I'm thinking something simple that can track people while they walk up that looks like it has a gun on it. I asked my city council and they said that if someone feels threatened on your property you could be charge with a crime.
I'd take the city council's advice and realize that you could be charged with a crime. Their job is to know the local laws and put them into place, as well as know how those laws relate to state law.
As for state law, the Revised Statutes of Missouri, RSMo Section 574.115 Making a terrorist threat says:
574.115. Making a terrorist threat, first degree — penalty. — 1. A person commits the offense of making a terrorist threat in the first degree if such person, with the purpose of frightening ten or more people or causing the evacuation, quarantine or closure of any portion of a building, inhabitable structure, place of assembly or facility of transportation, knowingly:
(1) Communicates an express or implied threat to cause an incident or condition involving danger to life; or
(2) Communicates a false report of an incident or condition involving danger to life; or
(3) Causes a false belief or fear that an incident has occurred or that a condition exists involving danger to life.
2. The offense of making a terrorist threat in the first degree is a class D felony.
3. No offense is committed under this section by a person acting in good faith with the purpose to prevent harm.
A fake gun turret on a porch in the public view that tracks people who walk by could be interpreted as making a terrorist threat because it (3) Causes a false belief or fear that an incident has occurred or that a condition exists involving danger to life. And, it's probably safe to assume your turret has the "the purpose of frightening ten or more people."
The fact that the gun turret is on private property doesn't mean much; it is in view of the public and your intent is for it to be seen by the public and you want to invoke fear in the public members who walk by. And it's not going to be seen by the council as some sort of security; threats are not security.
If you did put up such a turret, and the state didn't take action under 574.115, and there is no local law on the books that applies, the council can easy put one in place at their regular council meeting with a simple motion and vote. Since you already asked the council, they may already be considering such a law.
And, depending on the county, the council could invoke a law addressing threats to the public that has more severe penalties that the state law, because Missouri is a home rule (Wikipedia) state.