SIGSHR said:
My understanding of the M1911 is that it is the position of the ejector-which on mine are machined into the frame-which determines what rounds you can fire and only rounds that have the same case head size-i.e. 9MM and 38 Super-can be interchanged.
Your sentence is in conflict with itself. First you say it's the ejector that controls, but then you mention different case head size. Both statements are partially correct ... sort of.
Slides: In general, 1911s use three breech face cuts, one for .45 ACP, one for .40 S&W and 10mm, and one for 9mm and .38 Super. BUT ... I have it on good authority that Para-Ordnance used the same breech face cut for 9mm, .38 Super, .40 S&W, and 10mm. That was tested for me by a friend who owns a Para P18.9 -- he was able to fit a .40 S&W case into his slide. End of discussion. Nonetheless, it's better to stick with the three groupings I initially spelled out.
Ejectors: The position of the ejector is always the same, meaning that the positions of the two holes in the frame are always drilled in the exact same locations. However, 9mm and .38 Super ejectors are thicker than .45 ACP ejectors, and the extra material is added on the side toward the centerline of the pistol. These ejectors have a visible step in the surface facing the centerline. The offset brings the nose of the ejector closer to the centerline, to allow for the smaller case head. I don't know which type is "right" for .40 S&W and 10mm. It may well be the both types will work. IIRC, all Para-Ordnance ejectors were the offset style, even on their .45 ACP pistols.
I have never heard of a 1911 with the ejector machined as an integral part of the frame.
Feed ramp: You didn't mention the feed ramp. The feed ramp on 9mm 1911s is cut to a different angle than on .45s, and it's set slightly farther to the rear of the pistol. It is possible to convert from .45 to 9mm, but it's sometimes tricky, and not always successful.