This also means that for the vast majority of weapons (those that have a Critical rating of Damage +1), the extra hits caused by auto-fire can never be criticals.
Normal damage can't kill you by itself (allows for a coup-de-gras), so extra hits will just disable the character quicker.
This can lead to a situation where a full-auto attack with an MP5 (RoF 5) gets an incredible roll of 5 6's for 6 hits (for 6 damage in total) but, because I only rolled a single success, leaves the target (while suppressed) rather surprised that he's not taking the room temperature challenge.
Player: "You telling me that all six bullets just grazed him? Are you serious?!"
I agree - I originally read the paragraph differently, but re-reading it does seem to say that ammo hits are treated as additional hits (only if the base attack dice hit), and that they only do base damage. I thought that they added to the initial attack dice damage, which would have made crits much more likely. It seems like a much simpler approach too - I'm not sure I can be bothered to roll hit location for every ammo dice hit, and calculate armour and damage separately, only to find it's not going to cause a critical anyway. Hmm.
I actually think that the way you understood the rules is better:
1.) It's probably more useful from a mechanics perspective to add to damage base damage (as it increases the chance of a crit) than to have a separate base-damage hit.
2.) It allows the Severe Injuries rule to come in effect more often, which fits the idea of someone getting riddled by bullets rather nicely.
As it stands now there are some weapons that can never cause a Severe Injury... and it's not the weapons you might think!
A trusty Glock 17 has a damage of 1 and a Crit of 2. To get a Severe Injury you need to cause 4 points of damage. That requires 4 successes (the maximum possible) on your attack roll (1 to hit and the other 3 to add to damage for a total of 4). This only possible if both your dice are at least a D10 or higher.
Now, let's take a M240 GPMG, which has a damage of 3 and a Crit of 4. That means you need 8 points to cause a Severe Injury. Not possible I'm afraid... 3 damage + 3 for rolling 4 successes is only 6 damage.
M2HB 12.7mm HMG? Nope, no Severe Injuries here either (with 4 Damage and a Crit of 4, you can't get to 8 damage).
Now *if* ammo-dice hits counted towards damage instead of causing individual hits things would change dramatically!