Guys, if you read the OP's first post, he's clearly talking about long range rifles, and does not mention the M2HB machinegun.
I was a designated marksman in Afghanistan for two deployments with Marine Recon. I carried the Barret (M107) probably about half the time. It all depended upon what type of terrain we were going into otherwise I would just carry a .308 (M110). I like to joke that my fifty cal did more harm to me than it ever did to anyone else. Big, heavy, and LOUD!
Loadout-wise, I normally carried the rifle itself, fully assembled on my shoulders. I also carried 5 magazines loaded with a total of 50 rounds of Mk.211 "raufoss" ammo which is a high explosive, armor piercing, incendiary round.
I carried two spare mags in a SAW-pouch on my left side, so at any time I had access to 30 rounds.
I also put 2 sets of ear plugs on the rifle itself by jamming them into the sling attachment points, as well as having some in my pocket anytime I carried it.
Beyond that, I carried a personal radio and whatever food/water was required for the mission, as well as an incendiary grenade in case any enemy weapons needed to be destroyed. With a 35 pound rifle (loaded, with optic) I was definitely one of the heavier guys, but not my as much as you would think. Our machinegunners carried tons of ammo as well as anyone with a grenade launcher, especially the 6-shot M-32's. Those things go though 40mm in a hurry.
Only once did I split up my gear and that was when I broke the rifle down and "T-boned" it on my ruck so that I could carry my M4A1 as well. The only thing I gave to someone to carry for me were 3 magazines of ammo. I'm not even a very big guy. 5'10" 170lbs. It did help that we normally ran shorter duration missions <7 days but as long as you ingest enough "can-o-man" then you'll be fine.
It actually sits on your shoulders quite well, and with the bipods extended you have a nice grip and shoulder support for carrying it. It does start to dig into your neck after a while.
The reason that I never carried a sidearm is that at any time there are 7 other guys with me, and the only use for a sidearm out there is for the wild dogs and searching tight spaces. I stayed away from both.
I also never had a dedicated spotter (our unit snipers don't have them either) but whenever possible we tried to have someone on glass if we were taking a shot. With the ROE's today, rarely did we get to shoot anything until the enemy was firing first anyway.
The rifle comes apart into 2 halves kinda like an AR and some guys will split the upper and lower. It is also an unwritten rule that whoever carries the .50 gets to shoot the .50 as long as it is tactically prudent to allow him time to get on the gun.
Hope this answers some questions!
That's me with the rifle fully assemble on my ruck. I only had it that way for the pic and carried it broken in half as I described before on this particular mission. Note the ear plugs on the stock.
