I have big hands, and I really don't like the stock grip. A few people at games have asked me how to do this, and now that my friend is having me do one for him, I thought it'd be a good idea to make a quick and simple tutorial.
What you need (aside from gun and grip):
2x 12mm M3 screws
1 or 2 11 or 12 mm O-rings
Sandpaper
Circular file or drill
The reason VFC SCARs don't accept regular grips is the gearbox sits higher in the receiver than in a standard AR.
Stock VFC grip on the left, Magpul PTS MOE grip on the right.
Due to the height difference, standard screws are not long enough to reach the mechbox, and the motor cannot be adjusted high enough to contact the bevel gear properly.
Standard screw on the right, 12mm screw on the left.
First thing to do is to use a circular file or drill to widen the motor hole to slightly larger than 11mm. Make sure the center of the hole is in the same place, otherwise your pinion will not mesh with your bevel gear properly.
Modded grip on the left, unmodded on the right.
Take the spring off of your motor, and it should fit through like this. EDIT: A little higher actually, O-ring was on the motor in this photo.
Without the spring to secure motor height, O-rings will be used instead, similar to how people use O-rings to shim hop up units against mechboxes. One 11mm O-ring is what works in my Mk.16.
With most grips, that's all you need to do. However if you have a grip like the MOE, you'll also have to remove material from the back, otherwise it will not fit onto the receiver properly. I'm good at using sandpaper (used to work in a professional music repair shop) so I just took some 120 grit sandpaper and taped it around a burnishing rod to make a sort of circular file. I then finished with 600 grit sandpaper to get a nice finish.
Modded MOE grip on left, new one on right.
There you go, you no longer have to deal with the ugly, small and uncomfortable stock grip.
