I had a similar problem with my Beretta. The strikers were too long, and they caused a severe punch to the primers, but did not pierce them. I reduced the length without changing the diameter. I didn't have any issues after that.

Someone also mentioned that the mainsprings might be a little too stiff and that they should be reworked to reduce the possibility of damage to the primers. But I did not try this route.

Now, I don't think this is a particular problem with the 20ga frames of certain guns, but something that can happen to any frame size, as it has to do with the strikers, not frame size.

Having used factory loads for your test, I would rate the problem you experienced as a "design issue", that could be avoided by correctly measuring the original strikers. If longer than 1,5mm (projecting out of the face of the action), the strikers may require some filling.

More than that, once the first barrel presented a serious problem, the test should have been aborted immediately. Maybe the gun could have been fixed at that point, before further damage.

Fred