I have a Remington 870 with a 26" rifled barrel that I use for pigs at close ranges (10-75 yards out) in thick brush. I use 770 grain full-bore cast lead slugs. I have a Leupold VX-3- 2x8 scope which I keep dialed down to the 2 power setting most times. It works great. The VX-3 runs about $399 so it may be a bit pricey for your needs but Leupold has the VX-1 and 2 and Redfield brands that run cheaper. Just make sure that you know your slugs accuracy potential because many become inaccurate over a hundred yards. So find a scope which fits your needs with a low power objective and you should be good to go.
best wishes- oldandslow