It's funny, in SR3 I felt the combat damage could be gauged MUCH more accurately from each gun.
Basically it went liket his:
Every serious runner got 9 Speed, and consequently 9-10 Ballistic armour. That meant that he was "safe" from bascially anything below an AR without special ammo firing bursts. That tended to be my target. As a GM, I had the bad guys use AKs or shotguns to endanger, but not seriously injure the runners. If I wanted to hurt then, I would use XX or 6 round Full-auto.
As a runner, what I used to do was use up to four pool dice to dodge, and usually 2 pool dice to help with resistance. (Serious runners tended to have 6 or more body dice.) Usually it was quite safe to walk into a room full of mooks shooting SMGs, as long as you got into some sort of cover, or used tumble dodging to get into melee range. The one house-rule we always used was that damage never went beyond deadly. We just capped at D, which could create some weird things with being basically unkillable in one attack when using the old Trauma/Platelet combo. BUT it was still nice to have this kind of certainty.
The other possibly non-standard rule we used without ever actually mentioning it was adding ALL the rounds of a burst to the base power level, and not just those above the first. So a three-round burst produced +3Power/+1Damage. This produces the possibly funny numbers in the example in the spoilers.
[ Spoiler ]
For example, one notable encounter that just popped up in my memory, by one of my SR3 characters, "48".
48 enters a room within the hideout of some criminal organisation, the details elude me now. He is wielding a Katana and has his custom AR carried on a sling. He slips into the door to find himself in a dorm room with four beds, and four goons wielding SMGs. In IP one he decides against melee, drops into full cover and switches to his AR. The goons open the hostilities by ducking into cover, too, and each firing one three-round burst of XX ammo, for something like 12S total. Two of them manage to hit while 48 is still diving into cover (well, we used a system like that). 48 dodges one and soaks the second burst completely.
He then nails one of them with a three-round burst of his 9M, XX firing AR, switches to the next guy and misses or does some damage, I forget. Not all of the goons have two IPs, well, I think he gets hit once more with a 12S burst, using the last of his combat pool he soaks that. The final hit for the second IP leaves him with light damage for a resulting medium damage hit, probably a 12D soaked with only four 3s rolled with his 8 body dice.
He then kills at least one more of the guys in IP3.
Next round he finishes them off, possibly even with his sword, because he missed twice due to cover in IP1, but takes another light wound I think. So he came out with 2 boxes of physical and one box of stun, and no mods due to Damage Compensators. A single application of first-aid later he is uninjured.
So I had him fight four goons with 7M SMGs firing XX ammo, addmittedly in burst mode only, and the outcome was actually very predictable, because he could reliably resist the 12S to D hits they gave him with only little damage taken. And do note these guys were rolling only 4-5 dice against 6s or more due to cover, so they were rolling well.
A 12D hit in SR3 translated to SR4... is already quite a bit of damage IMHO.
So the rather reliable interaction of power, damage and armour made it possible to taylor encounters against the capabilities of your runners. As a GM, I could simply tell my players: I want you to be able to withstand at least 12D without taking serious harm. That's the bar if I don't want to really hurt you. Then if I did want to hurt them a little more I would use a heavy pistol with APDS or three-round bursts with 8M ARs and XX., or even 6 round full-auto with standard ammo in an AR. In our system that produced 14D with a reliable chance of a serious wound, but a very improbable death of the runner, since he, on average, had to roll 5s to resist. Another possible option to hurt them without killing them is hitting their impact armour. Always nice.
About light pistols: Well... they suck, and you can't do anything about that. The problem is that even when used at point-blank against a non-moving target, there is just a hard limit to their power.
I see it like this:
The single success hit of an L calibre pistol does an uncomplicated shallow flesh-wound against an unresisting target
The single success hit of an M calibre heavy pistol or AR does an uncomplicated full penetration flesh wound against an unresisting target
The single success hit of an S calibre weapon does a serious wound with all kinds of tissue damage
The single success hit of a D calibre weapon... well, you know it. It just causes enough trauma to put you out of it.
In this respect, SR3 is actually very accurate. For example: An untrained shooter (skill 3, speed 3) taking potshots at an unskilled target (body 3, no armour) will only need about a 6M weapon to kill his target with several shots. Soaking 6M is already difficult without armour. If you did manage to roll the four 6s, then you were just very lucky, and just got a graze, or a freak richochet off a bone that only produced a scratch. Heavy pistols get increasingly difficult to soak, hence the probability of dealing at least medium damage with a single success goes up significantly.
The problem is this: There aren't really many weapons with high base power but low damage. So most light pistols are easily completely resisted by a guy in armour, which makes sense.
As a house-rule to make light weapons more deadly in the hands of a truly competent shooter would be to increase Power first, and then damage. For example, give a 6L light pistol a max augmented power value of 9 (1.5x base, rounded down), to be increased on the same scale as damage with +1 per two hits. After hitting that power, improve the damage. Or increase BOTH at the same time, with the possible side effects of really powerful bigger guns.