Gunsmith Ronnie Holt works on a new M4 Carbine assault rifle at the FBI’s Gun Vault. The FBI abandoned the 9mm round after a deadly 1986 shootout, but the bullet is far more effective now. (Ricky Carioti/The Washington Post)

The Federal Bureau of Investigation is returning to the ammunition caliber it labeled ineffective and blamed for the deaths of two of its agents during a 1986 shootout in Miami — the 9mm jacketed hollow-point luger.

In addition to the new bullet, the FBI has decided to purchase a new pistol to fire it, something that could be in the hands of the FBI’s approximately 13,000 agents by 2016, according to bureau officials. The decision could also have far-ranging implications for local law enforcement agencies because they often model their procurement decisions on those made by the FBI.

The bureau dumped the 9mm bullet after the Miami incident because it failed to penetrate far enough into the gunman’s torso.

The shooter, former Army Ranger Michael Platt, then went on to kill two agents and wound a third. Though Platt was shot multiple times, an autopsy revealed that he died from the wound suffered from that first shot — one that penetrated his chest cavity but stopped just short of his heart.

In response, the FBI fielded a new pistol round, one they hoped would have better penetration: the 10mm. In the following years, the 10mm was ditched in favor of the .40 S&W, a stubbier round that could fit into pistols designed for small calibers.

Currently, the .40 S&W is a law enforcement favorite, but after recent studies on new 9mm rounds by the FBI’s Ballistic Research Facility, the 9mm is slowly finding its way back into the hands of police officers across the country.

According to FBI Special Agent Ray Cook, the current unit chief of the FBI’s Defensive Systems Unit, the bureau, which continuously tests various types of ammunition, began considering a return to the 9mm round in 2007 in part because of advances in ballistic technology.

“During our testing, we found that the [9mm] rounds used in the Miami shootout tested the lowest on our scale,” Cook said in a recent interview at the FBI Academy, referring to the bureau’s ballistic standards and testing methods put in place following the shooting.

The new 9mm round — known to gun aficionados as the 147 grain Speer Gold Dot G2 — is significantly more effective than what FBI agents carried into the field in 1986. According to Cook, the bullet has been rigorously tested and has received high marks in the FBI’s most important category for bullet selection: penetration.

Cook says that the lighter the bullet, the faster the gun can “drive” the round into the target. For the FBI, that translates into 12 to 18 inches of penetration into the human body. The 9mm’s weight, Cook added, also increases an agent’s accuracy in a gunfight, according to the findings of a 2014 FBI report that was leaked online last year.

According to Cook, the bureau’s ability to research and test weapons in ways that other law enforcement agencies cannot gives it great sway over many police departments.

“When we do something, local departments take note,” Cook said. “They see that if it works for us, it’ll work for them, too.”

For Jorge Rodriguez, a police officer in the Houston suburb of Baytown, Tex., his department is testing the idea of going back to the 9mm for many of the same reasons the FBI is leaving the .40 S&W.

“The 9mm has changed,” Rodriguez said. “The FBI report came out and basically affirmed that the 9mm isn’t a weak round anymore.”

The Los Angeles Police Department recently transitioned from a Glock .40 to a Smith & Wesson chambered in 9mm, while the New York Police Department issues a hollow-point 9mm round to its duty officers.

In early October, the FBI issued a request for proposal for a new pistol, a contract worth up to $85 million. Cook would neither speculate nor comment on what firearm manufacturer the FBI might select.

In 1996, the FBI adopted a Glock pistol chambered in .40 S&W and has since fielded a number of variants.

The FBI’s elite Hostage Rescue Team (HRT), however, uses others weapons.

“We are on a completely different program,” one senior HRT operator said, who spoke on the condition of anonymity to discuss the team’s arsenal.

It includes using Heckler & Koch 416 carbines — a favorite of top-tier Special Operations forces around the globe — as well as an array of different pistols. The HRT’s armorers, however, are among the 11 technicians who maintain the rest of the bureau’s weapons in a building basement adjacent to the FBI Academy’s Jefferson Dormitory — known as the Gun Vault.

The vault is half safe and half armory, a place where the bureau stores approximately 7,000 firearms and also a place where the bureau’s 60,000 firearms — from pistols to sniper rifles — are maintained and repaired, according to Kenneth A. Fennema, the FBI’s lead gunsmith.

“These are imperfect machines though,” Fennema said. “They break.”

The armorers who work on the bureau’s weapons are a mixed bag of prior military and old hands who have been with bureau’s weapon program for decades. For Al Neff, 61, an armorer who has been with the bureau for more than 40­­ years, he sees the FBI’s change to a new but older caliber bullet and the adoption of a new service pistol as a mandatory evolution to stay up with current technology.

“We want to see what’s out there,” he said. “We want to make sure that gun goes bang every time.”