The Glock 45 Gen4 in 9mm is a full-size frame built on a compact slide -- the same combination Glock developed for law enforcement crossover duty. The MOS (Modular Optic System) cut on the slide lets you mount a red dot without a gunsmith. 
 

Key Specifications 

• Caliber: 9mm Luger (9x19 Para) 
• Barrel Length: 4.49 inches 
• Capacity: 17 rounds 
• Weight: 30.34 oz (2.5 lbs) 
• Frame Material: Polymer 
• Finish Color: Matte Black 
• Manufacturer: Glock 
• Model Number: PR45509MOS 
   

Detailed Features 

• Modular Optic System (MOS): Machined slide cuts accept a range of red dot optics for faster target acquisition. 
• Glock Marksman Barrel (GMB): The improved barrel geometry produces tighter, more consistent groups. 
• Safe Action System: Three independent safeties -- trigger, firing pin, drop -- all disengage in the normal draw and fire sequence. 
• Grip Design: Interchangeable backstraps and aggressive texture let you dial in the fit for your hand size.

The Glock 45 Gen4 MOS draws consistent praise for reliability -- owners report running it through a wide variety of ammunition without significant issues, which is in line with the platform's general reputation. The MOS feature gets specific mention from shooters who run red dots; the mounting process is straightforward and the available adapter plates cover most popular optics.

Ergonomics come up often, positively. The full-size grip on a compact slide works well for a range of hand sizes, and the matte black finish holds up well to holster wear. The 17-round capacity is considered one of the better capacity-to-size ratios in the duty-size category.

The trigger gets mixed notes. It's smooth and consistent, but some shooters who come from striker-fired pistols with lighter or shorter resets find it less refined. Cosmetics are a minor point of debate -- the utilitarian look is exactly what some buyers want and a non-starter for others who prefer more visual customization from the factory.