I served in Marine Recon from '08-'12. Over 2 deployments to Afghanistan, our bread and butter was helo-inserting into a totally non-friendly area, securing said area, and patrolling it on our own for up to 1 month at a time. We also got to do some high speed drug raids, as well as boring mounted patrols in the middle of nowhere.
Because we needed to be completely self sufficient without vehicles or ground unit support of any kind we were very heavy. If a firefight broke out and you had your full 90lb ruck on, you ditched it if you knew you could get back to it, but generally people kept them on. We had all of our battle necessities on our kits, and the only person who really needed to keep their pack on regardless was the radio operator.
I normally carried a ruck with med gear, an M72 LAW, batteries, spare personal radio, and water. Maybe some food. I cinched it to me so tightly that it was like an extension of my body armor. It was only about 25lbs so I could move really well with it on me.
On my second deployment I carried the M107 (.50 cal Barrett) and normally a small backpack with water and ammo. Maybe some kind of thermal or binos too. I didn't really have to carry much platoon gear since I had the big gun.
That's me back in 2010 just off of a short patrol. Note that my waist strap and chest strap are fastened and tightened. My shoulder straps are also as tight as they can be because I knew that I wanted to keep my gear with me and not have to try and recover it later, or worse, put that rocket launcher or radio in the hands of the enemy.
This is prior to inserting into our area of operations. We planned to be there a week, but our birds took too much fire on the way in and we had to abort. The .50 cal was separated into 2 parts for the actual insert but is assembled for this pic so you can tell what it is. It weighs about 35 pounds loaded, and I carried 4 extra mags at 5lb each plus my required food, water, batteries etc... I was very, very heavy. Still, in this instance I would have kept my ruck with me in a firefight if at all possible, and only dropped it once a rally point had been established.
Once we inserted and we had established a patrol base, I could then pick and choose what I needed for any particular patrol. If I took the fifty cal, I generally took only the fifty cal and ammo, but it depended on where we were patrolling and the surrounding environment.