If you want to buy armor piercing ammo, it's probably too late.

The assault rifle bullet M855, known popularly as the "green tip" ammo, has flown off the shelves of gun stores in the last couple of weeks. The bullets can pierce vests and body armor worn by police officers.

"It's gone," said Jesse Bonner, owner of Jesse's Gun Shop in Corsicana, Texas, who had 2,000 rounds in stock recently. "It went pretty quick."

The Bureau of Alcohol, Tobacco, Firearms and Explosives recently announced a proposal to include M855 in its long-standing ban on armor piercing ammunition in order to better protect police officers.

The proposal has fueled a run on the bullets, which were already more expensive than other types of ammunition.

"People were definitely buying it up," said Justin Chancey, manager of Red Hills Arms in Tallahassee, Fl. "We can't get our hands on it anymore. Distributors are out of it."

Related: ATF proposed bullet ban upsets gun supporters

The green tip ammo costs about $1 a round, compared to other assault rifle ammo, which can cost anywhere from 30 cents to 80 cents a round.

The M855 is commonly used in the popular AR-15 assault rifles.

Handgun bullets made of steel have been banned from public use since 1968. But the M855 has managed to avoid the ban because it is used in assault rifles, which aren't technically handguns, and fall under the category of sporting rifles used by target shooters and hunters.

The latest proposal will take away that exemption.

Related: Cops want guns that don't kill

Any threat of a ban usually causes a boon in firearm and ammo sales. This is what happened when President Obama tried to restrict assault rifles in 2013 following the killing of elementary school children in Newtown.

"We are hearing and seeing reports that ATF's proposal has caused a run on M855," said Larry Keane, senior vice president and general counsel for the National Shooting Sports Foundation, the gun industry group. He said the ammo is clearing out nationwide.

The ATF has invited people to submit comments on the proposed ban up until the March 16 deadline.

Gun advocates argue that the ban will do little to protect cops.

The gun shop owner Bonner said that all assault rifle ammo can go through personal body armor, not just the green tips.