Prop firearms are not really consumable items. They are just rented over and over again by production companies from rental companies. And when they break, the rental companies would first repair them before buying new ones. They could be decades old and have been repaired over and over.

I assume the shitty reliability of prop guns has more to do with their age and how much they are used rather than low manufacturing quality.

Did you try backing the plywood with a piece of hardwood while drilling? If you can clamp the two together that will work even better.