics. The Mk108 has a terribly short barrel and that is one reason for the very slow velocity and weak ballistics. The NS37 has a long barrel and very flat trajectory thanks to its much faster speed.

Now it's not always a matter of making barrels longer or shorter. The recoil of the cartridge into the spring is what helps reload and recock the gun, and unless the timing is perfect you can mess things up. You need the bullet to exit the barrel as the next shell is being readied. That means if you take an existing designe and lengthen the barrel you need to make the spring stronger so the recoild keeps those gasses in play longer. You cannot load the next round with gasses still pushing a round down the tube, and you can't load and fire another with the first still firing.

It's all very interconnected and complex, but generally as a rule of thumb the longer the barrel the better the velocity and therefore the flatter the trajectory.

[Urchin]

Longer barrels give the propellant gases more time to act on the projectile. If the barrel is too long, the extra friction can actually slow the projectile down.

The Mk108 does have a slow muzzle velocity and that would lead to a lot of drop, but not 'dispersion'.

[Krusty]

Longer barrels give the propellant gases more time to act on the projectile. If the barrel is too long, the extra friction can actually slow the projectile down.

The Mk108 does have a slow muzzle velocity and that would lead to a lot of drop, but not 'dispersion'.

Like I mentioned it's all interconnected... You need the larger powder charge and the stronger springs for the longer barrels, and you're right after a certain point that does happen...

But you have to make it pretty long for that to happen.

[Urchin]

Yea, it would have to be real long for that to happen.

I guess if the barrel wasn't long enough to get the projectile to a stable spin rate that could lead to all kinds of bad things, but then you'd get bullets flying backwards and sideways and stuff. Not just a big pattern.

[Krusty]

You forget that the barrel design itself can also lend to accuracy or not.... B-25 strafers would fire until their barrels were white hot, and then keep firing. They liked the scatter gun effect as the barrels lost accuracy.

So low tolerances, or just different designs, different grooves (more twists, less twists, etc?) can all come into play.

EDIT: As well as projectile shape itself... Look at the major improvements when they added the boat tail to rifle rounds. Then look at a German MG151/20 round. Flat, squared off almost.

There are many reasons something may or may not be "accurate"... and then something may be "accurate" while having poor performance, or be a great performer but have terrible consistency. Also the "cone of fire" does reflect actual WW2 specs. The guns flexed, the mounts flexed, even when stationary tested on the ground, Bf109Es had a "cone of dispersion" for both MGs and MG/FF cannons. You got as much of the bullet holes into the zone as you could. They didn't all fall into the same bullet hole.

[mtnman]

The barrel length doesn't play into the accuracy equation all that much, as long as it's within acceptable limits.

Regardless of barrel length, the "fit" of the projectile to the inside of the barrel matters a ton, and not all barrels are equal.  A short barrel can easily prove more accurate than a long barrel.  Another deciding factor is the overall length of the cartridge, and how close the bullet sits to the back of the rifling.

Cartridges are purposely sized a bit "small" so that they're sure to fit in the variably-sized chambers (due to tolerances, and often differing manufacturers) they're designed for.  As a result, two things happen.  First, the cartridge sits tipped "nose-down" a bit, resulting in it hitting the rear of the rifling at an angle, deforming the projectile.  Second, the projectile actually moves forward a bit before it engages the rifling, again resulting in hitting at a slight angle, and deforming the projectile.  That effects accuracy.  And it can effect it a lot.

The action itself effects accuracy as well.  A less-rigid action is generally less accurate than a more-rigid action.  And then there's the rifling.  Projectiles are stabilized by RPM, not by how many inches they travel per revolution.  That means the projectile velocity needs to be matched to the degree of rifling.

The cartridge itself-  how consistent are the rounds loaded.  What are the tolerances for powder weight?  How consistent is the powder composition?  Possible air pockets in the projectile, effecting overall weight and balance?  Are the rounds consistent enough to result in the same velocity?  What's the range of velocities?

There are more factors than that of course, but it's a start...

The barrel length is just one little variable.

[moot]

Yea, it would have to be real long for that to happen.

I guess if the barrel wasn't long enough to get the projectile to a stable spin rate that could lead to all kinds of bad things, but then you'd get bullets flying backwards and sideways and stuff. Not just a big pattern.

Pardon my french but- natura non facit saltus - there ought to be be an intermediate "big pattern" regime between flying sideways and high enough speeds that the rounds' dispersion due to insufficient spin/speed/etc are negligible.

[save]

Difference from RL is that we shoot at distances fighter vs fighter off angle at 600 yards, whereas a shooting distance of 250 would be more normal.

[Urchin]

Moot,

I'm definately not a small arms or direct fire expert, but the way I see it there are only a couple factors that come into play at ranges that are as short as what we have in AH.

One factor is muzzle velocity. Obviously, the faster a projectile is moving, the faster it will get to whomever it is going to hit (our or limit of ~1.4/1.7k). The quicker it gets there the less time gravity has to act on it, so the less it drops. From our point of view, these bullets 'fly straighter' (.50s, Hispanos). This has absolutely nothing to do with dispersion. Dispersion can be thought of as the variation in impact point of a group of rounds about the mean for the group.

There are different things that cause dispersion. Obviously if the projectiles are different in some way this will cause a great deal of dispersion. Another factor is flexing in the gun mount or the barrel. Another significant factor is little changes in met conditions along the trajectory - these cannot be controlled for and will always exist and the best way to 'counter' them is to have a high MV so that the met has a little time to effect the projectile as possible.

If I had to guess, I'd say that HT went with a model that assumes these factors are the same for all guns and mounts, and that the quality control guys at the factory were doing there jobs so that the projectiles are all functionally identical (within type - so a 20mm MG 151 projectile will fly the same as any other 20mm MG 151 projectile. This leaves random disperion, which is either a constant for all projectiles, or varies so that the higher the MV the tighter the pattern.

There might be some difference in dispersion between ground mounts, fixed air mounts, and flexible air mounts but that can be tested.

Actually all of it can be tested, I just don't have the desire

[moot]

So you're saying that projectiles shapes (from projectile type to type, not within a type) and their various velocities combined are a non factor in dispersion?

[Urchin]

Well dispersion is the measured variance from the mean point of impact for a given group of rounds. There would be an impact if we were shooting different projectiles in a given belt, but all the rounds in AH are an amalgam of the different kinds of projectiles (for example HE AP Mine) in a belt so there aren't different shapes.

The shape of a projectile impacts the drag, which effects the amount of drop, but it wouldn't effect the dispersion.

How you want to model it depends on how accurate it needs to be. For AH purposes, a simple point mass system is close enough with some randomness thrown in - basically you could just wrap up everything that causes dispersion into a fudge factor or two.

One factor would be easiest, but I'd go with two - one for met and one for 'everything else' (gun mount rigidity, barrel flexing, minute differences between projectiles) because the muzzle velocity would tend to decrease the dispersion due to met (which is mainly due to changing wind conditions - not a big factor anyway over the ranges we are talking about).

[moot]

What I was thinking, and I'm at least as much of a layman on this, is that some rounds' shape could be more susceptible to tumble, or something.  I wasn't thinking of it in principle as the discussion has bent towards, but in terms of the difference between NS37 and MK108.  Maybe the 108 had a combo of projectile shape and too little barrel to result in more dispersion.  Or maybe HTC based the difference in dispersion on historical data..