This is indeed the Philco-Ford competitor for the GAU-8 cannon on the A-10, which lost out to the 30x173 version of the GAU-8 manufactured by GE.
There was also a GAU-9 cannon that was developed by Hughes as a back-up in case the GAU-8 program failed. It was a licensed version of an Oerlikon cannon (I don’t have my references handy and I don’t recall the model number), which also fired a 30x173 round. The 30x173 for the GAU-9 had an electric primer and a metal rotating band, compared to the percussion primer and nylon rotating band(s) on the GAU-8 30x173 cartridge.
I have a 30x167 durable dummy that is very similar to your round. I also have an inert-loaded round with a blue MHR (Honeywell) projectile and a gray painted case (not anodized), as well as a dummy that is painted entirely gray.
I would post photos but my collection is currently packed away. If you’d like I can try to dig the rounds out and post a photo.
The gun was the Oerlikon KCA revolver cannon, as used by some models of the SAAB Viggen (which recently left service) and available in the Hughes Model 34 gun pod (although I don’t if anyone bought that). This gun was rated by the British as the best aircraft gun in the world in around 1970, for its combination of weight, compatness, muzzle velocity and hitting power.
The KCA’s ammunition also differed from the GAU-8/A’s in being steel rather than alloy cased. It was actually the original from which the GAU-8/A case was derived, since its development started in the 1950s IIRC.
I attach a pic of my Philco-Ford round (from the Ammunition Photo Gallery on my website). It’s shown on the far right (although I have it as a 30x164 - or 5!)