I used screws that are about a cm/cm 1/2 long. The plastic of the stock seems pretty robust, and I haven't had any issues with the screws backing out, or the mini-rail getting loose.
At this point, I'm still terrified of really opening up any airguns for that exact reason......maybe I'll sack up and build a spring compressor sometime, but for now, I'm so satisfied with the gun, I don't wanna mess with it.
I've been getting good results with Benjamin HP's, Crosman HP's, and the RWS super H-points.