The GAU-7 was actually a 25mm weapon, originally developed to be the weapon for the F-15A/B. The gun development fell behind because of excessive variation in muzzle velocity because of difficulties with gas leaking around the projectile before it moved far enough forward to plug the barrel. To prevent the gun from delaying the aircraft, the GAU-7 development was cancelled and the F-15 design changed to accomidate the old 20mm M61.
Of course you are correct. There was considerable experimentation being done in 27mm in the same time period. Your description explains those wide driving bands.