With actual QUALITY airsoft guns, you'll get a nice yelp.
Most of the time, we rely on the player's honesty. That's what airsoft is mainly about lol.
That M16/M4/M83 sure looks real, minus that large CO2 tank you gotta attach to the rear.
And I've seen those, but the cost of all the equiptment can be extreme. The gun can be well over $300 for a quality one, $20-$40 for a CO2 bottle refill, then a bunch more for quality paintballs that won't burst in your gun...
also you cant forget about the big round hopper that goes on top to!