Massachusetts is the only state that I'm aware of that doesn't treat the receiver as the firearm.
There are a whole host of states that don't treat the receiver as the firearm. For example in NH, the definition of a firearm is rather vague and only mentioned in a couple places.
Here is the definition of a pistol:
"Pistol or revolver, as used herein, means any firearm with barrel less than 16 inches in length."
Section 159:1 - Definition.
Definition of firearm if a person tries to steal it from a cop:
"(a) "Firearm" means any weapon, including a starter gun, which will, is designed to, or may be readily converted to expel a projectile by the action of an explosive. "
Section 642:3-a - Taking A Firearm From A Law Enforcement Officer.
Definition of a firearm for hunting purposes:
"V-a. Firearm: Any weapon, including a starter gun, which will, is designed to, or may readily be converted to expel a projectile by the action of an explosive. "
Section 207:1 - Definitions.
As you can see nowhere does NH law get into details of what a receiver is.