I made this lego rifle when I was about 10. I just added the mini crossbow for the fun of it.
Step 1: Stock
The stock folds back and forth. I use a 1x2x4 thin square on both top and bottom to hold it in place.
Step 2: Red Dot Sight and Cross Bow Sight
I got a lego window and built a square around it with the window in the middle and put a dot on it with a red permanent marker.
The crossbow sight was made out of little tube like things and a 2x1 hole block.
Step 3: Cross Bow
I made a crossbow from https://www.instructables.com/id/A-1-Light-Lego-Crossbow. He/she made one and I just put it on my rifle.