I dunno. The original Merc had an Ingram LMG, and no background fluff to explain what a mercenary actually was in the world of Shadowrun.

Players and GMs just have to set the tone and understand what is and isn't acceptable in their games. Just because a toy was outlined in Fields of Fire didn't mean the GM had to allow it. The GMs for our games set rules about which sourcebooks could and couldn't be used all the time. And common sense (or a couple brushes with Johnny Law to explain what common sense should be) sets what is and isn't acceptable to just walk around with. An assualt rifle is very difficult to conceal, even under a long coat. People are going to notice you walking all stiff with three to four feet of inflexible rifle "hiding" at your side.

There's really been no huge advancement in the firearms of the game. Assault rifles are still assault rifles. And the players can only have what the GM allows.

If the GM is allowing these gigantic firefights with every kind of weapon imaginable, that's either bad storytelling/imagination from the GM, or Pink Mohawk.

I'm not a fan of arbitrary punishment, even for something like this. Collateral damage of the sort described in that story will definitely get law enforcement to look for you. Especially bad behavior will get them looking very, very hard, since bagging a terrorist is a major PR coup. After they start looking, it will come down to a dispute between the agencies' ability to track people down and the runner's ability to erase his tracks. I'd pay more attention to the careless mistakes made by the runner, and note the clues he leaves behind that could conceivably be found by Lone Start or whoever. After enough time has passed without new incidents, the heat will die down, assuming the runner is still alive and free at that point.

This is how you do it, if you want a reasonably realistic game. If you really have a personal problem with the way the player is acting, you talk to him and explain that this is not the sort of game you're running. Maybe penalize his XP for that adventure, or add one or two points of notoriety, but the talk is the most important part.

If, on the other hand, you just saying after the fact that one of Lofwyr's best buddies, or a contact's relatives, just happened to be in the blast radius, well, that's just passive-aggressive posturing, and will lead nowhere. If you expect mature, realistic and consistent behavior from your players, you should hold yourself to the same standard.

Lofwyr's best bud, or your contact's granddaughter? Yes, that's a bit much.

But, having your fixer all beat up give you a call and say, Hey.. there's a runner team looking for who ever did the Benzer Hit. I didn't tell them who you were.. but they looked pretty teed, and they knew I was approached to broker the deal. You guys better find out what's going on.

that's reasonable.

You hire your runner team, to go after a runner team whose as big into collateral damage as they are. For doing somethign they've done in the past. That gives them a warning that if you keep doing this, someone will send guns after you.

That's more similar to using law enforcement, as I described above, and is indeed reasonable (unless you have it happen on the same day. These things take time!). As for giving them "a warning", I would strongly prefer to actually deliver the warning to them verbally, before the game, as part of setting their expectations. Just because you, as GM, control most in-game events, doesn't mean you can only communicate with your players through in-game events .