I would like to point out that the components on that gun are not assembled correctly and they are not all pressure rated as you say. The components on the barrel are not assembled correctly. Those are also the components that are the most sensitive to blowing out because they receive a large pressure shock when the gun is fired.
You use a tapered bell reducer to go from the valve size to the barrel size. These components are not available in pressure ratings. While they are Schedule 40 components, that does not mean that they can be run at the same pressure as other schedule 40 parts. They take a large amount of force pushing them apart and because of that, they do not rate them for pressure.
Additionally, the components are not even pushed in all the way, this is very bad as the strength of the joint is based on the amount of material which is bonded together.
I would recommend you reconsider your design. I believe you are mistaken that this is acceptable for pressure. It is a great design and I would love to see it work, however, it is not safe and I would hate to see someone get hurt because of a explosion.