However, it's not all bad news - the F-35's stealthy external gun pod, which will be carried on the Marine Corp's F-35B and the Navy's F-35C, has finally been tested!
The gun pod, officially known as the MGS or Missionized Gun System, contains the same GAU-22A that is mounted internally in the F-35A. The MGS, however, contains slightly more 25mm ammunition for the GAU-22A with 220 rounds in the external gun pod instead of the 182 rounds for the internally mounted cannon.
While the 25mm GAU-22A does back a bigger punch-per-round than the M61 Vulcan 20mm cannon that is standard on most American fighters (F-15, F-16, F/A-18) it also has a slower rate of fire. The M61 fire at 6,000 rounds per minute, while the GAU-22A fires at only 3,000 rounds per minute - about 50 rounds per second.
This lower rate of fire has at least one advantage - the ammunition will last longer. While 220 rounds for the MGS seems like a lot, this works out to about four and a half seconds of total firing time. The F-35A, with its smaller ammunition load, only has around three seconds. The slower rate of fire also means the GAU-22A is more suited to attacking slower or stationary ground targets, rather than fast-moving aircraft.
The gun pod enclosure is designed to be mounted on the center pylon, in between the F-35's weapons bays. While it is designed to be as stealthy as possible, it will likely compromise the jet's radar cross-section at least somewhat.
It's not currently known whether the gun pod will limit the aircraft's maneuverability, speed, or range, although its impact in these areas would hopefully be minimal.
Here's the video of the test:
IOC Still a Ways Off
Currently, the gun pod, while cleared for testing on the ground, is unable to be used in the air, since the software to employ the cannon - known as the Block 3F software suite - is still quite a ways off.
The USAF originally stated 2017 as the goal for the 3F software suite to be up and running, but that is looking more and more unlikely. Right now testing for the 3F software suite isn't expected to begin until early 2018.
Originally, the Air Force was planning on giving the F-35A its Initial Operating Capability (IOC) when the 3F software was functional, but it appears that the USAF will not wait that long anymore.