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Basically the schedule has always been (since 2005; before the first F-35 flew) that the gun would be a part of the software update Block 3F.
Because of that, the gun ammunition manufacturer, a company named Nammo, has been spending several years developing a new type of ammo for the F-35’s gun. The reason it needs new ammo is because the only ammo that does anti-infantry, anti-aircraft and anti-armour well uses depleted uranium, which several partner nations have banned from their inventory. The ammo is actually pretty neat / advanced; [here’s a PDF / slideshow which shows it’s features.](http://www.dtic.mil/ndia/2014armaments/Wed15439Sande.pdf)
Anyway, the ammo has just finished R&D and as per the schedule, they have 2 years left before 3F is to be released. The gun begins ground firing (from in the jet) this year to make sure there hasn’t been some really fundamental issue and then they’ll move onto airborne testing. That will involve firing the gun in different atmospheric conditions, airspeeds, turn rates / G loads, etc so that they can look for problems and also map out exactly where the ammunition flies under x airspeed, y altitude, z G-load, etc. That in turn is because fighters since the 1980’s have had digital displays that show exactly where their bullets will fly at the precise moment of fire, and also where their previous bullets would have hit. Back in the 1940’s or 50’s they actually found that gyroscopic gunsights increased the hit rate by something like 10x, hence why you don’t want just a basic sight for your gun.
Also there was a recent article (which was incorrect) by the Daily Beast that claimed that the gun wouldn’t work until 2019, according to their anonymous sources. It blew up so much that the Joint Program Office had to put out a statement saying how those dates were incorrect, etc, etc.
tl;dr – as per the schedule planned back in 2005, they were waiting on the new ammo to be developed; the gun begins firing from the plane this year and it’ll be operationally ready by 2017 after they’ve mapped out where the bullets fly in different conditions.