Your best bet for an outdoor shotty and large CQB is the Marushin M500. The build is all metal and the internals are steel. The only weak points on the gun are the pump arms break over time, but can be replaced for $7, and the threads that connect a stock are prone to breaking if tightened to much. If you can get your hands on the M500 6mm, then you'll have the best gun for any CQB. No shells.
Everything I've seen and read about the Tanaka looks impresive, but you still have to deal with the shells. I will eventually get the m870 but my first choice over all is an M500.
Azn— My m500 8mm was recently discontinued at TAC for being too powerful. Hot weather made the propane quite strong last time I was there so Brian and I agreed it was too much of a threat to front teeth
He did agree to let me test it out with duster gas, so I may be back with it soon, but gladly not causing the kind of pain it was before.
Steve