If you do not have a chance to fire it before you go out then you should not be using it. This is not really a debateable subject.
But once you go out and try it you should find that 100 yards shouldnt be a problem. With iron sights I have no idea. My 870 w/ choke tube and brennekes are good to almost 100. I retired that gun and my bolt action w/ fully rifled bull barrel, scope and hornady sst will shoot dead on, 2" high at 150 yards. A tack driver that left me shocked.
I would invest another $100 and get a lead sled-best money you will ever spend if you want to really site in guns. Good Luck