The Saiga, or one of its variants, is the best magazine fed platform for an SBS. Good selection of magazines, easy to modify, easy to clean, etc.
If I had to pick a tube-fed auto, I would personally choose the Versamax. It has a pair of short stroke, self-cleaning, self-adjusting gas pistons that function similar to an AK. The pistons/gas ports are at the chamber so dwell time is not affected significantly by barrel length. While I haven't converted one to an 8" barrel, I fired both the shorter tactical versions and longer sporting ones extensively to the tune of 1000's of rounds. They are extremely soft-shooting and I have never experienced a single malfunction, running everything from buckshot to slugs to the worst Win Universal bulk pack.
I have a Saiga 12 and a Versamax, and the versamax is MUCH softer shooting and more reliable.
