Example Weapon M16, ROF 6, MAG 30.
Avg PC, Agility D10, Ranged Combat D8
Choosing to roll 6 ammo dice.
Result D10=7, D8=6, A1=1, A2=2, A3=6, A4=3, A5=5, A6=6
* You got two successes with your base dice. This will result in weapon damage plus one (2+1=3) to a body location. This is one bullet (well, not 100% clear on this but since you the rules say that if you are using zero ammo dice, you spend one round, I assume this is the case).
* You got two ammo successes, They can be used to either be hits on their own, either to the target you originally aimed for (another hit location doing weapon damage) or another target in the same hex (that is not harder to hit that the originally target) or they can be used to increase damage of a hit by one
You spend a total of 1+23 ammo, leaving 6 rounds in the magazine for the next round.
If you during the next round, you spend more than 6 rounds, then you simply empty your magazine and you can't get more ammo hits than the number of rounds you have. In this case (if my guess is correctly regarding ammo) this means that one round you are using because you shoot, leaving 5 rounds, meaning that you can maximum get 5 hits with your ammo dice.
Do remember that if the base dice didn't hit, then nothing hits. The ammo hits in this case still count as suppressed fire. It is very risky to use such a high ROF, it will most likely stop you from pushing the dice (many dice menas many changes to damage your weapon).