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Let me throw some gasoline(OK...JP-4!) on this fire. No matter WHERE the round exits a rapidly spinning barrel, the bullet has a vector which will take it off the line from which it was fired prior to impact. The gunsight (HUD)was set to take this into account so the gun cross was a death dot....but (for those of us old enough to remember selectable firing rates!)ONLY at high rate...if you fired at low rate the gun cross had to be offset a couple of mils to the LEFT of the desired impact point (at approx 2000 ft slantrange) to adjust for the gun NOT turning at the same speed. After the gun was set at a single speed that aim offset wasn't as critical...in fact that happened while I was at the Pentagon...so some gunshop expert may know if the guncross position on the HUD was reprogrammed. Does anyone even remember LOW RATE?????....Cheers, All!

Col R. C. Lemon

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On God's wing, to Heaven i fly.

I absoulutely remember low rate. I started working the GAU-8 in 1982. For three years I worked nothing but the gun and ALA (GFU-7). I changed the ACP's in many jet from the selectable high/low to the high only.
By the way what you said made my point. Lower rpm on the gun had to be compensated for. Higher rpm did not. Well imagine no rpm and you see the round is fired sooner than 9 o'clock. Actually there are zones on the firing cam.
The gun has seven bolts and each one is in motion as the gun is turning. The cycle has the bolt picking up a round from the transfer unit and pusing it forward into the breech end of the barrel. As the bolt enters the firing sector it enters into several zones. B,C and A. The cocking lever ramps up on the firing cam in zone B and this applies pressure to the cocking lever forcing it back against the firing pin spring. As the bolt approaches zone A (the actual shear point) the aft end of the bolt enters the lock/unlock cam and turns the bolt about an 1/8 of a turn locking the bolt lugs into the forward rotor. If the gun pin is removed and the safing cam
is actuated buy the arming soleniod the cocking lever will be allowed to fall forward and fire the round while the bolt is locked. As the bolt contiues on into zone C. Once again the cocking lever is ramped backwards so the bolt can be unlocked by the lock/unlock cam. A locked bolt cannot be unlocked unless the cocking lever is pushed backwards.
This is where some maintainers have accidently fired the gun trying to clear jams. If you couldn't get the safing pin in the gun most times it because the safing cam was held out by a cocking lever in zone A. So they would remove the lock/unlock cam and put a allen wrench in the back of the bolt. Then attempt to pry back on the cocking lever at the same time and unlock the bolt. Dangerous at best and more than once a round was fired off. First one I heard was at RAF Bentwaters around late 82.
Anyway after the bolt is unlocked the elipitcal cam path forces the bolt backwards along the tracks with the spent case and around to the transfer unit which hands off to the turnaround unit and elements where it is returned to the drum assembly.

And one last thing I think coach said about looking down the barrel of an unsafe gun. I have seen it done many times. Damn if I know why? I have had wing safety drive in front of an aircraft with an unsafe gun. I have had O-6's walk in front of and look down the barrel of an unsafe gun. When shit like that happened I took my crew and walked away fromt the aircraft. I had more than one more learned person than I ask me what I was waiting on? I would say "I am waiting on you to leave my hot gun area." Got many an angry look however no one on scene trumps the T.O. Just something I learned from a really good tech rep at Myrtle Beach. He'd say just let them look a while and then when they leave lets go to work. Never before they left. You know what? I never hurt anyone to include myself and I never fired off a round.

Fender
"A woman drove me to drink
and I hadn't even the courtesy to thank her".
W.C. Fields

HOT-GUNS,
Leave those to the experts and the guys in the back shop area,Fender did alot of time in there,I'd go with his answer.
But like it started out,It depends on What point your coming from,Pilot or Maintance. Just like "left engine,right engine,#1 ,


Goose


LIVE FREE OR DIE
<img src="http://img396.imageshack.us/img396/5917/scan00020br.jpg" border=0>

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\"Live Free Or Die\"

I've been kicked off the hot gun pad before, so no problem, Fender. And yes, I have a few passes with low rate under my belt.

But I stand by my earlier explanation, the bullet is fired at the 3 o'clock position and like RC said, the spin imparts an up and left vector to the bullet which is accounted for by the HUD.

Enough from me.

Coach

just to add a quick explanation of the old "low rate" HUD offset.....the impact point of the "left thrown" bullet Coach described above was accounted for in high rate on the HUD gun cross..in low rate, the slower rotating barrel doesn't impart as much left vector...so at the same slantrange the bullet would impact to the right of the aimpoint...hence, the operator had to "pull" the impact a few mils to the left by offsetting the guncross left to get the bullet impact back on the target. Hope I haven't told you more than you wanted to know...but it is an interesting phenomenon! Cheers, All!

Col R. C. Lemon

_________________
On God's wing, to Heaven i fly.

I can confirm that the location of the gun was selected to get the firing barrel on centerline. The discussion above about how complicated the physics really are should tell you something about how the gun was located. If it is on the centerline (or thereabouts) that is one less thing to worry about. The recoil is not in line with the CG vertically so there is an automatic deflection of the elevator that occurs at the first trigger detent (am I right about that?). Why worry about it in the yaw plane too? It is quite amazing that the gun was able to hit what it was aimed at, considering the potential for complications that exists. Most of the "improvements" that were tried, made an inferior system. (I am thinking of all the gas deflector systems that were tried). We just tweaked the depression angle of the gun a degree or two to match the comfortable dive angle and it was working well. Later on, the fireball problem led to propellant tuning, but that seemed to be acceptable if the engines got washed enough. Increased gun bay ventilation was also needed (how do you like those scoops?) The "Beta-dot SAS" fixed a criticizm that it took too long to stabilize the pipper on the target. All the fixes took about two years to get going, but I am still impressed how good it was "out of the box". The basic concept must have been sound. To my knowledge, no pod mounted gun has ever been satisfactory, no matter when it has been tried. Are there any AV-8 people out there that know there gun performs? Even the A-9 "semi recessed" concept had its problems, I've heard, but that may be rumour.

It ain't the heat it's the humility.

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