Converting a stock Colt .45 ACP M1911 or 1911A1 to 9mm requires the following:
1. Replacement slide and extractor. The breech face cutouts have different dimensions for the two cartridges. Ditto for .38 ACP Super. Firing pin diameters are different too.
2. Replacement magazine. Different cartridges, naturally.
3. Replacement ejector. The radii are different because the cartridges have different diameters.
4. Replacement barrel and new feedramp throating job.
5. Replacement of shortened recoil spring.
Years ago, I played extensively with converting M1911A1 pistols to use 9mm/.38 Super cartridges. The conclusion was that changing the pinned ejector was a PITA and that caliber conversion for any serious use was not practical. It was a fun project, but I would not consider it for any serious use.
Hay Cal: Like John, I did this a number of years ago but I did it a bit differently.
The first thing I did was to get a complete slide in 38 Super - you can just drop in a 9mm barrel later to shoot that caliber. By complete, I mean complete, with barrel, bushing, firing pin/spring, extractor, etc. Don't forget the sights - its a PITA to try and hit something without those.
As for the ejector, I did something different. I installed a 38 Super ejector and then polished it sufficiently to accept the old 45 slide. That way, all I had to do was to change slides to shoot the different calibers. It worked reliably for me. I didn't have any problems with either the 45 or the 38 Super/9mm using that technique.
You may have to work the rails on the slide a bit to get it to fit properly but that's about all there was to it. It is actually easier than most think. Your biggest task will be to get the ejector right but that isn't all that difficult either.
It is a fun project and you should have fun doing it and shooting the results. BTW, my 38 Super conversion wound up with a Bar-Sto barrel and it is incredibly accurate with any load I shoot through it. The 9mm takes a factory barrel jsut because I didn't want to spend the money for another Bar-Sto for just that caliber but even that shoots acceptably to 50 yds.
At one time Colt made a 9mm conversion kit for the 80 series 1911, if you can find one. I have never had the kit, but do have the Colt instruction manual that came with one and it is published by Colt and so marked.
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