You don't need to rip apart a tank in order to take it out of action. Cassetted turrets are a hot item now, and they lack the old-school targeting periscopes tanks had for over 90 years. In these tanks the gunner sits in the machine's bowels, surrounded by LCDs. Destroying the target acquisition modules renders him blind. Sure, the entire turret is easier to swap for a functional one, but critical time is lost.
Modern tanks are pretty well protected against missiles. Not even the old nemesis, the top-attack munition, is a guaranteed kill. The Armata Common Tracked Platform has zenith mounts specifically designed to protect against missiles with a top-attack envelope (seen
here and
here, the small rear-most trapezoidal boxes on the vehicle). Only high-supersonic missiles (that can hit before the ADS has either acquired or fired) or saturation strikes (which overwhelm the ADS's ability to manage threats) are viable, and either of the two is considerably more expensive than the classic ATGM (be it air or ground-launched). Guns are immune to close-in interception, and can damage or completely destroy these systems (along with other electronics and ERA bricks, greatly facilitating ulterior efforts to disable the tanks).
I'm not a fan of gatlings. I think they're too bulky, too heavy and the motor takes too much internal space (compared to a standard revolver cannon). And larger calibres allow for smarter shells and more complex warhead arrangements.
Irregardless of the effectiveness of guns, the issue still remains that in order to effectively employ them against armor you need to get low and slow. And assuming that we're talking about effectiveness against Gen 4+ MBT's, that brings up critical questions of what kind of systems are in place to protect the armor in the first place. I mean a Small Diameter Bomb moving at about Mach 0.85 has roughly the same kinetic energy as a 120mm kinetic energy penetrator round (~5.5 megajoules), but exerts that energy over a wider area due to it's size. On the other hand though, it does also have a 93kg warhead, which will obviously do some damage. Would a SDB kill, say, a T-14's crew? Hard to say, but I'd lean towards no due to the unmanned turret. Would it disable a T-14 and render it combat ineffective? I'd say it would, and even with active protection you're still getting the explosion and the rain of shrapnel that would neuter most exposed systems. And not to mention you get to do this at 20,000 feet at mach 1.2.
Moving up a notch, against larger munitions (1000lb or 2000lb JDAMs) or bunker-busters any tank, regardless of any active protection system is screwed; having one of
these go off a few meters early isn't going to help you much. Of course, using a 2000lb bomb on just a tank isn't exactly ideal, but it's an option in a worst-case scenario.
Besides, any half decent IADS is going to inherently limit the engagement ranges of any aircraft, especially non stealthy airframes. If the S-400 can engage a conventional fourth generation airframe all the way out to 240 nautical miles, you'll never see the gun employed. Furthermore one has to consider attached assets such as SPAAG or even infantry packing MANPADS that can make any aircraft bleed during strafing runs. Frankly if the airspace gets sanitized to the point of being able to strafe tanks with relative impunity, we've probably already won the war.
UCLASS is flailing because of the moronic "single airframe" requirement (or, rather, "minimal number of airframes", instead of the classic "task-appropriate airframe" that was the norm until the late '80s). It doesn't fucking work, it never did, yet they're still trying to turn a hacksaw into a hammer.
Well actually the reason for UCLASS's recent cancellation is not due to some misconceived notion that dedicated task-appropriate airplanes are not allowed now, but rather because the actual capabilities offered by the platform are largely redundant if you're already equipping the navy with F-35 C's anyways. After all,
according to the navy, motivation behind re-jigging UCLASS are informed in part due to the very lack of having dedicated airframes. With, 20-30% of
all hornet missions are dedicated to tanking, it makes a heck of a lot more sense having a dedicated tanker platform rather than having two ISR/Strike platforms on deck. Not to mention the fact that we can get C's on flat tops faster than we ever could get the UCLASS fully certified for operational readiness.
Also I never quite got this attitude that somehow multi-role aircraft are these horribly compromised machines. I mean, are people going out and arguing that the F-16, F-18, Rafale, F-2, and Su-35 are fundamentally compromised warplanes?
I'd wager that all those planes have worked and then some, especially with the hornets.