I'm thinking about having it slung in front of me for 18 hours during a 2 day shotgun class. If you do a couple of those, I guarantee you're gonna rethink your accessories.
http://3gungear.corecommerce.com/8-Shell-Side-Saddle-p23.html
I have some extra 3 Gun Gear 8 round sidesaddles with die cut velcro if you'd want one. $24 with free shipping, just send me a PM. They want something like $8 for shipping
Just stick some industrial velcro on your buttstock then stick an OSOE or Raven shell card on there. Problem solved.
Take a defensive shotgun class, and you'll see running brass on your buttstock isn't advisable.
Stick some velcro on the receiver and a Raven Concealment 4 round card and it's not heavy at all. The buttstock is too far away from the receiver to do quick portloads/slug changeovers.
Just to expand on what truckcop said, the M3 has the ability to be both pump and semi auto because many of the less lethal rounds don't have enough oomph to cycle a semi auto. Not very practical for civilians, which is why you don't see them very much. The M2 is the superlative choice between the two.
I can't shoot something until I know who/what it is, know your target and back stop and all that.
Night sights do nothing for target identification. If it's dark enough to need night sights, it's dark enough to need a flashlight. And if it's dark enough to need a flashlight, night sights get washed out from the beam.