I have a 941 for my Beretta Mod 92FS, it really wasn't big enough for my MP magazine, although I haven't measured them I would take a wild guess that the frame is the same between the MP9 and MP40, if the frame is the same the external dimensions of the magazine should be the same. The MP magazine can be forced into the 941 loader, but it doesn't work well at all.
I had been looking for a magazine loader for my MP40 since the 941 I already had didn't work well, so I went to a dealer and asked them, we tried the HKS GL-940, it fits the MP40 pistols, it is intended for Glock pistols.
The HKS 943 is for single stack magazines, no way it would fit an MP model.
I used my 941 till I found the HKS GL-940, to me the GL-940 fits the magazine better, you have to force the magazine into the 941, it doesn't drop into place like the Beretta magazines that it's designed for.
The GL-940 isn't perfect, but IMHO it's a much better fit than the 941.
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