What does "10-3...he's got a gun mean"? All information I couyld find is that 10-3 means "stop transmitting" but that doesn't say anything to me.
Why would anyone say that just because of a gun?
I'll take a stab at this question, as there are many variations of radio codes.
Mostly, it would signal radio control/dispatch, and others on the same freq to stop transmitting, as this officer has an emergency situation, involving a firearm.
If this situation worsens, the officer will need to contact dispatch control with additional emergency information, ask for a supervisor, medical assistance, supporting officers to control the scene etc. And, if the officer is injured, waiting for a break in radio traffic is difficult to accept.
The Dispatch-control operator would direct other units to an alternative frequency for radio traffic...AFTER advising all to stop broadcasting, due to an emergency at: (giving the location so some units can head that way, as they listen for more details).
This, would be my first inclination of what was meant by the circumstances you describe.
All ten-codes are not the same, in part, or whole.
Some agencies commingle a ten code with a four-code...which references the specific issue at hand.....as in crime designation. Or, like in California, they use a variation of state statutes as a type of four-code.
Some agencies are so very busy, that becoming a minimalist on the radio is needed. There is a constant flow of radio traffic.....even with computers in the car.
And, some seasoned veteran officers get a bit 'salty' and use short-speak, or "cop-speak" on the radio, which can get creative, as well as annoying.
Anyhow, if you discover it is something other than my take on it, please let me know....