AT Guns don't tickle Panthers at all. Go and calculate the penetration chances of 57mm AT Gun, or even Riflemen AT Snare for maximum argumentation, as it was shown in the shoutbox at the time of writing this post.
ATGs and RF nades don't Peirce the FRONT of the panther. It does Peirce the sides and rear tho. If he's going in for your calliope, he'll have to drive past you and expose his sides and rear which is the perfect chances to snare him. I agree with Australian magic. ATG+RF stops panther. The only tanks it doesn't stop are tiger and KT but they're so slow you won't have a problem keeping your calliope away from them.