The can't modify guns they sell or offer the service to sell them .
http://www.atf.gov/firearms/faq/manufacturers.html
Q: May a person engage in gunsmithing under a dealer’s license (type 01), or do gunsmiths need to be licensed as “manufacturers” of firearms?
Generally, a person engaged in gunsmithing requires only a dealer’s license (type 01). There are circumstances in which a gunsmith might require a manufacturing license. Generally, a person should obtain a license as a manufacturer of firearms if the person is: 1. performing operations which create firearms or alter firearms (in the case of alterations, the work is not being performed at the request of customers, rather the person who is altering the firearms is purchasing them, making the changes, and then reselling them), 2. is performing the operations as a regular course of business or trade, and 3. is performing the operations for the purpose of sale or distribution of the firearms.
Below are examples of operations performed on firearms and guidance as to whether or not such operations would be considered manufacturing under the Gun Control Act (GCA). These examples do not address the question of whether the operations are considered manufacturing for purposes of determining excise tax. Any questions concerning the payment of excise tax should be directed to the Alcohol and Tobacco Tax and Trade Bureau, U.S. Department of the Treasury.