One thing to remember is this. A partially exposed firearm such as an IWB holster with part of the gun concealed inside your pants would be considered a concealed weapon. As such if you don't have your CCW permit you could be in violation of the law.
According to what statute? The only state for which that is true, that I am aware of, is Mississippi:
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Summary
Mississippi is an open carry state and the state constitution appears to guarantee this right. Further, localities are generally preempted under state law from enacting local gun control ordnances that might restrict open carry. However, Miss. Code 97-37-1 & 45-9-101 operate together to ban carriage of a handgun "concealed in whole or in part" unless the carrier is inside a motor vehicle or possesses a License to Carry a Concealed Pistol or Revolver. As the Mississippi courts have said that a handgun in a holster is concealed in part, a License to Carry a Concealed Pistol or Revolver is required to openly carry a handgun in a holster in Mississippi.
In most states, if the average person can tell that the exposed part of a firearm IS a firearm, it is considered openly carried.