SAAMI max service pressure is the same for 2 3/4" and 3" 12's: 11,500 psi. However, I agree that a factory 3" mag is more likely to be pushing that max than is a 2 3/4" load. And because there is going to be at least a slight pressure increase with the longer hull (per Bell), you might very well exceed it. But in a pump or auto, most likely the greatest difficulty would arise with the ejection port etc. A 2 3/4" chambered gun probably would not cycle properly.