Movement and position will get you more kills than spraying ammo everywhere. Learning to be where and when is gold. And it doesn't matter how big/slow you are. Speed def helps, but positioning can be done by anyone.
Rant aside, most mags seem to work ok these days for AR unless there is something on the gun side that is unique. If you like quality, I still love my Magpul pmags. They aren't cheap if you can find them but after starting off with high caps, going to metal mids, I love the pmags the most. They make all the other mags feel cheap.
Learning to play with limited ammo, managing mag changes, playing with semi auto is where you start developing skills that will help with the body count, esp if you play indoors. Full auto is stupid indoors.. imho.