Arizona's new law (2009) is a model for the nation
Warning an attacker that you're armed is protected by law.
A.R.S. §13-421. Justification; defensive display of a firearm; definition
A. The defensive display of a firearm by a person against another is justified when and to the extent a reasonable person would believe that physical force is immediately necessary to protect himself against the use or attempted use of unlawful physical force or deadly physical force.
B. This section does not apply to a person who:
1. Intentionally provokes another person to use or attempt to use unlawful physical force.
2. Uses a firearm during the commission of a serious offense as defined in section 13-706 or violent crime as defined in section 13-901.03.
C. This section does not require the defensive display of a firearm before the use of physical force or the threat of physical force by a person who is otherwise justified in the use or threatened use of physical force.
D. For the purposes of this section, "defensive display of a firearm" includes:
1. Verbally informing another person that the person possesses or has available a firearm.
2. Exposing or displaying a firearm in a manner that a reasonable person would understand was meant to protect the person against another's use or attempted use of unlawful physical force or deadly physical force.
3. Placing the person's hand on a firearm while the firearm is contained in a pocket, purse or other means of containment or transport.
The law is described in plain English here.
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