In practice the current design works and no tubes explode. I choose not to overthink it.

But if you are going to overthink it, remember the gasses are compressible so the pressure analysis is not literal. The payload section is designed to slide off with about 10 pounds linear force and you can overthink it and back analyse the pressure at which it moves. I am guessing 20 psi or so.

So no matter the theoretical peak pressure from an oversized charge (1.2g vs 0.5g), the payload releases the pressure when the pressure rises to about 20 psi.

Overthunk.