Minigun physics

I’ve been wondering about this for a while, but haven’t been able to find a good explanation. How does a gunner deal with the lateral motion imparted on a bullet by the rotating barrels of a minigun? In a minigun firing 6000 rounds per minute with six barrels, the whole contraption is traveling at 1000 rotations per minute. For a barrel assembly, say, 20 cm in diameter, this translates out to a speed of somewhere around 10 m/s per second at the middle of each barrel. A bullet fired out the barrel also has a rotational speed of 10 m/s, which it retains as it leaves the barrel. If the bullet is traveling at 1000 m/s (I’m throwing out a rough number for a 5.56 round here), over the course of a 300 meter flight, it would move 3 meters laterally. I guess I thought this would affect precession of small-caliber high-velocity rounds. Is this sort of inherent inaccuracy built into miniguns, or is there some method to reduce the motion to discrete jerks (ie, stopping the barrel for some very small amount of time as it is loaded and fired)?

Two things, though neither is a “real” answer:

A) The “Minigun” is not a surgical tool. A “cone of fire” is a useful effect for the gun’s designed anti-personel role. One does not “snipe” with a Gatling.

B) Keep in mind that all projectiles will follow the same (or close to the same, see above) trajectory. While that trajectory may not, indeed, be the same paths followed if the barrels were “stopped” instead of turning, but they will still be relatively consistent with one another.

Clear as a sack of nails, right?

I don’t believe gatling type guns have a constant barrel rotation but hesitate briefly for each barrel to be locked into battery. IIRC the M61 vulcan gun in F-14s (20mm and hydralically powered but the same principle as the electric minigun and manually cranked 45-70 Gatling) was boresighted static with no correction for rotation. Mind you I was an AWG-9 tech but mayby an aviation ordinanceman could correct me on this.

Well, the bullets are going to be off by a consistent amount, and so you can adjust your sighting device to compensate for this.

The gatling type guns do operate in constant rotation. Each barrel has its own breech bolt. For an example, during a firing cycle of the M61A1/A2, the breech bolts move fore and aft on slide rails on the main rotor with rollers riding along a campath on the stationary housing. At forward travel the cartridge is chambered and as the gun rotates a cam forces a locking block on the bolt down into a well. Then a contact on the bolt passes under a firing cam which pushes down on the contact moving the firing pin forward and at the end of the cam is the electrical contact which lets the firing pulse through. The entire gun recoils against recoil adapters and a pair of cams pull the roller up, bring the locking block out of the locking well. At this point the projectile has exited the barrel and the roller begins the aft travel of the cam path and the bolt pulls the empty case and “finger rails” engage it, pulling it out of the bolt extractor. The percussion guns operate in a similar manner.

They do not, however, “throw” the bullets laterally as much as you think. At 100 meters, the dispersion of 80% of the projectiles is 15 to 30 inches depending on the application of the weapon, with the center of the burst being only about 4 inches from boresight of the barrels. (When the M61 was first designed, it was TOO accurate. The dispersion at 100 yards was about 8 inches. So “inaccuracy” was built into the gun, and can still be adjusted for purpose). The main reason that the projectiles don’t move laterally that much is a combination of the angle of the barrel compared to centerline, rifling style and the exit angle of the rifling help to counteract this. The main inaccuracy is when the gun is coming up to speed because the projectiles are not leaving each barrel in the same spot. They tend to follow a pattern like the number 6, with the “tail” occurring during ramp up and the circle being the remaining rounds (that’s why 80% dispersion is used rather than 100. To account for the short ramp up and the inevitable “flyer” rounds).

Wow, Turbo Dog, thanks. Could you recommend a book or technical manual that would explain it in greater depth?

Greater Depth???

Peace,
mangeorge

Try this US Army link for some info on gattling-type guns. I don’t want to try to compete with Turbo Dog, but occasionally I have worked on some gun / helicopter integration. I don’t think you’ll find your OP answered, however.

Don’t forget, it’s not like you’re using sights to fire the gun by. You’re shooting it, seeing where it’s hitting, then correcting as necessary. So any deflection from boresight isn’t even noticed probably. Plus there’s the tracer rounds that let you know exactly where your bullets are going.

Trucido, no problem. The only things that I know of that go into theory of operation, etc will be the Navy or Army tech. manuals. You can get a good listing of them if you do a search on Navy AO Advancement Manuals. You’ll come across a few of those in PDF that will list the gun manuals, at least for the M61. The army calls it, IIRC, an M198. I doubt you’ll come across any of the actual manuals online though. I have seen a few civilian books like “History of the Gatling Gun” and such, but I haven’t read them so I don’t know how deep they go. Jane’s may have something as well. I would list some of the manuals that I know of but that would probably twist some panties into knots, Nat’l Security and all blah blah blah. Feel free to email me if you have any questions or want some more breakdown though on the gun or ammo. The gun itself isn’t classified and very little is sensitive/proprietary.