Just checking since the M79 is less familiar to me than the M203.

My understanding is that the receiver is the NFA item like a M203 and you can purchase a barrel assembly over the counter like any other barrel?

Strictly asking about the legal ownership of the barrel alone. Not the receiver or if having a receiver and barrel is a gray area  or anything else.

Barrel is not a restricted part

The 40mm barrels are not a restricted item. I will say that in regards to receivers... it depends if the receiver (m79/m203, whatever etc) is coming to you as a registered item or not. I purchased my M79 few years ago as a receiver first, came to me as a title 1 firearm. The 40mm barrel assembly I kept at a local dealer to not have the two items together so I don't have "constructive intent" until my form 1 cleared.

As mentioned, neither the barrel nor receiver is NFA alone. The receiver is a title 1 firearm, which (legally) can be purchased through a standard FFL, form 1'ed, then barrel added.