Marine Corps Infantry...
Your ruck has most of your gear, but your LBE (load bearing equipment) has the basics for a firefight, which consist of spare magazines, water, and first aid/immediate response items. Depending on the environment, you may also carry a gas mask, although again - that is environmentally dependent.
[Caveat: Most of what I'm saying is environmentally dependent. A 4 hour patrol through an urban built-up area is a completely different assignment than a 2 or 3 day patrol through mountain passes separating Pakistan from Afghanistan. The Marines who initially went into Iraq were equipped with MOP gear to fight in a chemical weapons environment. Your load will vary accordingly to where you are, when, and why - and what your unit is expected to face.]
But generally speaking the shoulder straps on your ruck have a break-away fastener. If you are moving with a full load and are taken under fire, you drop the pack and proceed to fire and maneuver in response wearing only plate carrier and LBE. You return to the packs afterwards.
Hope that helps.
Treat everyone you meet with dignity and respect....but have a plan to kill them just in case.