The GLOCK 21 Gen5 MOS: A .45 ACP Standard Bearer

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After exploding on the scene in the early 1980s with the GLOCK 17, GLOCK pistols quickly became the benchmark for personal defense, law enforcement, and military handguns; this has been sustained for the past 40 years. It is hard to fathom for those of us who witnessed the uproar caused by the then-upstart Austrian company from the beginning.  

GLOCK handguns are ultra reliable, accurate, tough, and simple to operate, all at a competitive market price. Many detractors would have you believe that GLOCK has grown complacent by not introducing any new designs that depart significantly from the template laid out by the G17 in 1982. GLOCK handguns are actually in their fifth generation of refinement, including the GLOCK 21 (G21) Gen5 MOS.

When the first G21 was introduced in 1990, it was heralded as the new age platform choice for the venerable .45 ACP. Remember, the 1986 FBI Miami Shootout had the 9mm coming under scrutiny for terminal performance. With the G21, you get 13 230-grain JHP “flying ashtrays” in your magazine — in a pistol weighing less (38 ounces) than an unloaded steel-frame 1911 (40-42 ounces).

The G21 Gen5 MOS continues to provide a viable option for today’s .45ACP fans by allowing them to mount the red dot of their choice without modifications to the gun. 

Let’s do a quick rundown of what sets each generation of GLOCK handguns apart so we can better appreciate the Gen5 pistols.

The Gen1 G17 was created to satisfy the Austrian Ministry of Defense’s request for a new service pistol to replace the aging Walther P38. That first G17’s grip was smooth with little texturing, and there certainly was no accessory rail included in 1982, because there really weren’t many practical accessories for handguns yet.  

Gen2 GLOCKs had serrations added to the front and back straps of the frame. A steel plate stamped with a serial number was embedded into Gen2 frames to satisfy new U.S. firearm regulations.

Gen3 GLOCKs were introduced in the late 1990s with several updates, including the addition of a standard under-barrel rail (Universal GLOCK rail) for attaching lights, lasers, and other accessories. Thumb rests were added on both sides of the grip and finger grooves were added to the front strap. Later Gen3 GLOCKs included an extractor that also served as a loaded chamber indicator. 

This pistol series also had an additional crosspin added in the form of a locking block pin located above the trigger pin. The locking block is also bigger on Gen3 GLOCK — both changes were made to increase durability by distributing force across a larger area.

The Gen4 Glocks were unveiled at the 2010 SHOT Show. The new pistols reflected a combination of both ergonomic and internal changes. Responding to public demand, GLOCK added interchangeable grip backstraps of different sizes to make the pistol fit more customizable. Gen4 GLOCKs also have a larger magazine release that’s reversible for left-handed shooters.  

Internal changes included the addition of a dual recoil spring, which the gunmaker says reduces felt recoil by 10% to 30% while also increasing the lifespan of the pistol.

Finally, GLOCK introduced the first Gen5 models in 2017 in the company’s continuing quest for “perfection.” The finger grooves from previous generations were deleted, and an ambidextrous slide stop lever and a flared and beveled magazine well were added. Another big addition was the new GLOCK Marksman Barrel, made with tighter chamber specs and a more aggressive polygonal rifling with a target-style crown. 

Additionally, the new Gen5 firing pin and trigger spring enhancements give these pistols the best out-of-the-box GLOCK trigger to date.

The most obvious self-defense-handgun upgrade introduced in the past 10 years has to be compact red-dot sights. When the G21 Gen5 MOS was introduced in 2023, .45 ACP shooters could finally benefit from the red-dot revolution on the GLOCK platform. 

The advantages are obvious: with a red-dot, the user doesn’t have to switch focus from their target to the sights and it eliminates the need for a front sight focus, which can be difficult to master for new shooters. They can focus on the threat without lining up their sights — they just have to find the dot.  

And because indexing a red dot is far easier than obtaining a perfect sight picture with three pieces of steel, users find they can take shots beyond what was considered feasible with iron sights, and faster.

GLOCK’s Modular Optic System (MOS) made its first appearance on select Gen4 models. The system lets users choose from interchangeable plates that attach to the slide and accommodate the various footprints of different red-dot optics. The G21 MOS ships with a #6 MOS adapter plate for the Trijicon RMR mounting pattern with other plates available from the GLOCK Store.

GLOCK’s decision to mill 0.196 inches from the slide enables the red dots to mount as low as possible, even with the interfacing MOS plate. 

GLOCK’s “secret” to success is minimal operating controls, a high ammo-capacity-to-size ratio, and relentless reliability. However, simplicity should not be confused with a lack of refinement. As my experience and knowledge have expanded, GLOCK reliability and solid performance have become more entrenched in my mind as the standard.

A manufacturer’s reputation and a gun’s price range serve as no guarantee that a handgun is ready to perform immediately after purchase. Yes, many deficiencies are quickly remedied by “breaking in” a pistol or making slight adjustments to various parts. This does not have to be accepted. I have found GLOCK to be one of the few handguns you can count on being ready to go straight out of the box. 

GLOCK’s solid performance over the years is sometimes taken for granted by the shooting public, and frankly, by gun publications as well. I believe this stems from the constant focus on the newest product introductions. Rest assured, if it is a polymer-framed semi-automatic pistol, the GLOCK influence is there.

The G21 represents the quintessential balance of size, weight, capacity, and cartridge potency.  A loaded G21 magazine with one in the chamber adds up to 14 rounds of potent .45 ACP firepower in a package only a bit larger than a classic 1911 commander style pistol. 

As mentioned earlier, the G21’s frame is a Gen5 type, meaning it is not as large as the previous Gen 3 frame that proved such a handful — no pun intended — for anyone without extra-large hands. The G21 Gen5 comes with two interchangeable backstraps that allow users to adjust the grip size to fit their hand better.

I tested this pistol with Federal, Winchester, Black Hills, and Hornady ammunition with loads ranging from 185 to 230 grains, and including JHPs and FMJs. 

Approximately 450 rounds were fired in total, and I experienced no malfunctions. After the G21 sights were verified, I proceeded to monotonously hammer steel plate racks and popper targets at 7, 15, and 25 yards. Man-size steel targets were also occasionally engaged at 50 yards. 

The premium loads tested from Federal, Winchester, Black Hills, and Hornady generated sub-2-inch groups at 15 yards. From a more practical perspective, the G21 produced a 100% rating over a 60-round law enforcement proficiency test frequently used to evaluate handguns. The test is timed fire from the holster at various ranges from 5 yards to 30 yards. 

An unscientific assessment would be that the “snap” of the slide in its rearward travel is indeed reduced with the Gen5 G21 compared to the Gen3 version, which I shot alongside it. This has to be attributed to the dual recoil spring in the Gen5. Another factor could be the smaller grip radius fitting my hand better compared to the older model. 

Proven GLOCK ergonomics combined with the G21 slide’s smooth track during recoil and manageable .45 ACP muzzle signature allow for a very stable sight picture no matter how many rounds are fired or how rapidly.

It is best to think of the GLOCK lineup of handguns as a “system” stretching from the compact G26 to the G40 with many stops in between. The GLOCK advantage of similar operating controls, triggers, and “feel” is a huge bonus for moving between firearms from full-size duty weapons to more compact firearms. 

The G21 Gen5 MOS does not require a new manual of arms compared to that of a petite G43, only different magazines. The advantage of this is really appreciated when training spans several years and thousands of rounds. Intimate familiarization achieved by numerous draw strokes and pressing out to engage the target cannot be over-emphasized. The staging and prepping of the Glock Safe Action trigger with its short reset can be honed to maximum effectiveness across the many different GLOCK models. When under stress, this is a valued trait.  

I can’t readily come up with any similar handgun models on the market offering the same advantage across a similar range of dimensions and calibers.

The G21 Gen5 MOS fits right into the GLOCK modus operandi. The pistol allows for the time-proven .45 ACP cartridge to shine in a modern firearm while benefiting from more than a century of cartridge refinement.  

The G21 Gen5 MOS represents portability, reliability, controllability, and lethality. This is a perfect combination for a personal defense handgun.

LOAD                                       VELOCITY        ACCURACY     

Federal 230 HST                  872fps               1.25                                         

SIG SAUER 230 V Crown    855fps               1                                         

Black Hills 185 JHP              964fps               1.75                                     

Hornady 230 XTP                 909fps               1.25

Bullet weight measured in grains, velocity in feet per second by chronograph, and accuracy in inches for average five-shot groups at 15 yards.

H&B's THE BOTTOM LINE is a rating system combined with useful commentary on the following aspects of the test gun. Each of the seven criteria gets a 1 to 5 score. Max score is 35. 

Ergos and Handling: &&&&&
Solid GLOCK feel with interchangeable backstraps allows individual fit. G21 Gen5 grip girth is tamed while housing a double-stack .45 ACP magazine.

Recoil: &&&&
While the dual recoil spring and polymer frame absorb recoil, making it better than expected, the G21 is still a .45 ACP.

Action and Reloading: &&&&
The flared magazine well and larger baseplates further simplify intuitive semi-auto reloads — hand to grip. The trigger is the best out of the box for any GLOCK generation to date. However, a 6-pound trigger is hard to reward a perfect “5.”

Sight Mounting Ease: &&&&&
The advantage of GLOCK is that almost every after-market accessory manufacturer caters to them. The MOS models brings red dots into that fold, making mounting a red-dot extremely easy and intuitive. 

Intended Purpose: &&&&&
The GLOCK striker-fired polymer frame weapons have set and maintained the standard that others follow.

Overall Versatility: &&&&&
The addition of MOS capability takes the GLOCK 21 Gen 5 over the top by being able to mount a red dot.

Value-Price Point: &&&&&
GLOCK not only sets the tone for performance but also pricing.

• CALIBER: .45 ACP
• ACTION: Semi-automatic GLOCK Safe Action
• BARREL: 4.61-inches
• OA LENGTH: 8.07-inches
• WEIGHT: 25.9 ounces empty
• SIGHTS: White outline rear and dot front/MOS capable of mounting red dot
• FINISH: nDLC
• CAPACITY: 3 13-round magazine included
• MSRP: $745 ($620 shelf)