The boy lost control of the weapon while firing it Sunday at the Machine Gun Shoot and Firearms Expo at the Westfield Sportsman's Club, Police Lt. Lawrence Valliere said.
Police identified the child Monday as Christopher Bizilj of Ashford, Conn.
Police said Christopher was with a certified instructor, and they called the incident a "self-inflicted accidental shooting." The boy's father and older brother were also there at the time, a gun club member and school official said.
"The weapon was loaded and ready to fire," police Lt. Hipolito Nunez said. "The 8-year-old victim had the Uzi and as he was firing the weapon, the front end of the weapon went up with the backfire and he ended up receiving a round in his head."
The boy was taken to Baystate Medical Center where he died.
Francis Mitchell, a longtime member and trustee of the club, said he saw the boy's father supporting his son from behind when the accident happened.
"My reaction is shock," said Mitchell, who lives down the street from the club. "In the last five years, there has never been a problem or a bad accident. I've been sick all night."
Although the death appears to be an accident, police and the Hampden district attorney's office were investigating, officials said.
"We are going to review all the circumstance regarding what happened, who was involved, what authorities they may or may not have had, who was supervising," District Attorney William Bennett said.
Christopher was a well-liked third-grader at the Ashford School and played soccer in town, said Leonard Lanza, interim superintendent of Ashford public schools.
Christopher's brother, Colin, a sixth-grader at the same school, was with him during the accident, and crisis counselors spoke with students in both boys' classes, Lanza said.
"It's been a very nasty day for the whole community," Lanza said. "One does what one can."
It is legal in Massachusetts for children to fire a weapon if they have permission from a parent or legal guardian and are supervised by a properly certified and licensed instructor, Nunez said. The name of the instructor who was with the boy at the time was not released.
Authorities were investigating whether everyone connected with the incident had the proper weapons permits. Massachusetts requires licenses to possess firearms, and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives issues licenses to possess machine guns.
"We do not know at this time the full facts of this incident," Nunez said Monday afternoon.
"It's all legal & fun - No permits or licenses required!!!!" reads the ad, posted on the club's Web site.
"You will be accompanied to the firing line with a Certified Instructor to guide you. But You Are In Control - "FULL AUTO ROCK & ROLL," the ad said.
The ad also said children under 16 would be admitted free, and both adults and children were offered free .22-caliber pistol and rifle shooting.
Peter Robbins, the former director of the Barnstable Police Academy and a former firearms instructor, called the boy's death a tragedy, but said gun clubs try to teach children about gun safety.
"It's a wonderful experience to expose younger people to firearms safety through these clubs. That's what these clubs are about," Robbins said.
The sportsman's club was founded in 1949 and describes itself on its Web site as an organization that promotes "the interest of legal sport with rod, gun, and bow and arrow, both directly and through training."
It has eight firing ranges as well as archery and fishing facilities located on 375 acres in Westfield, about 100 miles west of Boston.
The boy's father, Charles Bizilj, is director of emergency medicine at Johnson Memorial Hospital, in Stafford Springs, Conn. The hospital issued a statement of condolence but declined further comment.
Connecticut state Rep. Bryan Hurlbert, whose district includes Ashford, said the Bizilj family is well known and highly regarded.
"This is a huge tragedy and the family is very involved locally," he said.