I originally purchased the Glock 21SF as an alternative .45 carry pistol.

The Model 21 has been around for what seems like ages, and it has become an icon of sorts among special ops personnel. It has been labeled rugged and dependable, and is acknowledged to be quite accurate in practiced hands.

The only difference between a regular Model 21 and the 21SF  lies in the frame. The SF monicker stands for ‘Short Frame’. This means that the distance between the trigger and the backstrap of the gun is shorter than it is on a regular-size Model 21.

You can think of the 21SF as a concession to shooters who have smaller hands. The shortened frame makes it easier for people with small hands to wrap a finger  around the trigger.

Glock 21 Versus the 1911

It’s not quite as large as the 1911, (7.6 inches overall against the 1911’s 8.5); still, the Glock 21 is not a small pistol, and requires a bit of savvy to carry concealed.

Its major attraction is its caliber, of course, plus the fact that each of its magazines holds 13 rounds of .45 ACP ammunition. Add 1 round in the chamber, and you have a total capacity of 14 cartridges, almost double that of the venerable 1911.

At 23.6 ounces empty, it also trumps the 1911 in terms of weight. (An empty Colt Model 1911 Government weighs 37.5 ounces.) What this means is that the Glock 21 causes less of a lopsided feeling when you carry it on your strong side. When you add a couple of magazines on the opposite side (in a pocket or on your belt) your gait returns to normal; you’re almost perfectly balanced.

Like all Glocks, the 21SF has no external manual safeties to fumble with. It actually operates single-action, as far as my definition of a single-action system goes. It is also striker-fired, meaning that there is no external hammer to snag during a quick draw from concealment. Aside from the trigger, the only other external controls on the gun are the very discreet slide release, the magazine release button, and a well-recessed takedown lever.

Generous Tolerances

Like a true combat side arm, the tolerances between my 21SF’s moving parts appear to be generous. This ensures that nominal amounts of dirt and grime getting into the pistol are less than likely to cause mechanical stoppages.

Personally, I find this comforting. It means that if I ever have to wade through waist-deep mud, my 21SF will still stand a decent chance of functioning. (I paid careful attention to the online “torture tests” performed on the Glock 21, and was duly impressed.)

Quite frankly, though, I don’t shoot as well with this gun as with my 1911’s at 25 meters. My Glock 21SF always seems to produce larger groups than either my full-size high-capacity 1911 or the Commander-size version that I have.

On the other hand, I am inclined to think that groups of 4 inches at 25 meters should suffice for purposes of personal protection. And hopefully, practice and more range time with the gun will help me shrink those groups in due time.

Souping Up the Glock 21

So what made me decide to turn my Glock 21SF into a semi-automatic submachine gun?

I have always been curious about how accurate the Glock 21SF could really be, so I decided to find out. Believing that the best way to do it was to add a shoulder stock and forehand grip to the pistol, I duly acquired a FAB Defense accessory sleeve that fits over the Glock 21SF.

The FAB Defense accessory sleeve is a clever device. The front of the pistol is held in place using the gun’s picatinny rail, and the rear is secured using the gun’s beavertail.

The pistol’s slide is completely enclosed by the sleeve. To cock the gun, you replace the back plate on the Glock’s slide with a plate that can be attached to a cocking handle. The cocking handle  extends outside the accessory sleeve through a slot on the left side, where it can be readily manipulated to retract the slide.

What the FAB Defense accessory sleeve gives you is a folding stock and a pistol grip up front, just a few inches behind the extended muzzle shroud.

The forward grip can be folded backwards when not in actual use. In the folded position, it actually covers the trigger and the envelopes the trigger guard.

With the Glock 21SF mounted in the FAB Defense accessory sleeve, it definitely looks like a submachine gun.

The accessory sleeve provides picatinny mounting rails on the top, on the left, and on the right sides. There is also a ring provided for connecting it to a body harness-type sling.

Accuracy Testing

To gauge the Glock 21SF’s accuracy potential, I mounted red-dot reflex optics on the top rail. The picatinny rail on the left accommodated a co-axial spotlight, and the one on the right was used to mount a green laser.

This arrangement was carefully mapped out, so that the butt stock did not touch any of the accessories when it returned to its folded position.

The red-dot reflex sight was zeroed first, and the green laser was calibrated so that its beam coincided with the red dot at 50 meters.

All that was lacking  at this point was an appropriate high-capacity magazine, to complete the Glock’s transition into a shoulder-fired weapon.

The answer was the 30-round magazine designed for the Kriss Super V submachine gun, which fit the 21SF perfectly.

Accuracy testing using this setup was conducted from the bench, using sandbags to support the assembled ‘submachine gun’.

That range session decisively proved that the 21SF was capable of producing 2-inch groups at ranges up to 40 meters. It also conclusively proved that I needed to practice more with the gun, since it was actually capable of accuracy I had never been able to exploit while it was in its handgun form.

Real World Application

Riding in the FAB accessory sleeve, the Glock 21SF becomes perfect for house clearing, just like a regular SMG. The finished product is still short enough to maneuver in tight spaces, and the mounted accessories add a new dimension to quick target acquisition.

The beam from the green laser can be seen even in broad daylight, and can be an invaluable tool when firing from awkward positions.

At night, the tactical light provides sufficient illumination for identifying targets up to 100 meters away, and can be used in tandem with either the green laser or the red-dot sight.

The use of a shoulder stock and forward grip virtually eliminate the recoil impulse from the .45 ACP cartridge. There is still a slight jump, but you never really lose sight of what you’re shooting at, the way that you do when you fire a pistol unsupported.

All told, it’s a very functional setup. The beauty of it is that when you want to carry the Glock as a pistol, you can simply remove the extended cocking handle and unfasten the gun from the accessory sleeve. This leaves all three accessories (light, laser, and scope) intact on the accessory sleeve’s picattiny rails. When you reconnect the gun to the sleeve, you simply re-check your zero, and the Glock is good to go again, in SMG form.

Right now, the Glock 21SF in this configuration is my favorite bedside gun. Set up this way, it’s not exactly a submachine gun, but it’s definitely more than a pistol.