The three basic groups of parts in a modern firearm are: the action (which includes the trigger, firing pin, and bolt), the barrel (where the bullet is fired from), and the stock or frame (which provides structure and support for the other parts).
The receiver is the frame of a firearm- in modern firearms, it is the part that the barrel connects to. Antique firearms, such as muzzle loading rifles, did not have a receiver- only the lock, stock, and barrel. (Yes, that is where the expression came from) On a modern firearm, if it has a serial number, it is stamped on the receiver. By US law, a receiver IS a firearm- everything else is parts.
The parts that make up a functioning firearm.
It is rare to find a modern firearm (last 50 years or so) that has NO markings at all. It is possible that those markings may have been removed from a modern firearm. Without some additional information, such as the type of firearm, we do not have enough information to offer a real answer.
Stock , Barrel & Action
Brownells are a brand of firearm suppliers. They sell gum parts and firearm accessories. They have a catalogue that shows their product.
1) the stock 2)the action 3)the barrel
A gun part is not a regulated item. However, the frame of a firearm, known as the receiver is legally a firearm. A convicted felon may not possess a firearm, nor may they possess ammunition.
It's possible to make a shotgun from common steel plumbing parts. It takes four parts and can not be identified as a firearm when disassembled. So you can throw the parts to the four winds and nobody can charge you with possessing a firearm.
Rifles, shotguns and pistols are the three main categories of firearms. A modern firearm consists of three main components: Actions that load, unload, and eject ammunition.
Any firearm using modern cartridges can be. However, it is an extremely dangerous thing to do with any firearm, and this should not be attempted.
The simplest would be a single shot muzzle loading firearm. In modern cartridge firearms, a break action firearm. They are made as rifles, shotguns and pistols.