Thanks Dave !
The plastic stocks I've seen in recent years are quite strong. Stonger than many give them credit for.
If the proper sized pilot hole is drilled, and the screw is not over-tightened, I don't see the screws ripping out of them anymore than a wood stock. The butt stock swivel on my Whisper, you'd need the leverage of a large claw hammer to pull it out.
As for the foregrip.
True Value and other hardware stores, Lowes, etc, carry larger outside diameter "fender" washers even in the smaller machine screw sizes such as #10 and #8. They are almost twice the diameter of a standard, or SAE washer. That spreads the load over much more area and makes for a stronger hold.
For that matter, cutting a piece of 16ga to 1/8 inch thick steel, the size of one of the foregrip's 'cavities' and drilling a hole through it makes an even stronger 'washer'. Or cut it out of thicker bar stock and drill/tap it for the machine screw that comes with the sling swivel.
There are many ways to attach a sling or bipod to plastic stocks that are strong and easy to do with simple hand tools.
Paul.