The present invention is a modular stock system for rifles. The system replaces the rifle's buffer tube with a modified one containing a mount for a constant cheek weld and a rail track for adjustment. A stock module then mounts on the replacement buffer tube. The stock module is interchangeable depending on the user's needs. Various configurations of the stock module are discussed, including adjustable stocks, fixed stocks, and specialized stocks. The stock module may be made even further modular by the addition of rail system.