My experience with the .45 GAP
I've carried the .45 GAP on duty when I worked for a Central East Florida Agency. It was a Gen 3 GLOCK 37 and at first when I started with that agency I was a GLOCK 22 .40 S&W fan.
9mm/.40S&W/.357 SIG Frame
Smooth and soft shooting; little recoil
Accurate as sin
The slide is a .45 ACP size slide so it will not fit in any kydex holsters meant for a GLOCK 17 etc.... My old Safariland Raptor Series holster for my GLOCK 22 will not fit a GLOCK 37
Capacity - 10rds versus 15rds in .40 S&W or 13rds in .45 ACP
Ammunition while not impossible to find, it's not easy. Yes, ordering online is a great thing but still....
I like the cartridge and I believe that if it was released way back when the .40 S&W came out then it would have hit main stream. The .45 GAP is a good cartridge and has a good service history. Both the Florida Highway Patrol and the Georgia State Police carry it and have had good reports along with the New York State Police and a couple of other state agencies. The main issue with the .45 GAP is that no one else chambers a pistol in it other then GLOCK. Yes, both Springfield Armory and Para chambered a pistol in it but both of those were dropped from their production. The XD and 1911 are only on the used market now.
I no longer work for that agency and went back to carrying a GLOCK 22 in .40 S&W with my current agency of employment. I still use .45 GAP ammo though.... in my S&W 625. I can find the good stuff cheap when my local shops clear out their .45 GAP stock and load it on the moon clips. Having a 180gr Speed Gold Dot JHP .45 GAP load on moon clips for a S&W 625 is no laughing matter for the bad guy or hog on the business end.
It's a cartridge that can be good but not in the US Market. GLOCK's main issue with the Model 21 was that it was built off the Model 20 which is a 10mm Pistol.... Hence the large size. The magazines for the .45 ACP are the same size as the 10mm mags.... hence the large size. It's easier from a production point to make on size instead of two for a large frame pistol. yes, the SF Frame is smaller but if GLOCK built a dedicated .45 ACP frame it would be smaller then the current Model 21 SF. But that is a marketing issue and they're doing the right thing. .45 ACP pistols aren't the number one selling pistols.... in fact their closer to fifth or sixth place in the market. .40 S&W is the number one selling pistol for LEO contracts and 9mm follows second place due to civilian sales. Followed by .380 Auto, .38 Special, and .22LR.
The GLOCK 38 in .45 GAP is a great compact platform in a great caliber. Except the whole 8rd thing bugs me... for that I carry my Ruger P345 (that I fixed and corrected many of the common issues with it) except that it's even slimmer then the GLOCK 38 grip, carries the same capacity and is chambered in .45 ACP. Oh, it's also the same size as the GLOCK 38 in barrel length and general size.
It's a great idea but on the wrong market and released in the wrong time period. If it came out back in the 9mm vs .45 ACP time of the late 1980s - early 1990s it would have killed. It is a good viable caliber for foreign markets that restrict the ownership of "military" calibers like Mexico and Italy.
In the end.... I might buy a GLOCK 37, GLOCK 38, and GLOCK 39 simply to round out the collection but they have to be DIRT CHEAP. If you want to carry it don't be worried of it under performing just simply understand that it's much like the 5.7x28mm. It has many positives and some negatives. But it will serve you right.
In your case though.... pass on it. GLOCK 38s are a dime a dozen and usually cheap.
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