FAB Defense FX4210B 4-Round Magazine Extension for Glock 42, .380 ACP, Black Polymer

The FAB Defense FX4210B is a magazine extension for Glock 42 .380 ACP pistols that transforms your single-stack carry gun into a higher-capacity option. If you shoot a 42 and want more rounds on tap without switching platforms, this floor-plate replacement adds four rounds to your mag—taking you from a six-round standard capacity to a full ten rounds. It's a straightforward capacity boost that doesn't require a different magazine; the extension works with your original springs and internals.

The real win here is ergonomics. A Glock 42 is a compact gun built for deep carry, which means the grip is snug. When you pin a magazine extension to a small pistol, you risk losing purchase on the frame. FAB Defense addressed this with a contoured design that gives your pinky and ring finger a proper ledge to wrap around. That matters at the range, in dry fire, and especially under pressure. The finger indents aren't just texture—they're engineered to keep your grip secure during magazine manipulations and reloads.

Construction is fiberglass-reinforced polymer, so it's durable without adding much weight. At 37 grams, you're barely noticing it on the gun. Black polymer finish is straightforward and blends with the Glock aesthetic. This is purpose-built for the 42; it won't fit other Glock models, so make sure you're running the right platform before ordering.

Key Features

• Increases magazine capacity from 6 to 10 rounds for Glock 42 .380 ACP
• Ergonomic contoured design with finger indents for secure grip and magazine handling
• Works with original magazine springs—no need to swap components
• Fiberglass-reinforced polymer construction for durability and lightweight performance
• Minimal weight addition (37 grams)

Specifications

Frequently Asked Questions

Will this extension work on my Glock 43 or other single-stack Glocks?

No. The FX4210B is engineered specifically for the Glock 42 .380 ACP. It will not fit other Glock models, including the 43, 43X, or larger frames.

Do I need to buy new magazines or replace any springs?

No. The extension uses your existing magazine and its original spring. It installs as a floor-plate replacement on magazines you already own.

What makes this extension's grip design different from a standard capacity extension?

The FX4210B features contoured finger indents molded into the polymer. These indents give your pinky and ring finger a secure purchase point during magazine handling and reloads, which is especially useful on a small-framed pistol where grip real estate is limited.

How much does the extension add to the gun's overall weight?

The extension weighs 37 grams, so it's minimal. You're adding capacity without a noticeable burden to your carry gun.