I just needed a simple stock model to toss on a nerf blaster that I'm working on. It's nothing terribly fancy, but it works. Good for both nerf and airsoft applications. Stock pushes up to fold, and pulls out to extend. If your stock folds too easily, you can just use a stronger hinge spring.

Hardware required:

• 38mm long 5mm rod (for the stock hinge)

• 8mm long spring with an ID of at least 5.5mm (for the stock hinge)

• 32mm long 5mm rod (to keep the stock from sliding off the arm)

• 30mm long 5mm rod (for the slider lock)

• 18mm long spring with an OD of 12mm (for the slider lock)

• 2x M3x15 machine screws (for the stock mount)

• 2x M3 heatset inserts (for the stock mount)

• [Optional] 155mm long 5mm rod (for extra stock arm rigidity)

I left generous amounts of space for the springs, so they don't have to be exact. Just close enough will do fine. I just buy the steel rods off of amazon to cut up and use as pins. These also don't have to be exact, perfectly fine to just eyeball it. 

the 5mm steel rods I used

the spring assortment that I pulled the springs out of