I have my own mentality when it comes to ammo caps: I can't carry any amount I wouldn't be able to with real rounds. So right now, with my current fitness level, I limit myself to 5 mid-caps, loaded with 35 each. As I improve my fitness, I will slowly carry more, up to 10 midcaps for a 300 (more or less, with hop up loss) loadout. Hell, half the fun of using the rifles is reloading in my opinion! Why rock a battle-rattle when you are playing airsoft? If you want massive ammo amounts, go play paintball. (LMGs are the obvious exception)
Peace through superior firepower.