25 mm GAU-8/A

In the mid-1970s, following the failure of the 25 mm GAU-7 and its combustible-case telescoped ammunition, a conventional alternative was proposed for arming the F-15 fighter - a 25 mm version of the 30 mm GAU-8/A.

The cartridge used was based on the 30 x 173 case, necked-down and shortened to around 136 mm (my measurement, from a poor-quality drawing). The case was therefore significantly wider than that of the 25 x 137 Oerlikon. Were any of these cartridges made (and, if so, does anyone have a photo of one?) or was it just a paper exercise?

Thanks for any help.

I have seen 25mm GAU-7 TP projectiles loaded in a necked down GAU-8 case.It was a light green case and the neck had the finish gone, probably scraped off during necking and loading. I am pretty sure I passed it on to Bill Woodin. It was considerably shorter than a normal GAU case by my recollection, going back a number of decades. I have no idea what it was intended for and frankly didn’t pay much attention to it. The Armament Lab at Eglin AFB routinely did their testing in 25mm because they had a very large quantity of 4 or 5 different GAU-7 TP bullets and they reckoned that the results could be scaled up to 30mm or down to 20mm with more accuracy than scaling up 20mm results to 30mm or scaling down 30mm results.

I knew about the 10mm round that preceeded the GAU-7 but never heard of a 25mm cased round being considered, but those decisions were made long before I got down there in 1978.

Sorry I couldn’t be of more help.


Lew, the anodization (very thin layer) of the case is very hard and brittle. Deforming the case “below” just makes the anodization come off immediately.

Then that must be what had happened. I recall the case also looked scratched where it was bare, but that is an old memory.


Thanks Lew, that sounds right - as far as I can make out the 25 mm GAU-8/A ammo did use the same projectiles as the GAU-7, and fired them at the same muzzle velocity. The gun was also modified to fire at up to 6,000 rpm, so in performance terms it was an exact match for the GAU-7. The downsides were that the modified GAU-8/A gun and ammunition were bigger and heavier than the GAU-7, but the upside was that it was a low-risk, low-cost development using entirely conventional technology, thereby avoiding all of the (ultimately fatal) problems of the telescoped-combustible-case GAU-7.

It seems that the 25 mm GAU-8/A was not taken any further because of concerns that the US Army was about to adopt a new vehicle cannon firing the 25 x 137 Oerlikon (which became the M242) which would lead to the Oerlikon round becoming a NATO standard, and that introducing two new rounds in the same calibre would be unpopular. The project leaders noted that if the 25 mm Oerlikon was adopted, the GAU-8/A would not make a good basis for a gun to fire it as it was too big and heavy. Eventually, of course, the GAU-12/U was introduced in 25 x 137, but for some reason was never adopted by the USAF as a 20 mm M61 replacement - only by the USMC in the AV-8B (plus very limited use in an AA system).

Reviewing the history of weapon development in general, I think that it would be very sensible, when proposing some advanced and untried technology, to develop a conventional equivalent alongside it, then test the two side by side to see which is best. So many advanced projects have collapsed into failure with the requirement left unmet because there was no backup.

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Tony, The story I have heard is that the F-15 program manager, Gen Ben Bellis got tired of the continuing problems with the GAU-7 and became convinced that it would never provide the consistent muzzle velocity required for accuracy without delaying the whole F-15 program, and was concerned that the alternative proposals would delay finalizing the F-15 configuration, or otherwise delay the program so he directed the use of the existing 20mm gun. I do not recall when that decision was made.

I heard this explanation from a number of the old Eglin AFB gun guys including Dale Davis. I could have misunderstood some of the finer points of the discussions, but believe this is basically what happened.

Parallel development has been done in some programs, but usually only in the very early stages when it is less expensive. Full scale development of an alternative engine or gun or radar or stealth technology is prohibitative expensive unless it happens to have a major application elsewhere. Instead, the answer is usually to have an “off the shelf” alternative picked out that can be used with only an acceptable degradation of the end item. This is a broad generalization, but is consistent with what I have seen in the military aircraft development process. The more “new & untried” technology introduced into a program injected into a program after the very early conceptual phases when the initial experimental “proof of concept” hardware is developed, the more risk, and too much risk is the best way to cause a program to be cancelled. I have seen this same process happen during the development of the AIM-120 missile.



That sounds about right, Lew.

The 25 mm GAU-8A was closely based on the 30 mm version, so the development costs were expected to be minimal - certainly by comparison with the advanced-tech GAU-7. I think that it was mostly a paper exercise, just examining the GAU-8/A and determining what changes would need to be made. If this study had been carried out in parallel with the start of the GAU-7 project, then it could have been moved into the hardware development stage when the GAU-7 began to run into trouble.

You may say with some justification that it’s easy to be wise after the event, but we keep on cancelling advanced-tech projects when they run into trouble, usually with nothing to replace them with.

My “rule of thumb” is that you can double the risk of programme failure for every innovation that it includes. So if a conventional tech project has a 20% risk of failure, one major innovation would make that 40%, two major innovations 80%, and after that, forget it!

I think we are in agreement!

Separate email on another approach!


A couple of drawings from the document about the 25 mm GAU-8/A (the bottom drawing showing the 25mm compared with the 30mm rounds). Sadly, the photocopies are too poor to read the measurements, but the overall length of the 25 mm round is given as 9.500 inches.



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I found this picture on the site www.big-ordnance.com
The left one


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Yep, that looks right - thanks!

The round on the left looks like the one I had, except I think the bullet in mine was seated deeper in the case. Case color and neck are as I remember mine.


Gentlemen, is there a case length known for the 25mm case?

They exist with 140 and 145 mm cases.

Fede, thanks!
Any chance to determine which one is depicted in the image above?

That seems likely, judging by the drawing. I think that the bottom of the rotating band should be at the case mouth.

That’s interesting. As I can’t read the measurements on the drawing above, the only case length data I have to go on is linked to the overall length of 9.500 inches (241 mm). Given that figure, both attempts I made to measure the drawings came up with a fraction under 136 mm case length. Of course, they may have made several amendments during development.

The document these drawings came from (sent to me by Bob Leiendecker) is: Feasibility Study to Investigate Modifying the GAU-8 Gun Design to 25 mm Caliber. The document reference is given as AFATL-TR-75-62 and there is an additional number stamped subsequently on the front page: ADB005834. If this helps anyone to retrieve a copy, perhaps it might be clear enough to read the measurements from the drawings?

Tony, thanks for the reference. I tried DTIC but it seems not to be available there.

I tried working the measurements the other way round - I measured the GAU-7 projectile I have, which is 117.7 mm long, with the distance between the back of the rotating band and the base of the projectile being 12.7 mm, giving a figure for the visible length of the projectile when seated of 105 mm. Take this figure from the 241 mm overall length and the answer is 136 mm, the same as I measured from the drawing.

So, unless they were using a different GAU-7 projectile from the one I have (I know that several types were tried) it seems clear that the case described in this report was indeed 136 mm long. I wonder if the 140 mm and 145 mm versions came before the 136 mm, or afterwards?