The standard 870 Express comes in a "combo" version, with both a vent rib barrel and a fully rifled slug barrel. The vent rib barrel is your choice, either 26 or 28 inches, the slug barrel 20". The gun & both barrels shouln't be much more than & $ 570 + tax - and cheaper than buying the second barrel separately. If you're after grouse, go for the 26" barrel and buy a "Skeet" choke to go with it ( the Express comes supplied only with a Modified Choke )
12 ga. - 1 oz trap loads of # 7-1/2 work great for grouse, and Remington's Core-Lokt Ultra Bonded 2-3/4 " - 385 gr. Sabot slugs work just fine in the rifled barrel. The Standard Express is chambered for 3" and will handle steel shotshells should you want to put it to work for waterfowl. IMHO, I find that the 3-1/2"' loads are not necessary.