The Hefty paid for itself right there.
The air bags would NOT have gone off though...as the barrel would have had to have decelerated you hard enough to slam you into the dash/wheel, hard enough to cause serious injury w/o the bag.
As the barrel probably didn't decelerate you at all, based on your description of the event (Didn't even feel it hit), the bags would have stayed packed etc.
The bumper, fascia, grill, etc, are typically what the barrel eats though...and that's what the Hefty saved you from replacing...plus any radiator damage, etc.
If it had been a deer, or larger heaver object, the bumper would have saved even more, maybe even kept it from being totaled, etc....but even deer, etc, don't decelerate you enough to need an air bag deployment. (I should know, I've mowed down too many of them to count...)
A MOOSE, a COW, etc...something that when you hit it, its like hitting a wall, THAT'S when the bags go off.
I had a steering wheel bag go off on me a few years ago when some chick late for work turned across three lanes of traffic w/o warning, and got t-boned by me and the car to my left, and to my right, plus, she went head on into a car in the driveway she was trying to pull into...
..The bag went off like a shotgun blast to the chest, cracking a few of my ribs...the company car I was in was totaled (Stock mistubishi on that one), as was every car in the accident.
NO ONE was seriously hurt though, the air bags went off like clock-work, and despite totaling 5 cars, we all walked away.