Gov. Charlie Baker is pushing back against Democratic Attorney General Maura Healey's crackdown on "copycat" assault weapons.
Healey said the sale violates the state's 1998 assault weapons ban.
Baker said clarifications are needed to protect the rights of law-abiding gun owners.
"I support the assault weapons ban that has been in place for nearly 20 years here in Massachusetts and support our country's Second Amendment," Baker wrote in the letter. "However, ambiguities in your notice require clarification for responsible gun owners who simply want to follow the rules and for the thousands of gun owners who were told they were following the rules for 18 years."
During his monthly "Ask the Governor" segment on WGBH-FM last Thursday, Baker initially said Healey has the "authority and the jurisdiction" to target weapons that skirt the state's assault weapons ban, but then later that day said her action "potentially leaves tens of thousands of law abiding citizens open to criminal charges."
A spokeswoman for Healey said it was surprising to receive Baker's letter given what appeared to be his initial public support.
"Many of the questions raised in the letter mirror statements by the gun lobby," Cyndi Roy Gonzalez, Healey's communication's director, said in a written statement Tuesday. "Dealers across the state seem to understand the notice perfectly well and have come into compliance."
Gonzalez also said, "In the days since announcing our enforcement notice, we have seen a dramatic decline in assault weapon sales and significant compliance by manufacturers and gun dealers."
Baker's letter accompanied a longer letter from his Secretary of Public Safety Dan Bennett.
In his letter, Bennett takes issue with Healey's description of a two-part test to determine if a weapon is essentially a copycat of an outlawed weapon.
"Depending on how your office is interpreting the two-part test you have articulated for determining whether something is a 'copy or duplicate' of a listed assault weapon, a large number of firearms, including pistols that have been sold here legally for decades, may be unintentionally affected," he wrote.
Healey said the enforcement notice sent to gun sellers and manufacturers was needed to clarify what constitutes a "copy" or "duplicate" weapon under the state's assault weapons ban, including copies of the Colt AR-15 and the Kalashnikov AK-47.
Healey said an estimated 10,000 copycat assault weapons were sold in Massachusetts last year.
Healey also has said the new rule will not be enforced against gun owners who bought or sold these weapons prior to the notice being sent out.
The letter from Baker comes after hundreds of gun rights advocates gathered on front of the State House on Saturday waiving "Don't Tread on Me" flags. One held a poster depicting Healey with a Hitler mustache and the words "Heil Healey."
Dozens of Massachusetts lawmakers have written to Healey saying she took action unilaterally and with little notice for gun dealers and owners.