I'm sorry, but I don't understand how mounting a scope on typical firearm could be considered a manufacturing process.
Additionally, mounting a scope is a very simple process. There is no reason to have a shop install it for you. Even if they do and bore sight it, you will still need to zero it yourself.
In contrast, a dealer-gunsmith may make or acquire his/her
own firearms, and repair, modify, embellish, refurbish, or install parts in or
on those firearms. If the dealer-gunsmith then sells or distributes those
firearms for livelihood and profit, the dealer-gunsmith is engaged in his/her own
business of manufacturing firearms. A person engaged in the business of
manufacturing firearms for sale or distribution is required to be licensed as a
manufacturer, identify/mark all firearms manufactured, maintain permanent
records of manufacture, submit annual manufacturing reports, and pay any taxes
imposed on firearm manufacturers. A licensed dealer-gunsmith who becomes
licensed as a manufacturer must also segregate all firearms manufactured for
that business separately from firearms for which gunsmithing services are being
performed.
The above is from the ATF
The act of mounting a scope for a customer is common in my shop
That falls under gunsmithing.