WATERTOWN, Conn. — With a blue Glock 17 pistol in hand, Joe Arnson stepped into the living room. He checked the first floor before heading upstairs. Just beyond the landing, a blue-skinned robber was pointing a gun straight back at him. Without hesitating, Arnson pulled the trigger, blowing a pink hole in the paper robber’s face.

It was good marksmanship, instructor Clint Hyduchak told Arnson, but if the blue guy had been a real robber, he would have shot Arnson before he reached the top of the stairs. That is, if the bad guy Arnson missed hiding under the staircase didn’t get him first.

“When you enter a room … you want your weapon in front of you and that’s all you want the bad guy to see,” Hyduchak said.

Arnson was the first paying customer at the first venture of Watertown safety training company Prepare to Act. The setting is a 2,000-square-foot shoot house at 24 Depot St. set up to train gun owners in home invasion scenarios using nonlethal bullets called Simunition. The guns are real, but are modified so they can’t shoot real bullets.

Simunition, a division of General Dynamics, has been around for military and law enforcement training since the late 1980s, according to Chris Fields, owner of King 33, another shoot house, in Southington, but it was only made available to the public through commercial range programs in August 2011.

King 33, a 5,000-square-foot warehouse on six acres of land set up with rooms to resemble a house, office, classroom, and pro shop, has trained 400 people since it opened last year, Fields said.

Until Prepare to Act opened its shoot house this month, Fields said, King 33 was the only dedicated nonlethal training center in New England.

The four-story building on Depot Street in Watertown contains nine video cameras allowing instructors and classmates to observe from their smartphones, and walls that can be moved to change the layout of the furnished apartment.

On his second run through the building, this time armed with a Smith & Wesson .38 caliber revolver, Arnson was more careful to check around corners, staying hidden behind walls until he was sure a room was clear.

As he turned the corner to a back room, a bright flash and crack of gunfire rang out. Clint Hyduchak’s brother, Andrew Hyduchak, had jumped out from behind a counter. Arnson, he later admitted, hesitated a split second before firing back at the real person. He managed to hit Andrew Hyduchak in the helmet and on his bare hands.

It stung, Andrew Hyduchak said later, but not as much as a paint ball. The fake bullets hit with barely enough force to leave a small dimple and colored mark on the wall.

Clint Hyduchak said training in a real house, as opposed to a shooting range, allows students to build muscle memory, look for targets, and react to someone shooting at them.

“When I go to a shooting range, there’s a little piece of paper that doesn’t move. When you do it here, it’s a little more dynamic,” said Arnson, a self-described gun enthusiast.

Hyduchak said Prepare to Act eventually hopes to set up scenarios for law enforcement officials to practice hostage and barricade situations, and for families to make plans in case they’re already in the home when someone breaks in.

At King 33, courses include home defense, anti-carjack, urban defense and rural defense for the public, private security companies and government agencies.

Fields said he doesn’t just train students to shoot targets. In fact, his first course is about situational awareness and doesn’t include any guns.

He trains them how to fight through an attack with only the necessary means so that they don’t violate the law in the heat of the moment.

“You don’t have to use (a gun) to defuse the situation, but it’s there if you need it,” Fields said.

He said self-defense might mean jumping out of a window, hitting someone with a pot, getting people to a safe room, or fighting out of a situation to leave the building.

“There’s a lot of variables and considerations in combat that civilians enter into in an armed combat that there previously was no training for,” Fields said.