I wanted some way to carry bolts, even broadhead bolts safely and noiselessly.

The white cup at the rear is simply the bottom of a (empty) can of squirt cheese snack.

The label is removed and a small screwdriver used to poke the rubber plug into the can to release the pressure. The table saw was used to cut the bottom to a length about 2". Nothing critical. A pill bottle would be far easier to work with and does not dent like the aluminum.

Next a hole to fit the PVC of the shoulder stock was made in the bottom. A half round file turned out to be faster than the Dremel tool.

A cylinder of foam rubber was cut to fit inside the cup with a 1" hole in it. Styrofoam would work too.

Slide the cup over the back of the shoulder stock tube, and use a stout rubber band to hold the bolts.