I also have an 870 w/rifled barrel and missed a gimme shot at a buck last year. Just like you, I couldn't believe it. When I test fired the gun, it was all over the place. What I found was that the barrel was loose. The screw-on cap that holds the barrel in place has a tendency to back off. If it's not that, something else is loose and moving. Check your scope mount and rings. If your rings aren't tight, your scope can slip back from the kick. You didn't say where your scope was zeroed. If it's at 150 yrds, you would be way off at 30 yrds as pointed out above - not sure why you would zero at that distance - 75-100 yrds is more normal for IN. If all else checks out, it could be that the reticle in your scope has gone bad and is moving around with each shot - slug guns have a tendency to ruin scopes. You'll need to check things over and test fire your gun. As stated, you may have just missed. Good luck figuring it out.