[Bloody Sam Roberts]

And how much harder would it be to buy just one Barrett on the street compared to multiple AK’s?

There's the rub. But I am unfamiliar with the procurement logistics for the M82. But it would represent a force projection orders of magnitude higher than the AK or a variant.
That might make it a worthwhile endeavor given the target.

[Kartographer]

You have to remember you need multiple teams because for this to work you have to destroy multiple targets. So say ten teams you need ten Barretts, say ten teams of 2 active shooters you need 20 AK’s. Now what do you think?