Sidewinder Mossbergs
Paul Helsinki over at GunsAmerica did a pretty good review of the Adaptive Tactical Sidewinder Venom kit, a $199 aftermarket that turns a Mossy 500 series pump into a military-grade 6 or 11-shot detachable box/drum mag scattergun by dropping the mag tube/slide/forearm/spring and adding a new one.
The 5 round straight mag is on the gun and the 10 round rotary mag is on the mat.
Sounds like an interesting alternative to a Saiga or KSG for a little less price (as you can find used but functional 500s and Maverick 88s around for about $100 ish)