What choke constriction has worked well for the rem 870 and winchester xtended range shells?

Best advice I can give is don't over choke it. Extended Range, like Hevi-shot, is very hard, and seems to like going out of a slightly more open tube a little bit better. Mine shoots them best with a .670, and I would go no tighter than that with it. My girlfriend is shooting them out of her 20, and getting the best results with an extended Improved Modified tube.