The Type 1 AK-47 was stamped. I've heard two different reasons why they switched to milled, either of one makes sense. First, there was limited number of stamping machines available and they needed them for more vital industries. Switching to milled also provided employment for skilled machinists. The second stated reason was that the stamped design had flaws. On the one hand, when they went back to stamped with the introduction of the AKM in 1959, the design was different from the Type 1. On the other, Type 1's have been found in working condition, still being used, in Iraq and Syria. What is undisputed however is that the stamped receiver rifles are much more economical to manufacture, use much less metal, and are substantially lighter.
Some may remember a posting in several forums by Doc Cheney regarding Battlefield Vegas's experience with high round counts on their rental machineguns. They keep meticulous records on rounds fired, parts breakages and when they can no longer put a gun back into use. The stamped guns would last more than 100,000 rds, but the milled guns were well in excess of that amount. The milled rifle fanboys were ecstatic, but the reality is a civilian shooter is unlikely to have the money or time to shoot a stamped gun to destruction (in reasonable use). In military use, the rifle is more likely to be a battlefield casualty before it wears out.