A modular system for adjusting the length of pull and drop of a long gun. The modular system enables a combination of any one of a plurality of comb portions with any one or more of a plurality of butt pad spacers for a versatile and robust system that provides a wide range of comb height and length of pull combinations. The system can be implemented by the user, avoiding costly gun fitters. The system provides these aspects in a sleek, streamlined way that does not mar the appearance of the long gun. The spacers for the length of pull adjustment include registration surfaces that provide a three-dimensional interface with the buttstock and comb portion for enhanced strength and stability of the assembly over that of conventional fit adjustment systems.