"Can I carry my loaded and concealed firearm (assuming I have a license to do so) in a New Hampshire business (public place) such as Costco that has a NO FIREARMS sign posted? NO, private property rights prevail. If you are caught carrying a firearm in a business that is properly posted prohibiting firearms (i.e. the sign was of a sufficient size-there is no law precisely defining the ―prescribed manner of posting‖ i.e. what size and type the sign must be-however some guidance may be found in New Hampshire R.S.A. 635:4 Prescribed manner of Posting 40) and posted at the entrance where you entered (thus you had notice) you may be asked to leave (only someone with ―authority‖ can ask you to leave, typically a store manager or even an employee of the store may be deemed to have ―authority‖) and you could be prosecuted under New Hampshire R.S.A. 635:2. 41 See State v. Gaffney, 147 N.H. 550 (2002). (When defendant refused to leave the police station after being asked to do so, there was sufficient evidence to convict defendant of criminal trespass under former R.S.A. 635:2, II (b) (now R.S.A. 635:2 III(b) because defendant received strong warnings to leave the station or risk being arrested.) You can be asked to leave even IF you do not have a firearm and even if you believe you are not doing anything wrong, as a business, even Wal-Mart is private property and has a right to control who is allowed on the premises (assuming they are not violating federal law, by, for example posting a ―NO X race allowed‖ or keeping a federally protected category of individuals from entering).
That being said what are the penalties for carrying in a posted place? None. They can ask you to leave. If you refuse to leave then you can be charged.