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

Latest upgrade to SHORAD program:

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The US Army is upgrading its M-SHORAD capability even as it enters service. It will now have the @MoogSDG RIwP weap... https://t.co/85ipQbmmY6

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-30mm

-Directed energy weapon

-Stingers and/or Coyote anti drone UAS