The barrel/slide length is slightly longer on the .45, no more than a quarter inch. The length of the grip is slightly longer on the .45, I think I remember no more than a quarter inch. The slide on the .45 is very slightly wider than the 9/.40. The .45 will fit some 9/.40 holsters and not some others. Some kydex holsters with the kydex moulded tightly around the end of the barrel/slide will not work due to the very slightly longer length of the .45. A closed bottom leather holster fitted closely to the end of the barrel for 9/.40's may not work either. Otherwise, as far as I can tell, the polymer parts of the frame on all these guns is very, very close to the exact same except for the length of the grip/magzine base plate and the very slightly wider width of the grip area on the .45.

For my sized hand, the .45 Shield fits me best. I shoot it better than I do my 9MM Shield. I like them both very much, but the .45 is my favorite. The .45 magazine is very slightly wider than the 9/.40 so the mag may or may not fit a closely fitted mag carrier for the smaller calibers. If it fits a leather mag pouch, it may stretch is slightly and cause the smaller caliber mags to fit a bit more loosely in those. You just have to check and see about holster and mag carrier fit. Good news is that these items are now pretty easy to find for the .45 if necessary.

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So long ... Ken