I am sooooo not trying to be an ASS here Dawg, but you might try using the search & archive engine on this site...
The Cast receivers are old news.
This is one of the biggest dramas you will ever encounter... I mean the drama between "ORF" and "SW/BW/TW/TN"
You see, there is a special reason why we cannot buy bare stamped receivers anymore....Bobcat is not the only reason.
If you really want to get stuck in some deep shit then you will pursue a custom MP5 build
Believe me on this one.
If you don't have parts yet and you really want an MP5 clone... then go buy a Bobcat and be done with it.
You will be taking a pretty big nose dive if you think this custom clone thing is going to be easy.
If you want a true custom gun without the worry of the parts count, receiver BS, and parts kit availability... then go to Top-Notch and buy a complete parts kit that includes the STAMPED receiver (1100.00), send it to your smith and there you have it.
Last thing.... the cast receivers have been used for many a good build, they are perfectly fine... their only drawbacks are that they don't finish out as well and that they are slightly heavier than stamped. Other than that.. and their inherent "out of spec. nature" they are perfectly fine for a clone build. With their weight comes strength too... they are a bit tougher than stamped. But that is the nature of all gun receivers that are cast/milled compared to stamped ones
Good luck.
-bones