Wire EDM's typically use water to flush out the cut during machining, at least ours does. It can leave behind a "scale" type finish that looks like the finish above. Sinker/spark EDM's use a oil that submerges the part.

Im of the school of thought that when dealing with wall thickness and monoblocks, your structural support/ pressure holding is all done with the wall thickness of the sleeved portion of the tubes, not the sleeved portion of the tubes and the thickness of the monoblock added together. My reasoning for this is because of the joint at the front of the monoblock and the tubes. The monoblock thickness cant add any strength at this joint, the sleeved portion of the tubes is what contains pressure at this point. Another issue with this joint is it is in a area where pressure is highest, and it also has a 90 degree angle. As im sure you are aware cracks are prone to form at 90 degree joints.

Do I think your gun will fail at normal pressure...Not really. But as a manufacturer I have to add additional safeguards in place to prevent from rare circumstances causing failures. As an individual doing work on personal guns, you cant hold yourself liable if failure occures...not without a good lawyer anyways.

None-the-less, keep up the work and tutorials.