Actually the use of FMJ was agreed to before the advent of modern "flak" body armor. It's not what it's meant for.
Excerpt from the 1899 International Peace Conference at The Hague, which entered into force on September 4, 1900:
Quote:The Contracting Parties agree to abstain from the use of bullets which expand or flatten easily in the human body, such as bullets with a hard envelope which does not entirely cover the core or is pierced with incisions. The present Declaration is only binding for the Contracting Powers in the case of a war between two or more of them.
Snipers use matchking because On 12 October 1990, another Memorandum of Law from Parks at the request of the Commander of the United States Special Operations Command (USSOCOM) and coordinated with the Department of State, Army General Counsel, as well as the Offices of the Judge Advocates General of the Navy and Air Force, concluded that:
Quote:The purpose of the 7.62mm "open-tip" MatchKing bullet is to provide maximum accuracy at very long range. ... Bullet fragmentation is not a design characteristic, however, nor a purpose for use of the MatchKing by United States Army snipers. Wounds caused by MatchKing ammunition are similar to those caused by a fully jacketed military ball bullet, which is legal under the law of war, when compared at the same ranges and under the same conditions. (The Sierra #2200 BTHP) not only meets, but exceeds, the law of war obligations of the United States for use in combat."
The use of the FMJ was agreed upon in 1899 because it was viewed as a more "humane" bullet, as opposed to expanding or shredding bullets, that cause excessive damage and "potential suffering", because FMJ bullets were considered "cleaner" and enough to cause sufficient deadly force.