renatohm said:

Or weight saving, as 20mm ammo is lighter.

Weight has to be a major factor.  The bare M230 is about 130 lbs, versus 115 lbs for the XM915 Gatling with feeder and drive.  Peak recoil for the XM915  is noticeably less than for the M230, which means less structure weight in the mount.   Ammo is also significantly lighter, especially if they go back to the aluminum-cased XM1031 20mm ammo developed for Comanche.  

renatohm said:

20mm will probably do the trick when rocktes and missiles are too much for any reason

Well thats the problem, its a really mediocre round for air to ground.

Its optimized for anti aircraft; the delay function means it will likely burry itself into the ground before exploding, and it produces a small number of large fragments vs the high number of small frags one wants for anti personell 

It's also not a particularly good anti ground vehicle, as its a steel body filled with an incendiary optimized HE mix. I wonder if it would penetrate the roof of a IFV much less a tank; I almost think the Tungsten cored 50 Raufoss could have better penetration.

The whole reason 30mm Proximity was first developed was that Apache rounds were marginally effective against insurgents because they were burring themselves in the dirt before detonating. This has even more of a delay action fuse, and at 100g is a fraction of the weight of the 248g 30mm. 

gatnerd said:

Do helicopters get in much air to air fighting?

Not to date.  There reportedly was a small number of helo vs helo actions during the Iran-Iraq War, but that seems to be it.

However, since current planning focus is on near peer conflicts, it may be the Army plans for FARA to engage enemy helos.

gatnerd said:

For air to ground, does the increased hit probability of the higher velocity 20mm overcome the greater lethal radius + airburst of the 30mm? 

It very well might.  Three decades ago, the M230 was called an "area weapon system", presumably because of its relatively large dispersion.

  30mm  --  https://youtu.be/5lLg24H2Wq4?t=135

  20mm  --  https://youtu.be/OM2gEkW8Vcw?t=40

Or maybe the decision makers want a 20mm Gatling because they subscribe to the gatnerd-schnuersi "high rate of fire" school of thought.

  M230  -----  625 rpm

  XM915  --  1500 rpm

Frankly, I am quite shocked and rather dismayed to discover that you would abandon your principles and advocate a low rate of fire weapon.    

gatnerd said:

I wonder if [20mm] would penetrate the roof of a IFV much less a tank...

Does it matter?  FARA has a much better option for engaging tanks and IFVs.

                                    https://youtu.be/DbIGhjEVVd0?t=250

20mm strafing runs

YouTube

M-197 Strafe event

Im not a great shot my first time out, but it was a blast! The GoPro was having a hard time holding the shot too...

stancrist said:

It very well might.  Three decades ago, the M230 was called an "area weapon system", presumably because of its relatively large dispersion.

That shooting in the 30mm video was indeed abysmal. Combat footage shows better accuracy (videos a bit grim be warned)

YouTube

30mm Apache Gun

The Hughes M230 Chain Gun is a 30 mm, single-barrel automatic cannon developed by Hughes and now manufactured by Alliant Techsystems. It is an electrically o...

https://youtu.be/o3lybSntpSw?t=145

YouTube

Apache's 30mm Cannon Takes Out Compilation

That said, I do indeed appreciate the higher rof of the 20mm. 

Some drone busting video, and discussion of improved proximity fuses and possibly even guided 30mm. Its a multi tweet thread so scroll.

Twitter

Air Power on Twitter

"We've got proximity ammunition, which is a really impressive ability to sense a drone in flight and airburst at th... https://t.co/fPHELgVhID

Read more from Twitter

The projectile does the work, I prefer the 30mm over 20mm as the better overall compromise of all factors involved (TOF, ROF, target effects, dispersion, weight, penetration, ammo packing density, etc) for a single feed system.

They did experiment with 30x113 on the cobra and my impression is marines would have much preferred it if it worked, but recoil was apparently excessive. Not surprising considering the airframe was considerably lighter than the Apache,  however it should be noted that a lot has changed since then and the current iteration of the Cobra is more than double the empty weight of the older AH-1G, and actually heavier than an empty baseline AH-64, so maybe it's time to revisit that.

It may be more an issue of nobody still being in the service who is also in a position of authority and knows this esoteric history or the potential, so unless the company brings it up as an unsolicited proposal (or better yet, borrows a cobra and demonstrates it working) institutional inertia is probably going to win out given the 20mm is already deeply embedded in the supply chain