Wearing black helmets and police T-shirts, their handguns levelled, a drug raid team inches along the outside of a compound, throws open the door, spots a man with a gun and opens fire.
With the pop-pop-pop of a Glock 9mm an armed suspect is shot and hits the floor as seven counter-narcotics officers fan out and clear the adjoining rooms, checking for any more armed and hostile drug runners.
The training operation over, the team leader had some stern words for his men.
"We must aim better and make every shot count so no one is injured unnecessarily and we go home safe to our families," he bellowed as some of the law enforcement officers stared at their boots.
As the men listened, a drug villain casually strolled out of the roofless plywood structure, pink paint splattered on his shirt.
The cops and the guns are real, but the raid and the bullets are not.
This Hanoi paintball op is part of a US-Vietnamese training exercise in which Drug Enforcement Administration (DEA) agents pass on some of the skills of their dangerous trade to their local counterparts.
Besides the drug raid drill, the course includes handcuffing practice, arrest scenarios using an interactive gun-and-video screen system, and first aid training for bullet wounds.
More than 80 Vietnamese officers from counter narcotics units, Customs, army and police academies are joining the two-week courses with the US agents in Hanoi and Ho Chi Minh City.
A few decades ago, these men might have fired live rounds at each other, but today's training is part of a new, joint war against a common enemy -- the syndicates that traffic heroin and synthetic drugs through Vietnam.
"Vietnam is a transit country for drugs going into the international market," said Jeffrey Wanner, the DEA's Vietnam attache.
"It's based on their proximity to the Golden Triangle, to some of the major producers," he said, referring to the Thai-Lao-Myanmar border area that has long been a hotspot for opium poppy cultivation.
"Some (traffickers) use the Mekong River, some use mountain routes, some use the main highways. Then it's compiled again in a central location and brought into the local market or to the ports for international distribution."
Most heroin in Southeast Asia comes from within military-ruled Myanmar, where syndicates are also increasingly producing synthetic drugs including methamphetamines and MDMA or ecstasy, Wanner said.
"The amount of heroin they are producing is decreasing, but the amount of synthetic drugs is increasing dramatically. It's easier to do, the profits are a lot higher and there's a growing demand for that."
Vietnam's communist government has mostly eradicated large-scale opium cultivation and imposes harsh penalties for drug users and smugglers.
People caught with more than 600 grams (21 ounces) of heroin or 20 kilograms (44 pounds) of opium, its raw material, are usually sentenced to death by firing squad.
Nonetheless, Vietnam has become a major trafficking country, in part because of its porous borders with Laos and Cambodia to the west and China to the north, as well as its 3,200-kilometre (2,000-mile) coastline.
Vietnamese police Senior Captain Luu Duc Cuong, a course participant from Cao Bang province on the Chinese border, spoke of drug gangs who use sawn-off Chinese-made AK-47s assault rifles.
"It is easier to conceal and causes more severe injuries," he said.
Cuong hasn't come face-to-face with the drug gangs yet but wants to be ready when he does. His most dangerous operation so far, he told AFP, was the arrest of a heroin addict who used his blood-stained syringe as a weapon.
The threat was serious -- the UN Office for Drugs and Crime Control says heroin has been Vietnam's most popular illegal drug since the 1990s and intravenous drug use now causes two thirds of all known HIV infections.