Except the Gifford's shooting. Or VA Tech. Or Columbine. Or 99% of gun homicides.
Your law enforcement friends must not have seen many ballistics tests, which show .223 rounds having similar penetration/fragmentation patterns to that of handgun rounds. Your choice of ammunition is more important in a home defense situation than your choice of firearm. A 12-gauge loaded with 1 oz slugs will penetrate a lot more than one loaded with buckshot. A 9mm loaded with FMJ will go through more barriers than one with JHP.
What's your magic number, and how did you conclude that your round limit would reduce gun crime?
Statistics and statistical analysis can either illuminate or befuddle, but there is (truly) a standard for an objective application of the discipline. Looking at today's LA Times, I see that there are all sorts of shootings or murders attributable to handguns that are "reported." But -- barring the analysis I speak of -- it appears that they aren't mass murders, noting the exceptions which you cited (i.e., VA Tech).
This last year, you'll acknowledge that we've seen a string -- a "block sample" in time-series if you will -- of mass murders or attempts at mass murder involving the particular assault rifle in question.
I have few illusions that the recent Feinstein proposal will work. There are already so many M-4's and AR-15's in the hands of people that simply eliminating them from the market may pose little in "risk reduction."
Feinstein's view is tutored by a personal experience dating back to the '70s. If one sees risk materialize first-hand, the risk or uncertainty or possibility carries greater weight in the attitudes and conclusions.
I'm personally skeptical that a ban on AR-15's will significantly reduce the risk. I'm more convinced that regulation of magazines and clips may have a useful impact. Crime and shooting statistics from Australia seem to show that regulation of magazines -- limited to maybe five rounds -- puts a dent in the carnage. The same approach is effective in Europe, with limitations of three to five rounds.