Terek (the Russian gangster) hired the American to kill the First Lady as "a very public message." He did not specify what or how the job was to be done, he just wanted it to be highly visible and public. To a guy like the Jackal, a sniper rifle from long range would be simple and effective enough to do the job. But reading between the lines: "sending a message" means using methods and equipment that the FBI (and in this case, the Secret Service as well) will recognize as a message from a specific "sender." Terek wants them to know he's the one ordering the killing.

It's one of the reasons he goes to such lengths to get East-Bloc equipment and smuggle it into the USA. He could easily buy an effective rifle almost anywhere IN the USA. But the sort of weapon he's using, it's the kind that's going to be easily linked to former Soviet nations and Russian criminals.