The last third of the book explains how to keep the muscle. Most people gain 8-10lbs on Mass Protocol, then lose it in 4 weeks of high-cardio ops. The book has a specific "Maintenance" protocol using just 2 lifting sessions per week that preserves 90% of your mass.
Stop searching for shortcuts. Buy the book, print the templates, and start the work. Your body will thank you when the mission requires both the mass to move a barrier and the wind to run a mile afterward. tactical barbell mass protocol pdf work