what bullet is on the poster of this film?

The cartridge most closely resembles the 12.7x120mm round: An obscure, anti-aircraft cartridge developed for the Vickers D Class Heavy Machine Guns. The lack of a protruding rim and the incredibly smooth, "slope" of the bottleneck are good indicators that the resemblance is, however, entirely superficial. It would also be easier to add the movie's logo on a CGI bullet than a real one as well!

Though it is also true that the bullet looks rather tiny when compared to the rest of its body, this is a typical phenomenon among most full sized rifle cartridges and even more so with heavy machine gun or autocannon rounds when range and penetration power are desired over the reduced recoil and control that their squat, intermediate cartridge brethren boast.

I think you're right, despite your confusion between "bullet" and "cartridge." The gently sloping shoulder looks like it wouldn't headspace properly, so I'm going with "computer-generated graphics" for that cartridge.

There's nothing new about large powder capacities behind a tiny bullet. The Remington Ultra Mag line is even more over-bore than this thing. But without a rim or belt, this cartridge would be inherently unsafe.

Its just a prop and not any bullet i have seen.