I've been meaning to make some tech write ups on some things I've been working on, more or less just to have the info in one spot, accessible, and organized to some degree. As well, to potentially answer questions on compatibility that others might be wondering.

So for this post, I was tasked with installing an aftermarket hop up unit into a KWA VM4A1 with the AEG2.5 gearbox. Owner originally wanted a Prowin installed, but due to the design of the receiver, there isn't room for the tall top portion on the Prowin unit. The stock KWA unit has a very low profile top area, where the arm is. For the Prowin to be installed, it would require a fair amount of modification to the upper receiver above the base of the outer barrel for it to fit.

The Maxx ME-PRO hop up unit fits down the outer barrel and in the receiver without and modifications to either.

Modifications needed:
-You will need to grind the ridge on the top of the proprietary nozzle flush with the rest of the nozzle for full movement within the hop up unit.

-As well for the bb release tab, about 1mm or so needs to be ground away from the bottom of the gearbox shell towards the front.
(without this material removed, magazines are difficult to lock in and cause a compression loss. This build, shooting 410 properly, was shooting 230-250)

And just some other reference to parts in the build, as its all working smooth now:
-Maxx ME-PRO hop up unit(w/o barrel spring)
-Maple Leaf 370mm barrel
-Modify Soft Flat hop bucking
Everything else is currently stock on the gun.

NOTE: This is only for the M4 models, not the QRF MODS, TK.45s or other non 556 models. The pistol caliber models all have proprietary hop up units to each design.