not sure what 870 your thinking about but my 870 did not take a shell it took a 40 rd capacity magizine witch are very cheap to order ($50 for 4 mags once they reached my front door)

i have an aug(hybrid model with the sight stuck on) myself, with a resently added 140 PDI spring and it works wonderfully. a full metel M92F is a great side arm to go with it(its the combo i use), just make sure you have a desent holster, or the damn thing will flop all over the place while you move around.

I actually have a 140 pdi in the gun already, yea i am looking at a beretta for my sidearm, mabye a Tac Master