I dunno Mixel, I'm not an aircraft cannon expert. Ever heard of Google? It's this really kewl thing of which you can ask it anything and it spits back answers atchya!

(Sorry, just had to rib ya a bit.)
In short: 25x137 (GAU-12/A, GAU-22/A) appears to have roughly twice as much "pop" on a shell by shell basis compared to 20x102 (M61 Vulcan) and half as much "pop" compared to a 30x173 (GAU-8/A -- A-10).
25x137 appears to have no issue with 8mm - 12mm RHA plate at tactically significant ranges (out to 9000', perhaps? That's a LONG shot). 25x137 APEX also appears to be able to handle 20mm RHA plate @ 45° (not sure of the range) and 50mm RHA plate @ 0° (i.e. perpendicular impact). 20x102 seems to handle 8-12mm RHA OK. Not sure how it does with 20mm armor plate.
Am guessing this means M61 Vulcan may be getting a little long in the tooth for the newest armored personnel carriers, but that the GAU-22/A should not have an issue with them. GAU-22/A may be able to handle older tanks, dunno. I read somewhere that even the GAU-8/A may not be quite up to snuff against the very latest tanks. If true, then dropping to 25mm didn't lose much. On the other hand, maybe the 25x137 round may be able to disable an MBT from the side / rear top if it can disable the engine. I don't know how much armor tanks have there. They can't have 500mm of armor everywhere.
After googling a bit, my overall impression is that the GAU-22/A offers roughly comparable (perhaps slightly better?) performance to the M61 Vulcan. The 20mm Vulcan spits out roughly 81% more shells per second, so perhaps slightly better chance of getting a hit from more BBs. But each 25mm shell packs more wallop. So maybe a single 25mm will do in a fighter, whereas a single 20mm will only damage it (badly?). I can's speak to TOF (time of flight), range, or accuracy. Both the 20x102 shell (Vulcan) and 25x137 (GAU-22/A) have similar muzzle velocities. TOF to 1000' is probably negligibly different. The 25x137 *may* retain velocity better at longer ranges. But that's a swag on my part. On the other hand, one parameter I have seen discussed as an air-to-air gun factor is "throw weight" which is total mass a gun system can project in one second. The M61 spits out 100 100g 20mm shells a second for a total throw weight of 10kg. Using a Nammo PGU-47/U 25mm round (222g) for ammunition, a GAU-22/A will dispense 12.2kg per second. So a slight "throw weight" advantage to the GAU-22/A.
One web page,
Modern Fighter Gun Effectiveness blah blah's on about a bunch of stuff, throwing out some numbers that I don't know how accurate or meaningful they are. However, they suggest the following:
30mm (30x173) == 2 x 25mm (25x137) == 4 x 20mm (20x102).
That is, the 30mm shell the A-10 fires is roughly twice as powerful as the 25mm shell the AV-8B / F-35 / AC-130 fires, which is roughly twice as powerful as the 20mm shell the M61 Vulcan (F-15, F-16, F/A-18, F-22) fires.
I am attaching below some Nammo PDF's I found. One I've seen before. The other appears to be the Nammo 2016 catalog. You can browse it for all sorts of munitions & projectiles. 25x137 APEX is on p. 89. 30x173 begin around p. 100. Happy surfing.
Some suggested Google search strings:
25mm apex armor penetration
armor penetration 20x102 25x137 30x173
20x102 25x137 30x173 compared
20x102 vs 25x137 vs 30x173
In my opinion, the F-35 possesses a gun system that is roughly equivalent to the M61 20mm cannon for air-to-air work, but offers roughly double the effectiveness in the air-to-ground role. A pilot with a good trigger finger will get 2-3 squirts with the -A model, and maybe 3-4 squirts with the -B and -C. The fat fingered pilot may only get 2 squirts out of each. However, comments by someone else in another thread a while back suggested the F-35 may be able to be programmed to fire precise round counts settable by the pilot via his gee whizzery flat panel display. The mental picture I have is that the pilot may be able to select a 10rd burst, or 20, or 30 etc. Combined with the JTAC wizardry being built in, it may be possible that the F-35 pilot can get / confirm target coordinates from the JTAC, and then have the aircraft fly a closed-system gunnery solution whereby the F-35 fires the gun with a commit authorization from the pilot. Then 10 or 20 or 30 rounds are precisely dispensed on the target. If true, IMO this would be an awesome CAS capability that no other aircraft possesses and could result in some truly special CAS abilities.
Take an F-16, stir in A-7, dollop of F-117, gob of F-22, dash of F/A-18, sprinkle with AV-8B, stir well + bake. Whaddya get? F-35.