Ammo for Box Mag-Fed Shotguns that Doesn't Deform?

[Dave Markowitz]

Question for the guys who own a box mag-fed shotgun because I have some interest in picking up a Vepr 12:

Have you found shells that can be left loaded in the magazine for an extended period of time without deforming out of round? A friend had a Saiga and left it loaded for HD, and the 00 buck rounds he had loaded squished down so they weren't round, preventing it from cycling. I don't know what shells he had loaded. Thankfully he discovered this before he ever needed it.

Thanks in advance.