Re: Uninformed guess.
As far as I can remember, the Barretts 82 (also .50 cal) could still kill someone through a brick wall after 3km. That doesn't mean they were ever aimed well enough to do so (edit: 1km "effective" range). Being an old fogey, I remember when 6 of them being found in Northern Ireland meant the army had a tempory halt on using helicopters, as they made such juicy targets. That was back when we had proper terrorism (as financed by the USA).