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The summary is that, in Virginia, it's up to the individual localities if they want to make it illegal. If you have a reasonable belief that you need a loaded shotgun or rifle for your job, then you are excused from any local laws. You should contact the Department of Game and Inland Fisheries (http://www.dgif.virginia.gov/contact/) and ask, as I was unable to find anything online (in a quick search) for Manassas, specifically. Alternatively, you could just keep it unloaded as this should be fine anywhere in Virginia.
Virginia law (http://leg1.state.va.us/cgi-bin/legp...cod+15.2-915.2) states:
§ 15.2-915.2. Regulation of transportation of a loaded rifle or shotgun.
The governing body of any county or city may by ordinance make it unlawful for any person to transport, possess or carry a loaded shotgun or loaded rifle in any vehicle on any public street, road, or highway within such locality. Any violation of such ordinance shall be punishable by a fine of not more than $100. Conservation police officers, sheriffs and all other law-enforcement officers shall enforce the provisions of this section. No ordinance adopted pursuant to this section shall be enforceable unless the governing body adopting such ordinance so notifies the Director of the Department of Game and Inland Fisheries by registered mail prior to May 1 of the year in which such ordinance is to take effect.
The provisions of this section shall not apply to duly authorized law-enforcement officers or military personnel in the performance of their lawful duties, nor to any person who reasonably believes that a loaded rifle or shotgun is necessary for his personal safety in the course of his employment or business. (emphasis mine)