The controversial tweet Sunday came as "American Sniper," the Oscar-nominated biopic of a former Navy SEAL and sharpshooter, won the weekend box office.
"My uncle killed by sniper in WW2," Moore tweeted. "We were taught snipers were cowards. Will shoot u in the back. Snipers aren't heroes. And invaders r worse."
The backlash soon followed. Former U.S. House Speaker and CNN contributor Newt Gingrich invited Moore to spend some time with terrorist cells overseas, then weigh in on the value of snipers.
"My dad was in the First Marine Division in the South Pacific in World War II. His brother, my uncle, Lawrence Moore, was an Army paratrooper and was killed by a Japanese sniper 70 years ago next month," Moore wrote
"My dad always said, 'Snipers are cowards. They don't believe in a fair fight. Like someone coming up from behind you and coldcocking you. Just isn't right. It's cowardly to shoot a person in the back. Only a coward will shoot someone who can't shoot back.'"
The "Fahrenheit 9/11" and "Bowling for Columbine" director has long been a staunch advocate for stricter gun control laws in the United States, and his statements are known to court controversy.
But Moore stressed that his comments were not a jab at the movie "American Sniper," which earned $105 million
in weekend ticket sales and nabbed six Academy Award
nominations last week, including best picture. The box office take trumped previous records for a January opening.
"I didn't say a word about 'American Sniper' in my tweets," Moore wrote on Facebook.
"Here's what I think about American Sniper': Awesome performance from Bradley Cooper. One of the best of the year. Great editing. Costumes, hair, makeup superb!"
Actor Seth Rogen equated the war drama to a Nazi propaganda film shown in Quentin Tarantino's "Inglourious Basterds."
Rogen later clarified his statement, tweeting, "I just said something 'kinda reminded' me of something else. I actually liked American Sniper. It just reminded me of the Tarantino scene."
"American Sniper" is based on the best-selling memoir of the same name by Chris Kyle
. Kyle served four tours in Iraq with the Navy SEALs and is lauded as "America's deadliest sniper."
He was not without controversy of his own, including disputed claims in his book that he punched former Minnesota Gov. Jesse Ventura
and killed two armed carjackers.
Kyle was shot and killed by a former Marine in 2013.