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.
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