What worked, what didn't
A number of years ago I did an impromptu test out in the desert with just those same calibers: 7.62 NATO vs 5.56mm, all GI issue ammo (M855 didn't exist then...)
We set up several scenarios, an old full-size American car (complete), roofing tin, and cardboard.
We put a sheet of cardboard in the front seat of the car, and another behind a large creosote bush.
We also put that sheet of tin behind another like bush.
Distance: 100' to the windshield or bush.
Weapons: HK91 and Colt AR-15 carbine.
Quantity: 20 rounds each.
Results: 5.56 had a half dozen solid hits on the cardboard in the car. Some dozen either stuck in the glass or just bounced off.
All 20 7.62 came out either the trunk lid or bumper of the car.
12 each 5.56 hits on the cardboard hiding behind the bush, 10 full penetrations in the tin, several dents.
20 each 7.62 holes/hits in both 'bushhiders'
We used to call AR's "poodle shooters".
DG