I couldn't get my .38 super mags to feed 9mm. Constant nose-diving.
I think shimming the mags internally will still leave the release point too far back for reliable presentation of the round to the feed ramp. Best idea I've heard so far is a shim fitted into the the back of the mag well itself that permits the use of a small frame magazine in the large frame opening. This shim would rest against the inside surface of the back strap, and would push the mag forward enough so that it will engage the mag release and put the feed lips release point close enough to the ramp that the round can feed in and roll up under the extractor correctly. Alternatively, a shim fitted EXTERNALLY on the magazine should do the same thing......?
I haven't had a chance to try this mind you, but it does sound like a workable solution.
I have been shooting a Match Elite, Gold Custom & Gold Custom Frame with 9mm top end all year. The other 2 guns are also 9mm. I had the same problem. Thought about adding the shim in mag well also. I found a way to tune the mags so they work. I prefer the 38 super mags as they do not have that front spacer welded in them. If you would like to send me 1 mag I will tune it for you. If it works for you I will tune all of them for you.