The Umarex Glock G17 Gen5 MOS Gas Blowback Pistol (Umarex - 2.6563) is a highly authentic, officially licensed airsoft replica with a full metal slide, realistic blowback action, and an optics-ready slide. It is designed to mirror the look, feel, and dimensions of the actual GLOCK 17 Gen 5 firearm.
 
Key Features and Specifications
Official Licensing: The pistol is fully licensed by GLOCK, featuring authentic trademarks and a true 1:1 scale design, ensuring compatibility with standard duty holsters and accessories.
Modular Optic System (MOS): The standout feature is the factory-prepared slide cutout for mounting miniature red dot sights. It is compatible with popular optics footprints like Docter, Trijicon, C-More, and Leupold (adapter plates are typically included or sold separately depending on the specific package).
Realistic Blowback: The heavy metal/aluminum slide provides a strong, realistic recoil impulse, contributing to an immersive shooting or training experience.
Gen5 Ergonomics: It incorporates modern Gen 5 features, including the removal of finger grooves on the grip for versatile handling, improved grip texture, a flared magwell, and an ambidextrous slide stop lever.
Accessory Rail: An integrated Weaver/Picatinny rail on the dust cover allows for easy mounting of accessories such as weapon lights and lasers.
Safety Features: It includes the authentic GLOCK trigger safety, which prevents accidental firing unless the trigger is fully depressed.
Power Source & Ammunition: The airsoft version typically uses green gas and fires 6mm BBs from a magazine with around 20-23 rounds capacity. Other versions firing 4.5mm steel BBs or pellets and powered by 12g CO₂ cartridges also exist under the Umarex license. 
User and Expert Opinions
Realism: Reviewers and users consistently praise the high level of realism and authentic feel, noting it's very close to the real firearm in weight and handling.
Performance: The GBB system is generally reliable and offers a satisfying, crisp blowback action.
Trigger: The trigger pull on the airsoft versions is often described as feeling more like a double-action trigger due to the internal mechanism of feeding the BBs, which is heavier and longer than the real steel single-action trigger.
Accuracy: Testers found it best suited for reactive targets (like steel plates) due to the trigger pull, but one rapid-fire group at five yards measured 1.4 inches. The pellet version with a rifled barrel offers better precision than steel BB versions.