High Penetration:
It has higher penetration. There is a rule in the game to define something like penetration level, such as: usual gun, snipers with penetration feature. But anti-materiel rifle just has higher than everything we have.
For example, it can penetrate 1 layer of wall (but not floor), or multiple layers of usual objects (or humans). And it could set vehicles on explosion with 1 shot. I don’t know how the game calculates the penetration, but we could make it like this: The penetration power of the bullet is 1.9, the power cost of all walls is 1, usual obstacles 0.5, low obstacles 0.2, etc.
Since its fire power is high, making some restrictions on other sides would make it more characteristic. And we don’t want it to be a super perfect weapon in the game.
Loud Noise:
It has much louder noise level, which would alert people in longer distances. The sound of bullet hit would have a louder noise, too. This means that the penetration would alert nearby people on both side of the wall.
Fixed Zoom:
It has a normal zoom level and it’s fixed.
Longer Reload Time:
It takes more time to reload. And in reload time, the scope view would be automatically off.
Longer Conceal Time:
47 needs more time to conceal it back to the case. And he must finish the reloading first before he can conceal it.
Container:
Since most of anti-materiel-rifles are not able to be disassembled into pieces that are small enough to usual suitcase, we might need a larger case especially for it.
Or we could consider to make the rifle un-concealable, or even harder: you can only hold it or drop it - not even carry it on your back.
Gameplay:
Maybe I should put this part in the first place. The purpose of this equipment is a rewarding item that provides some fun to the player after he farms the main story (or main mission) enough. The player is supposed to use it to accomplish the Sniper Assassin challenge, or to finish contracts.