According to the old Winchester loading guide, the 1200 fps 1/2-ounce load, pushed by 14-grains of 296, in the compression formed AA hull was 9800 psi. The load I currently use in Remington STS hulls, 13.2 grains of Lil'Gun, is 10,800 psi. The SAAMI spec for 2 1/2-inch .410-bore is an average max of 12,500 psi. Modern day factory loads are probably not much lower.
SAAMI Specs.jpg