Complex sentence: The U.S. Fire Arms Mfg. Co. Custom Shop produces custom-finished firearms, a type of functional art. Custom finishing of guns is a type of firearm modification.
Split into simpler sentences: The U.S. Fire Arms Mfg. Co. Custom Shop customizes USFA production firearms. Through a variety of processes, production pieces become a type of functional art. Custom finishing of guns is a type of firearm modification.