I am currently preparing to becoming a Combat Rescue Officer (CRO) or Special Tactics Officer (STO), which is a officer position of Para-rescue/ Combat Controllers.I was wondering if I would be a mistake to take mass gainer in order for muscle growth? I have seen built Navy Seals, SWCC, RECON, MARSOC, and etc. How do they achieve such mass, yet pass their physical portions? Currently, I have passed the Air Force Physical Fitness Assessment test. I am running, swimming, weight lifting, and doing pushups/situps/pullups. I feel I need to take mass gainer to not burn my muscle from working out so much. I workout 6 days a week.

Body type: 174lbs 6'2" under 10% body fat