Here's how America's top snipers take the fight to the air, firing from helicopters with deadly accuracy

• Shooting from a moving vehicle is no easy task, and that is especially true for helicopters.
• To fire precisely from a helicopter, aerial snipers have to battle the aircraft's vibrations, determine wind speed and direction when the wind is whirling all around them, and take into consideration a dozen other things before taking the shot.
• Marine Corps Staff Sgt. Hunter Bernius, a scout sniper and instructor at an advanced sniper training course that teaches this skill, talked to Business Insider about what it takes to do this job.
• Visit INSIDER's homepage for more stories. 

It can be hard to take a precision shot on the ground. It can be even harder to do in the air. Helo-borne snipers are elite sharpshooters who have what it takes to do both.

"There are a million things that go into being a sniper, and you have to be good at all of them," veteran US Army sniper First Sgt. Kevin Sipes previously told Business Insider. When you put a sniper in a helicopter, that list can get even longer.

Read more: Shoot like a sniper - top Army marksmen reveal how they take out targets from far away

"Shooting from an aircraft, it is very difficult," US Marine Corps Staff Sgt. Hunter Bernius, a Texas-native who oversees an advanced sniper training program focused on urban warfare, told BI.

"Getting into the aircraft is a big culture shock because there are more things to consider," he added. "But, it's just one of those things, you get used to it and learn to love it."

"Eyes in the sky"

Helo-borne snipers are called on to carry out a variety of missions. They serve as aerial sentinels for convoys and raid teams and provide aerial support for interdiction missions.

"As far as taking the shot, it is not often that we do that," Bernius explained to BI. "Our primary mission is reconnaissance and surveillance, just being eyes in the sky for the battlefield commander." But every aerial sniper is prepared to take the shot if necessary.

'It can throw you off'

Helo-borne snipers typically operate at ranges within 200 meters, closer ranges than some ground-based sharpshooters, and they're not, as Bernius put it, "shooting quarters off fence posts." That doesn't make hitting a target from a helicopter any less of a challenge.

Either sitting or kneeling, aerial snipers rest their weapon, a M110 Semi-Automatic Sniper System (SASS) in the case of the Marines, on a prefabricated setup consisting of several straps the sniper can load into to reduce vibration. "We're constantly fighting vibration," Bernius said.

Like resting your gun on the hood of a big diesel truck while it's running, the helicopter vibrates quite a bit, Bernius explained. "If you're talking about a precision rifle, it's substantial when you are looking through a small scope at a hundred meters. It can throw you off a few inches or even more."

The vibration of the aircraft isn't the only concern. Aerial snipers also have to take into consideration rotor wash (the downward pressure from the rotating blades impacting the bullet as it leaves the barrel), wind direction and speed, altitude, and distance to target, among other things.

Communication with the pilots, who often act as spotters for these elite troops, is critical. "Going in without communicating is almost like going in blind," Bernius explained.

Before a sniper takes his shot, he loads into the rig to take any remaining slack out of the straps and dials in the shot, adjusting the scope for elevation and wind. Breathing out, he fires during a brief respiratory pause. If the sniper misses, he quickly follows with another round, which is one reason why the semi-automatic rifle is preferred to slower bolt-action rifles.

Helo-borne snipers can put precision fire down range regardless of whether or not the helicopter is in a stationary hover or moving. In cases where the aircraft is moving, the aerial snipers will sometimes use a lagging lead, counterintuitively placing the reticle behind the target, to get an accurate shot.

'Very familiar with being uncomfortable'

The urban sniper training that Bernius oversees is an advanced course for school-trained snipers, Marine Corps sharpshooters who have gone through the preliminary basic sniper training at Camp Pendleton in California, Camp Geiger in North Carolina, or Quantico in Virginia.

In the advanced sniper program, Marine Corps snipers go through four weeks of ground-based sniper training before transitioning to the air. "It's primarily 600-meters-in combat-style shooting from tripods, barricades, and improvised positions," Bernius told BI.

"The first three days is laying down in the prone, and then after that, they will never shoot from the prone again," he explained. "These guys get pretty good at putting themselves in awkward situations. They get very familiar with being uncomfortable," which is something that helps when the sniper moves into a cramped helicopter.

Nonetheless, moving from the ground to a helicopter is tough, and a lot of snipers get humbled, Bernius said. Fighting the vibrations inside the helicopter is difficult. "Some guys can really fight through it and make it happen, and some guys really struggle and they just can't get over it and can't make accurate shots," he explained.

In many cases, Bernius told BI, aerial snipers have to rely more heavily on instinct than the guys on the ground. That takes repetition. That takes practice.

But once a sniper has mastered these skills, they can use them not only in the air, which is the most challenging, but also in any other vehicle. The skills are transferable.

'I'm doing this for the love of my country'

Not everyone can be a Marine Corps sniper, and each person has their own motivations for serving. "I grew up in a small town in East Texas hunting, playing in the dirt, hiding in the woods. It was a lot of fun. I could do that all day, day in and day out," Bernius explained to INSIDER.

That's not why he joined up, though.

Bernius had the opportunity to play baseball in college, but in the wake of the 9/11 terrorist attacks, he decided to join the Marines instead. "I don't regret it one bit."

"I'm very patriotic," he said. "I'm doing this for the love of my country. I've been in 13 years. There's been a lot of ups and a hell of a lot of downs. But, I would say love of the country is what's keeping me around."