Proof?
sander did the test, he ask him
I did the test, what i found is that it's unreliable.
Load a custom games with cheatcommands and make any AT gun be with 2 models only. Now, you can start by making it aim from left to right and you will see the cover indicator disappears depending on the angle and the type of AT gun.
If this is the case it means that one of the models has no cover whatsoever.
Now, EVEN if the cover appears to be been granted, i can with 100% accuracy kill a 2 model AT gun with a Rifle grenade from any side of it's arc when on neutral position (aiming dead center). At least with a pak40 (same thing happens with all other AT guns but it might change a little bit).
In this case if you are towards the left, center or slightly center right, if you aim towards the left model you can kill it 100% of the time. From center right, to right of the arc of the AT gun, you aim towards the right model.
This is so specific because the Rifle grenade has a smaller AoE. In the case of normal grenades or bundle nades, they have enough AoE that even if you aim dead center frontally you will decrew them.
If both models would be benefiting from heavy cover, you wouldn't be able to kill them as either grenade does 40 or 60 dmg.