hired gun

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One, especially a professional killer, who is hired to kill another person.
• noun One hired to fight for or protect another.
• noun One with special knowledge or expertise, as in business, law, or government, who is hired to resolve particularly difficult or complex problems.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person who is employed as an armed guard, enforcer, or mercenary and who is prepared to use gunplay or similar violent methods in order to assert the interests of his or her employer.
• noun idiomatic, by extension A person who is employed to advance the interests of his or her employer, especially in a vigorous manner using such methods as political lobbying, legal advocacy, or persuasion.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a professional killer who uses a gun