There is a known problem with shells deforming in box-fed shotguns. I am not sure, however, if this is due to the vertical spring pressure in the magazine, or if it is due to the design of the weapon itself. Supposedly it's the pressure against the bolt in the Saiga that causes this, and not the magazine itself.
Either way the plastic shell was never designed with a box magazine in mind.
I'm still holding out hope for the Calico shotgun, not seen since SHOT 2012. A helical magazine is the best solution for high-capacity detachable 12 gauge magazines.
It's also bulky as hell. In my opinion, it would be worth the attempt to make detachable tube magazines that you can *also* top off via the lifter/loading gate. Thus providing the ability to both top off the tube and change it out if shit hits the fan and you need more shells ricky tick.