Hi caps are fed from a well and when you "wind" them if takes BB's from that "BB well" and puts some under tension in a feeder. When you shoot the BB's come out and as a result eventually there won't be any BB's left in the "feeding track" so now you have to wind it to take BB's from the well and load it into the feeder.
Mid, Low, and real caps on the other hand are all fed from a "feeding track" with spring tension holding them. In this case there is no room for the BB to rattle inside the track therefore you get no rattle.
For outdoors I have real and low (68 rnd) caps that I use. Keep in mind that you do not have to load a mag to it's full capacity so you can load a mid cap (120 round) to 30 or 68 rounds if you want or anything in between. So mid caps are probably the most versatile mags which is what I suggest you pick up, either that or low caps (68 rnd).
If you have a hi-cap it's OK to use for the first few games since you probably don't have too much stuff but it's a really good idea to pick up some more mags for future games.
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