Accuracy vs. rate of fire in automatic weapons?

Factual Questions
1

I’m thinking about multi-barrrel weapons like the Vulcan 20mm and miniguns. I’ve seen some video of range testing of these things, and it always struck me that the bullets scattered like water out of a hose rather than beads on a string.

Why is that? I understand that only one round is actually firing at a given moment, and that should (I would think) contribute to a narrower pattern, not a wider one.

One thought I had was that wake turbulence of one bullet might screw up the path of the next one. What kind of muzzle velocity do the shells have? At a maxed out firing rate of 6k rounds per minute (100/second), how far does the first bullet get in the next hundredth of a second? Also, is it conceivable that the first bullet(s) could create a partial vaccuum behind, and subsequent bullets could pass up or hit the first bullet(s)?

2

I can’t answer any of your questions, but your rate of fire one reminded me of an article I read a little while ago. Electronic gun fires million rounds/minute.

3

Just a WAG, but perhaps its just that the insane amount of rounds fired contribute to make the pattern seem wider. In reality, each bullet doesnt deviate any more than it would when fired by any other weapon, but with 6-8 times more rpm than a regular machinegun, you get an apparently wider pattern.

Just a theory…

4

I’ve heard about Metal Storm before, and if wake turbulence would be an issue withanything, it certainly would to that.

There’s at least one error in that article, which is the gap between bullets in a conventional gatling gun. If the round travels 100 feet (roughly equivalent to 30 yards) in 1/100 of a second, the shell is moving nearly 10,000 ft./sec. No way, Jose! I figure maybe 3,000 ft./second, probably closer (and maybe even below) 2,000.

5

Also note that with the increased rate of fire, the recoil is increased in tandem. Sardaukar3925’s theory and the amazing amount of rock induced on whatever frame is holding the weapon causes a loss of accuracy that is made up for by the sheer number of bullets exiting the gun.

Interestingly, one of the most accurate long-range shots was made by a man sitting on a hill, firing single shots from a .50 caliber machine gun.

6

The Vulcan cannons are convergent, that means that they focus on a point X meters ahead; that´s so to counteract the centrifugue force (let´s keep this simple, shall we? :slight_smile: ) the rotation of the cannon applies to the exiting bullets. Past the focal point the bullets start to diverge from each other, thus scattering quite a bit.

7

“Yet after 15 years of trial and error in his tropical Queensland home, O’Dwyer came up with a gun prototype that recently fired 180 rounds of nine-millimeter bullets in 0.01 second during a demonstration before military officials in Adelaide. Metal Storm’s bullets leave its barrel so quickly that they are only microseconds apart–when one bullet is flying through the air, the next is just 10 centimeters (four inches) behind. For current machine guns, the gap between bullets is 30 meters.” - I think there is no mention of the time duration between bullets for “current machine guns.” Maybe I missed something, though.

8

Ale - I beg your pardon? You lost me there with that centrifugal part. I thought the barrel was stationary when it is fired, if only for a very small time frame. I also thought that the “convergence” is due to the range for which the arm is “sighted in.” If a firearm is “sighted in” at 100 yards, for instance, the bullet should strike the target in the same spot (theoretically - I’m working on that with all my rifles!), but closer or farther distances will give a greater dispersion of bullet strikes. Is there some other force at work here that I have not considered?

9

I make my assumptions based on the convergence angle of the GAU8 cannon, which AFIK works on the same principle ans the Vulcan, you can see by yourself here http://www.wpafb.af.mil/museum/research/attack/a6/a10arm-2.jpg that the convergence is so great that the focal point would be a few meters ahead of the gun.

By the way, that´s a bif MoFo indeed.
http://www.wpafb.af.mil/museum/research/attack/a6/a10arm-3.jpg

10

I’ve shot fully-automatic rifles and have been in close proximity to a (live firing) GE minigun on a number of occasions. (Just last week, in fact.)

A hand-held fully automatic rifle (e.g. M16, FAL) can be difficult to control, especially larger calibers (e.g. 7.62 x 51). This, in my opinion, is the primary reason for spray-n-pray; the shooter simply has difficulty controlling the rifle.

Not surprisingly, a tripod mounted fully automatic rifle is a bit more accurate, since the rifle is easier to control. A GE minigun, in fact, has a rather small pattern. I know this because I’ve seen a GE minigun shoot w/ 100% tracers at night. The stream of tracers looks like water shooting from a water hose. :cool:

11

Please excuse my ignorance on this subject. I thought that each barrel fired only when it was lined up with the firing mechanism. That would mean to me that each barrel fired when it was in the same position as the proceeding and following barrel. If this is the case, it would turn out that the trajectory of each bullet would be very similar to the bullets being shot out of the other barrels. There would, of course, be differences due to characteristics of each barrel, but the key word here is similar.

12

I’m hanging out with Crafter_Man from now on. He knows where to get the really fun stuff.

13

Perhaps I have not done a good job of explaining my understanding of the Gatling / Mini-gun / Vulcan / Chain gun -type of firearm. The barrel is rotating, right? The firing mechanism is stationary, right? Just checking…

14

I spent 10 years doing R&D/T&E of 20mm and 25mm guns and ammunition, primarily firing the M61 variants and the GAU-12.

The accuracy is dependent on the muzzle clamp. When the M61 was first developed, it was found to be entirely too accurate, so a different clamp was developed to “loosen” the barrel cluster to increase a probability of kill on a fast moving aircraft. By using different clamps and even by varying the torque on the clamp cam can drastically change the dispersion. There are even clamps which can create an oval group. Depending on the platform, the dispersion requirements can change, but the general requirement is that it must place 80% of the projectiles within a certain diamater circle, with the center being a certain distance from the average boresight of each barrel, depending on the range. With the right clamp, it is possible to create a small amount of “convergence”, but that difference is very small compared to the slightly different flight paths of the projectiles cause just by barrel heat. Basically though, on most applications, there isn’t a convergence because the barrels are not angled compared to the center point of the gun. In other words, if you put a laser boresight in each barrel, without moving the cluster, there is not a point in space where they will converge. Once the gun is firing “at speed” all of the projectiles follow the same basic flight path. I can’t speak for the GAU-8 though.

The velocity of PGU series of 20mm ammo is 3410 plus/minus 50 feet per second by spec. That’s about 34 feet per .01 second. Although there is some effect of turbulence, it has, in the grand scheme, very little effect on the path of the next projectile. It could possibly be measured, but it would be so small that it wouldn’t really matter. A change in wind speed in the middle of a burst has a bigger effect, and that isn’t very large. For a bullet to be passed by a following one, it would have to be travelling much slower to begin with, so slow in fact, that the cartridge would have begun it’s extraction phase, exploding the case and plugging the barrel.

radar ralf yes. The barrels rotate with their own breech bolt, and depending on the gun reach a sear point or an electrical contact, both of which are stationary to the housing of the gun.

15

Interesting. So the muzzle clamp just allows a little “planned” jitter into the gun, and the bullets scatter slightly due to to this?

Is the barrel stationary for the entire time the shell is transiting through it?

16

Boyo Jim - Well, sir, I now understand what you meant by the inaccuracy in that article. You are absolutely right on the money. They should have said something like 30 feet. I also see the point you’re making about the bullet’s transit time in the barrel. Perhaps Turbo Dog can give us some more info on that.

17

God don’t I love this message board!

Turbo Dog was that you in some other thread that knew about aircraft in general.

Nice post by the way…

18

Boyo Jim no, jitter isn’t exactly the right term, though it is sort of correct. Perhaps my use of the term “loosen” threw you off. It also ties into your next question a little bit.

All gun barrels vibrate to a certain extent when fired. The term used is harmonics. When you fire a gun rapidly, the barrel gets warm and the vibrations are more pronounced. I’ve taken high speed films of single barrel guns where you can actually see the barrel do the “stadium wave” and look like rubber. With an M61, you can put on a clamp that will make the projectiles converge at a certain point, as well as a clamp that will widen the pattern, somewhat like a choke works in a shotgun. On all platforms that I am aware of, with an M61, there is no convergence. The muzzle clamp is designed to just slightly spread the barrels at the muzzle to increase the spread of the barrels and maintain a certain level of vibration dampening. The clamp in the middle of the barrel cluster called, get this, the mid-barrel clamp, isn’t adjustable and is simply for stability.

Here is where it gets tricky. Each barrel fires in the exact same location, and the ammo is designed for nearly identical action time (time from primer initiation to projectile muzzle exit). So why is the muzzle clamp important? The muzzle clamp only actually torques three of the six barrels. The other three are contained but, have a bit of free play to them. Those barrels are the “scatterers”. The mid-barrel clamp helps keep them within “limits” as far as the harmonics go, but esentially you have three extremely accurate barrels and three “sort of” accurate barrels. Why is this important? Remember the part about the gun being “at speed”? During a long burst, they all bascially even out. But in practice, a burst is pretty short (in terms of air-to-air events), less than 1 second. In an F/A-18 system, it takes on average .25 seconds for a gun to reach full rate (6000 spm). In that time, the gun starts with a small group, speads large, then drops back down when everything balances. On a target, the gun basically draws a number 6. That is where the 80% dispersion comes into play. When you eliminate the tail of the “6”, you have the majority of your rounds. The software for the aircraft firing the ammo takes this into account. Does that help?

For the second question, ignoring the dynamics of the barrel when firing, and going on “location”, with all “gatling” systems, each barrel has its own dedicated breech bolt. Imagine 6 bolt action rifles in a rotating drum. As each one reaches the 12:00 position, you close the bolt and pull the trigger. They aren’t stationary, because they are always moving, but they are stationary as far as their breech bolt, cartridge and firing contact is concerned. Make sense?

19

Carlos Hathcock, right?

Good question. Wouldn’t the bullet have an angular momentum if exiting from a swinging barrel? Or would this have too little effect to care about? Or does the gunsight account for this?

20

But the barrel is moving around an axis as the shell is moving through it? I suppose that shouldn’t matter to the divergence so long as each barrel fires as it moves throught the exact same portion of an arc. But wouldn’t it affect the overall accuracy of the gun? Wouldn’t the shells would have some degree of lateral velocity imparted by the moving barrel? And the barrel cluster is spinning pretty damn fast – what is that?, maybe a 6" wide cylinder spinning at a thousand RPM?