In my time overseas I never once saw or captured an enemy machine gun with a spare barrel. We did however find a number of machine guns with warped barrels, a sign that they were overheating from too much use and no barrel change.
So technically as a light machine gunner, I should have been the second senior guy in a team of four, and the 3rd or 4th guy would carry a spare barrel. In reality, they gave me the machine gun because I was the new guy. I carried the spare barrel for about half the deployment and then I just stopped. I replaced it with 400 more rounds of ammunition. I never changed my barrel even once on deployment. A few times I probably should have, but I was young and bold.
A Scout means many things, but in my particular case, I was with a Light Armored Reconnaissance unit and my job was to ride in the back and provide infantry support to the vehicle (LAV-25, badass vehicle) to check out IEDs, patrol local areas, perform route reconnaissance surveys (to determine what kind of vehicles could safely pass through a route), etc etc. One of our preferred uses was to have the vehicles drive us to a location where terrain masked our movements, slow down to 5 MPH or so, have us jump out the back, then the vehicle would keep going while we hid until nightfall. We'd then conduct observation post operations until we were discovered, took contact, or were told to walk back to base. IT was a fun job, but it was a lot more walking than I thought I would get in a vehicle unit.
In Iraq we were mortared ALL THE TIME, but very inaccurately. Where I was in Afghanistan, we never received a single mortar round, which was weird. But instead Taliban would angle RPGs and try and fire them over the COP (you're right- Combat Outpost or Company Operations Post) walls. They almost always missed but we had a few close calls. They did the same thing with AKs and PKMs angled up like crazy then firing them into the COP. We had several wounded aid workers who were staying with us to get human terrain data this way. They all survived, but they never sent any more back to us.
So, RPG-7s actually have anti-personnel warheads that are very effective. But I guess they use them against vehicles because US mobility gives us a distinct advantage to close with, engage, and destroy the enemy through fire and maneuver.
When I was in Afghanistan, we went through two phases. First phase was paying the locals to grow alternative crops in exchange for letting us burn their poppy fields (although they were allowed to continue growing marijuana), and then we had a drastic switch where we stopped even trying to stop poppy growth. The rationale I was given was that the locals were far more willing to cooperate with us when they could put food on the table (via the local narco-economy) than if the Taliban was the only ones who could provide them money (via paying them to spy on us, emplace IEDs, or attack us). It was a lose lose situation.