I should have looked up "E4" before I opened my big fat mouth. Most of my experience with Marines is with officers, aviators in particular. An F/A-18 pilot has training schedule that is so completely filled, there is barely enough time between deployments to get it all in. I can't see expecting Capt Butch or Major Fred finding time to maintain a high level of proficiency with an M4. Basic familiarity, perhaps yes. But not anything more than a couple of hours a year.
But an E4... sure, I can buy that. If you guys say the 500 meter targets are a reasonable requirement for a Lance Corporal, I believe you.