For a slick SBS(short barrel shotgun) you can take in a daypack or outdoor hike, I'd see; www.serbufirearms.com
. You can convert a 870 or Mossberg 500 pump shotgun. They have 20ga too. You would need a tax stamp & meet certain ATF requirements, www.atf.gov
For outdoor use, you may want to get a 12ga treated with Robar NP3+ or T2. Bearcoat, www.bearcoat.com
is a good value too.