Mandolin

Here's a 2017 Army PDF giving stats for Mk.244. The round is stated to fire 1,950 grain/126 gram projectile at 3,610 fps/1,100m/s There's also this 1975 DOD Appropriations hearing, Phalanx is stated to fire a 1100 grain projectile. I'm not sure how accurate that is, as this is an early version with the DU rounds not tungsten, but should be interesting. Dimensional sketch An older 2002 Mk149 brochure giving a bit more info, including penetration

No reason you couldn't use WDU-4, though I don't have the manual. AH-1Z uses WDU-4 per the 2008 manual and Mk149 came out sometime in the 2010s. So WDU-4 and Mk149, yes.

Few months late, but UH-1 cannot use M255. 1. M255 can only be fired from the M260/M261 rocket pods. This is due to these being the only pods capable of interfacing with M255's remote-set M439 fuze. 2. M260/261 pods are only used by the US Army. 3. Army helicopters use a different stores-management system than USMC. The combination of M260-series launchers and the stores management system allows a helicopter to mix and match rocket in up to five zones with a full load of four M261 pods. Additionally, the Army system included a range-finder/range-setting allowing the user to set the M439 distance fuze either automatically via the laser rangefinder or manually. 4. USMC only uses LAU-61 and -68 rocket pods. 5. As of 2008, the AH-1Z's only flechette rocket is the WDU-4 (~2200x 20 grain). Since then, Mk149 has been introduced. Mk 149 flechette uses the payload of M255 (~1180x 60 grain) with the time fuze of the WDU-4 to increase lethality without needing to upgrade the entire system. There is also a WDU-13 flechette head (~700x 60 grain), but that was only ever authorized for use by Air Force jets. WDU-4/-13 have a payload of red marker powder as a visual indication of function. This is pretty standard on all US flechette/APERS rounds, similar marker powder is found in 106mm RCL (which actually uses yellow) and 105mm tank gun APERS. I have not seen anything one way or the other about M255 or Mk149 having marker powder. An additional note, WDU-4/-13 and Mk 149 have a fixed range. The integral fuze will fire 1.8 seconds (a few sources state 1.6 seconds) after firing, so you have an extremely limited range at which said rounds can be fired. I've never actually bothered to do the math on actual range. One final note: M255 was intended for SF use from MH-60 DAPs and AH-6s, it wasn't qualified for the AH-64 until the mid-90s IIRC. OH-58D is qualified for it as well, not sure when it got the capability.