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The .45 GAP was designed to deliver .45 ACP bullets/ballistics from a shorter (but thicker) case which could be used in the standard Glock 17 frame. Some folks really want to carry/shoot the large .45 bullet but cannot get a proper grip on the larger/thicker/deeper frame required to host the .45 ACP (Glock 21) due to having smaller hands. Its only major problems are cost/availability of ammunition.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)