I don't thing anything bad is going to happen to your Modified choke. As far as 00 Buck, I've taken a few deer with 00 buck with my Remington 870 Wingmaster 30" barrel, fixed full choke, rib vent, 1970 vintage pump. A couple never knew what hit them, dumped them right in their tracks, No BS'n around. I love that gun. Deer, ducks, rabbits, pheasants, squirrels all taken with that full choke. That Remmy and a few other 870's and 1100's I have from that era are fixed choked guns. Great guns and none of my constrictions are hurt. Not all slugs are created equal. What kind of slugs are you talking about? Name, style, give some more info.