In the computer game Call of Duty, the AC-130 mission is mostly a piece of cake for the gamer as far as accuracy goes; the rounds go straight to where the cross hairs point and all the player has to do is lead the target a bit for the 25mm, 40mm or 105mm shells to score a hit.
In this YouTube video of an AC-130 operating in Afghanistan, though, the gunner is struggling to hit the target, especially around the 5:32 to 5:41 part.
Is this just the gunner having bad timing on his shots, or are the shells really veering quite far from where they ought to be?
Keep in mind that they’re firing from a moving platform that’s rather high up. The linked video doesn’t make it clear what the delay is between firing and impact but it looks to be a few seconds.
I don’t know if the AC-130 has a ballistics computer for its guns or if it has the equivalent of continually computer release point (where you tag the target, press the fire button not to fire directly but to allow the fire-control computer to fire weapons at the right moment). If it has none of those, shots could indeed be difficult.
I’ve seen websites that claim, “With its extremely accurate fire control system, the AC-130 can place 105mm, 40mm and 25mm munitions on target with first round accuracy.” Whether this is true or just marketing, I don’t know. As others have mentioned, the best tools can still fail when you consider the skill of the individual gunner and the conditions.
I would submit that in the video we don’t really know what the gunner was aiming at. I do not believe we are actually watching the “gun cam.” Rather, it appears we are watching an observer’s camera while the gunner shoots at something else. Without seeing the gunner’s view we really don’t know where he was aiming.
Airplanes dont move in perfect straight lines, there is ALOT of shift moving through the air. Even a few degrees of pitch or roll is tremendously magnified a thousand yards out.
If I recall correctly 1 degree of change in angle is about 50 feet at 1000 yards. So a little wind buffeting causing 2 degrees of roll will throw your aim point 100 feet off even without any errors in time to target delay.
An AC-130 basically does a steady-state pylon turn around the target. That’s a trivial motion to correct for, and even WWII gunsights could handle much more difficult situations.
While almost no correction is needed for a perfect pylon turn, all real-world pylon turns are imperfect. Plus, as others pointed out, a real-world AC-130 targeting computer has to deal with turbulence and other difficult-to-anticipate variations. However, gyro-stabilized mounts are a thing; tanks use them to fire their cannons while moving. They work in a way similar to optical image stabilization in an SLR lens. So it’s pretty straightforward to make a decent firing platform out of a plane performing a pylon turn, even with turbulence and vibration. It’s imperfect, of course. But how imperfect?
One helpful metric would be the Circular Error Probable (CEP) for each of the AC-130’s cannons. A quick google search yielded this PDF (of a presentation) but it seems to be only the first portion of the presentation…the results (which would help answer the OP’s question) aren’t included.
I thought I read somewhere that the plane’s minigun mount had some inherent and unintended wobble which enabled it to spray such a wide area but I didn’t see that in the Wiki article or linked reference. Note that the reference link doesn’t actually make the “football field” claim, which I saw elsewhere phrased something like “if you let a single rabbit run around a football field Spooky would kill it.”
Here’s a much older report about the AC-130E aka Pave Aegis of late Vietnam era, the first AC-130 version fitted with the 105mm. It says (pp. 30-33) that the trial 50%* CEP was 1 milliradian (ie 10’ per 10,000’ range) at 8,500’ altitude 11-13,000’ slant range, 2 mil at 14.5-15k altitude 18-22k slant range, and actual units achieved 2.4 mil from 10k ft range or IOW 50% of the rounds in less than a 25’ circle, v a 3 mil expected combat 50% CEP. The combat case seems to be speaking specifically of the 105mm; the general discussion might be either the 40mm (L/60 Bofors type, which along with 20mm Gatling guns were the other AC-130E weapons) or 105mm.
The report also gets slightly into the major uncertainty of the use of those a/c, which was mainly ‘truck hunting’ on the Ho Chi Minh trail. They were sometimes used in support of friendly forces in contact or general offensive use against irregular fighters, but to a much lesser degree than AC-130’s have been use in recent conflicts in those latter two missions. How many trucks were really being destroyed? This has not AFAIK ever been studied rigorously with benefit of Vietnamese accounts. It feeds back to some degree to CEP estimates in actual combat when you’re estimating it from the a/c, not measuring it on the ground against parked vehicles on a test range. http://www.dtic.mil/dtic/tr/fulltext/u2/a487279.pdf
*generally ‘CEP’ is assumed to mean 50% if not stated otherwise but this report gives CEP’s for other %-tiles.