I'll throw it out there. A 203 shell with BB's inside and piece of napkin or tissue paper stuffed in at the top.

Just the 203 shell, nothing else. Actuate your "secondary" by pressing the button on the back. :P

Or at most the S-Thunder Flaregun/pistol thing, but you won't be as high speed/low pro as people who run only the shell.

In my experience an M203 is not great for airsoft. They weight too much and just make your gun bigger. Each shell will usually only get one target, and may not get anything. There's no accuracy factor to them really, more of "for fun" factor.

As for shotguns, they are generally light but lack that accuracy and reliability you need. Also they are still a full sized weapon to carry around. High end, reliable pistols are where it is at. A good Tokyo Marui will shoot good and straight out of the box. There are a lot of upgrades for them, and with the right weight of ammo they are easily a 60' - 100' weapon.

a pistol with several mags is heavier than a short plastic shotty with some shells.
And all the shotties I've held were way more precise than most average pistol.I really think pistols are over rated in airsoft.

There's no question, many shotguns are mainly plastic and can be quite light. Pistols "may" weigh more, but I can't imagine many do. The weight in a pistol either way is more dense and compact, but that means easier to manage and carry. A hip holster is out of the way and doesn't increase you profile. Also you can mount it anywhere so weight distribution isn't an issue. Pistols are easier to wield by leaps and bounds, no contest really. I'm certain a shotgun can be made quite accurate; air nozzle, piston, hop-up, barrel, seal, etc. But as a backup, I really don't see the benefit over anything. I'd almost consider carrying an SMG just for firepower and deal with the weight. I go from my primary to my pistol when I don't even need to, just because I want to close in my profile and react quicker, especially indoors or during CQB. To each their own.

what pistols have you been using and what shotgun?

Shotgun way too bulky to comfortably run as a secondary for me, I'd take the heavy mags trade and have semi auto over a light plastic scatter gun any day

true !