I have assembled several BM59's on these receivers and always have the customer sign a release form when doing so as I have no idea of the quality in manufacturing these receivers. I do have some observations but take these with a grain of salt as I do BM59 builds so, full disclosure, there is a conflict of interest in giving my OPINION. I would also like to say there is no such thing as "perfect". You can look at one of my receivers, at least I can, and find things that I would have liked to do better.
Here's the areas I find that need improvement on these receivers:
1. The bolt stop window is cut out incorrectly on most. This wouldn't be a problem if it didn't cause malfunctions. In many cases you cannot get a magazine in the things because the bolt stop will interfere with the mag as the window is located wrong. The manufacturer has a "fix" for this but I'm in disagreement with that fix and have my own way of addressing this but it costs money to do it, further machining.
2. Key ways in the follower pin area for the front mag catch not deep enough. This is a hit and miss issue and is addressed most easily by filing down the keys on the front mag catch.
3. In many cases the inside of the receiver is just not milled wide enough and you can hardly get a mag in it. I would also note that the milling does not go far enough forward. Any thoughts of reasonably quick mag changes are gone when this exists.
4. The front right mag lip, on the bolt stop side, is milled to deeply allowing for a mag that can rock up to 1/4" backwards and forwards in the mag well. This has been the case in every receiver of theirs I've seen thus far. You can have chamber strikes at 11 or 12 O'clock when canting the mag up and forward on these. It's tough to have happen though.
5. Aesthetically the angles on the front legs do not go upwards on the inside/rearward portion far enough to give it the real look.
6. Aesthetically they use the Garand follower pin instead of the 8mm roll pin that the real 59 uses and, on several receivers, they did not send the spacer pin to fill the gap for the missing 8mm pin. FYI, using the 8mm roll pin does NOT make this machine gun ready. There are MANY more machining procedures needed to make this receiver full auto ready. The real reason that they aren't using the 8mm roll pin as the original escapes me.
7. The biggest issue I have seen? On the Austrian casts receivers, that are clearly marked on the underside, they have the same issues as any of the other CAI receivers, the oprod track SUCKS! The track is too narrow at the front, not allowing for full forward motion of the oprod. Then, at the rear of the track, it is too wide allowing for easy oprod dismount.
If you wanted all these things addressed, I would charge $150 alone just to address the oprod track. To fix all the other issues, if they were there, another $275. To fix item #4, you'd have to TIG weld the receiver to build up the cut out and then mill it in. I have no idea what the charge would be to fix item #4.
Not all of these items rear their heads on every receiver but I have seen receivers where every one of these issues existed. It is not a very pleasant phone call to make to the customer and every one of them has chosen not to mill out the inside for fitment, not to repair the too deep mag rest, and not to fix the oprod track. I feel for these folks and really think the receiver manufacturer should take the receivers back, upon request, and fix some of these issues.
These are my opinions from someone who is in the business to make a profit and am in direct competition with said manufacturer. Take all my words with a grain of salt. I'm not saying anything that others won't say during the course of this thread