A milled receiver is a receiver that is cut out of one solid block of steel and milled out to get the shape and design of the receiver.
A stamped receiver is one that is made from a sheet of steel thats pressed in a machine to the shape of the receiver.
Now most AK’s are stamped, the reason is that its quicker and easier to produce, but keep in mind that this is also the way the Russians make their AK rifles because thats how it was designed.
The milled are more expensive and some people will swear by milled receivers because they percieve them as being better for their rigidity.
But keep something in mind ……. watch a slow-mo vid of an AK firing sometime, what you’ll see is the barrel and receiver “Flexing” everytime it shoots. This flexing is a side effect of how the AK is designed.
Because of this flex a stamped receiver can have a longer lifespan as far as round count than a milled receiver especially for an active shooter, because under the stress of firing a milled receiver it is more likely to crack then a stamped receiver, whereas a stamped receiver will not because it flexes with the gun.
Some people will still try to say that a milled receiver is better but remember, the only reason there are any AK’s with milled receivers is because in the 40’s when the AK was designed (to have a stamped receiver) the technology of welding the bolt rails to the stamped receiver had not been perfected yet and they had failures.
So they moved to a milled receiver to where the bolt rails are just cut into the receiver which solved this problem. But as soon as the technology exsisted that they could reliably weld the rails, they moved back to the stamped receiver in 1959 and it was called the AKM (which is what most of the rifles on the market today are).
So to sum up, a stamped receiver is the way to go, it’s what the AK was designed to use.