I don't believe these figures you're using and submit that you've just made them up.
Let's say it takes 3 seconds for the average person to change a magazine, and a semi-auto rife can fire 3 rounds per second (you may make your own bids for these figures I'm just using something ballparky). Let's take a shooter with 2 30-round magazine and a shooter with 2 8-round magazines.
First shooter:
Second shooter:
over a 10 minute interval the first shooter could fire 1560 rounds while the second shooter could fire 1140, or 27% fewer.
Limit magazine size to 6 rounds and the count will be even lower. You cannot possibly try to pretend that that amount is not significant.