The riskiest place to store a shotgun in the off-season is in a vinyl soft case or foam-lined hard plastic case. Both can trap moisture inside, rusting the gun. On the other hand, I have also seen guns that have been kept zipped in the same case ever since Grandpa died in '78, none the worse for wear. It's a crapshoot. However, it's smart to keep a gun uncased and very lightly oiled to avoid rust. The best thing, of course, is to shoot and clean your guns often.