One of the gripes I often hear is that the current generation KWA rifles can’t take aftermarket triggers. This is only partially true, as there are many brands that fit without issue or simply require the opening in the receiver for the trigger to be enlarged. Bottom line, most of the issues boil down to manufacturer tolerances and chasing that ever-elusive “TM spec” that should really be considered a tongue-in-cheek statement these days.
One issue you can sometimes encounter with certain aftermarket triggers is having the trigger slip off the trigger trolley (or “shuttle”, as it’s sometimes referred to) at the end of the trigger pull. This is usually not noticeable in semi-auto as the cutoff lever is doing the job of tripping the trolley to stop the gun from continually firing as it does in full auto.
Full auto mode is where you will begin to notice the issue, as drawing the trigger fully back can cause it to slip off the trolley. The return spring on the trolley will do its job and retract it, thus causing the gun to not fire until you release (reset) the trigger. You may find that if you only pull it back halfway or so you can reliably get it to fire though. This is the “sweet spot” where the catch on the trigger is ideally situated and not falling off.
Again, this boils strictly down to manufacturer tolerances. The trolley catch on some triggers is higher than others. Some have rolled edges, some have sharp edges, etc. Anyone who’s fitted Retro Arms triggers, as an example, knows how much finessing they sometimes take to become 100% operational in even a basic-bitch Ver. 2.
As it applies to the KWA VM4 and RM4 series, there is an easy fix here that takes less than 5 minutes to implement.
The Fix
By installing a stop screw for the trigger trolley, you can fine tune the amount of travel, which in turn can prevent the trigger from slipping. I prefer to mount the screw on the right side of the trigger housing myself. This also allows you room to adjust the screw when the switch is mounted to the gearbox shell. If you use the trolley channel as a guide, you can get the location set perfectly without having the guess where to drill the hole. A 1.59mm drill bit is about perfect for this.
Keep in mind, you will also be drilling through the back of one of the trigger contacts – there’s no real way around that. The backside of KWA’s trigger contacts are substantially wider than traditional Ver. 2 contacts, so there’s plenty of “meat” to work with without weakening the assembly. The screw selection you use should be very small and not too long, lest it touch the inside of the gearbox shell and act as a short circuit. I found the screws KWA uses (6.25mm long) to affix the trigger switch to the gearbox to be ideal for this purpose.
You can also enlarge the hole in the contact to be slightly bigger than the one drilled through the plastic housing. This would ensure the screw doesn’t touch the contact at all. But if using the appropriate length screw, this isn’t necessary.
After the hole is drilled, you can thread in your screw and get to fine tuning the depth required. I’ve found that it really doesn’t need to protrude into the trolley channel very far at all; 1 – 2mm is often sufficient.
I’ve drilled a “witness” hole in this example so you can see the point where the leading edge of the trigger trolley is bumping up against the set screw (this is not necessary for the purpose of this mod). You have to set the screw where you’re ensuring the trolley can still make good contact with the trigger contacts. Obviously, if you set the screw too deep, it can prevent the trolley from reaching the contacts.
After you get it set, you’re pretty much done. Button her back up and test for function! I’ve used this mod with 100% success on any aftermarket trigger that proves to have slippage issues.