Plastic cases should be pretty easy. A shotgun chamber, be it 2.5" or 2.75", is of sufficient diameter to accmodate thick-walled (about 0.025") paper cases. If you use 2.75" plastic cases in a 2.5" chamber, the mouth of the plastic case is so thin (about 0.005") that even in the forcing cone area it will not impinge on the bore diameter. In other words, fired from a plastic case, the shot and wad will actually have to jump a short over-diameter area before finally reaching the actual bore. Find a bore gauge and prove it for yourself.
