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Fix My Glock

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My Glock doesn't Glock! Help!

Glock clones are enticing builds for beginners. There are lots of reasons but some of the top are:

  • the Glock itself is one of the most widely known handgun designs in the world
  • the design uses relatively few parts
  • the non-printed parts are widely available in the US
  • the frames are relatively easy to print

Despite the brilliant simplicity of the design, it's still a precision machine that relies on well made parts, correct assembly, and maintenance of precise tolerances. Beginners expecting a "cookbook" build often find themselves with a malfunctioning gun and no idea how to fix it. While this guide can help you find problems in a completed build, it's main focus is helping you not encounter those problems in the first place.

This guide is not meant to provide "quick fix" answers. If you have the attention span of a coked-out squirrel, consider purchasing a mass-produced Glock. Law Enforcement "trade-in" Glocks are widely available at prices significantly cheaper than buying all the parts for a build. However, if you're willing to take a methodical and detail-oriented approach, read on ...

Troubleshooting Your Print

Many print-related problems can be traced back to one of two things: Lack of experience with slicer settings, and not following the print instructions in the README file included with the frame distribution.

A Glock frame (or any firearm) should not be your first 3D print. Use commonly available test objects to hone your 3D printing skill. You don't need to learn every setting in the slicer, but you do need to understand the basics -- particularly with respect to strength and supports.

This section provides some general guidelines but developer's instructions should take precedence. If you downloaded a frame that doesn't include any documentation, find the frame design it was based on and follow that design's README instructions. If you cannot determine the original source of a design, you may wish to reconsider using it.

Enhanced PLA (PLA Pro or PLA+) is the least expensive and easiest to print filament that is known to be reliable in Glock frames. It is recommended that you DO NOT use any of the following: PETG, PETG-CF, PLA-CF, Plain PLA, "Artistic" PLA (silk, matte, wood, etc.), PC, TPU. Other common filaments that are known to work but require highly-tuned settings to get reliable prints include: PCTG, ABS (no CF/GF), and ASA (no CF/GF). Most engineering filaments will work if you have well-tuned settings and a printer that can handle them. These include CF/GF reinforced versions of: PA6, PA12, PA612, PPA, and PPS. PET-CF/GF is controversial. It can produce reliable frames with the right settings but it's lack of impact strength makes it prone to sudden failure. Do not use engineering filaments unless you have verified your settings by producing (and breaking) test parts to verify strength of the resulting prints.

General Print Settings

  • Print temperature and cooling: Use the maximum temperature recommended by the filament manufacturer and the lowest part cooling fan speed that still results in acceptable print quality. Use a test print such as a Benchy to tune temperature and cooling settings.
  • Strength: 4 walls, 100% rectilinear infill. More walls is acceptable, but testing has shown that with 100% infill there is no strength benefit to using a large number of walls. Historical guidance was to use infill percentages in the high 90s. This was due to some slicers not correctly handling 100%. These issues are long since resolved and current versions of Prusa, Bambu, Orca, and Cura slicers handle 100% infill just fine.
  • Supports: Manual tree supports is the most popular approach. Automatic tree supports will work, but will generate more supports that really necessary. In all cases, having well-tuned support interface settings will have a major impact on the quality of your print. Slicer "defaults" are rarely optimal, and no one can just give you the optimal settings for your printer/filament combo. There are many support tuning videos available on YouTube (e.g. https://youtu.be/1BXPPyk-CgI?si=FyPgpuvAUO6SUp_E). If you have never tuned your support settings, time spend watching some videos and doing support tuning prints is well worth it.
  • Orientation: Rails-up vs. rails-down (or grip-up vs. grip-down if you prefer) is the subject of much debate. In terms of strength it makes no difference. Rails-up is the traditional recommendation. Pros/cons are:
    • Pro: No supports in most of the areas where parts install (rail pockets, locking block pocket, etc.). Not a big advantage with tuned support interfaces.
    • Pro: Less chance of warping.
    • Con: More support material, longer print time.
    • Con: Support scars are on "visible" areas of the completed frame. Not a big disadvantage with tuned support interfaces.
  • Angle: Horizontal (rails parallel to build plate). This is the correct orientation for both rails-up and rails-down prints. Do NOT angle the frame up or down. Layer adhesion strength is directly proportional to layer surface area. Angled printing reduces layer surface area in critical places like the front rail and locking block pin holes. "Intuitive" arguments in favor of angled layers opposing the direction of frame stress are provably incorrect.

Calibration

You're going to be installing commercially manufactured parts in your frame so it's critical that your frame dimensions be correct. Many filaments have some amount of shrinkage as they cool. If you do not account for that, you may have difficulty fitting parts and/or getting the build to function. The solution is using a calibration print to measure dimensional changes and adjust scaling factors in the slicer. There are many tools available -- from simple calibration cubes to complex designs like Califlower and Calilantern. The infamous 3D Benchy is actually a calibration tool with detailed measuring instructions on the website.

In all cases, you need to print the calibration tool with the same settings you plan to use for your Glock frame. Changing things like infill or number of walls to speed up the print may give you incorrect results. Take measurements on your calibration print using a set of calipers that are accurate to at least 0.1mm. Inexpensive Chinese digital calipers like you can find at Harbor Freight are usually good enough.

If you have a Prusa-derived slicer (Prusa, Bambu, Orca, etc.) you will compute ((actual / expected) x 100) and use that for "Shrinkage" in the filament settings in your slicer. Some slicers have separate Shrinkage settings for XY and Z dimensions, so use appropriate measurements for each.

It is also recommended that you tune flow ratio since that can have a significant effect on strength. Ellis' Print Tuning Guide has a section on "Extrusion Multiplier" tuning that covers this. Ideally, you should run all the calibration tests in Ellis' guide, but shrinkage and flow ratio are usually enough to get a usable frame print.

Pin Holes

The four pin holes (front rail, trigger, locking block, rear rail/trigger housing) give many people grief. Even with good printer calibration, these holes will often print just slightly undersize. Designs keep the tolerances on these tight because you can always make the holes larger, but it's pretty hard to make them smaller. Always test fit pins before assembly. The pins should fit snug, but not tight. Installing the pins with any significant amount of force will lead to frame failure. If it doesn't immediately crack the frame, it will create stress that significantly weakens the frame around the pin holes.

If a light tap is not enough to get a pin through the hole, the hole must be enlarged. There are two recommended ways to do this:

  1. Use high-quality metric (3mm and 4mm) drill bits to cut the holes to the correct size. Do NOT use fractional inch sizes that are "approximately" the same. Do NOT use a power drill for this. Use a hand chuck or simply clamp the bit with Vise-Grip pliers so you can turn it by hand. Don't force it -- let the bit do the work. Work slowly and carefully to keep the hole straight and remove as little material as possible.
  2. Chuck the actual pin in a variable speed power drill. Align the pin with the hole in the frame and run the drill at low speed to "spin" the pin into the hole as though you were driving a screw. Friction will heat the polymer in the frame allowing it to deform to the exact size of the pin. Keep in mind that you do not want to actually liquefy the polymer, just get it hot enough to deform around the pin.

The second technique will produce the strongest holes, but takes a little more practice to get right.

Troubleshooting Your Build

Functional Testing

WARNING: DO NOT FUNCTIONAL TEST WITH LIVE AMMO. FUNCTIONAL TEST EMPTY OR WITH DUMMY ROUNDS ONLY!!!

If you do not have dummy rounds, you can print some: https://odysee.com/@spaceboundtactical:9/SnapCaps-Dummy-Rounds:9 . These won't function quite as well as commercial dummy rounds (a.k.a. snap caps) but they're good enough to get through most functional tests.

You're going to start with your frame and slide completely disassembled, so it's best to go through these tests as you do your initial assembly. If your gun is already assembled, take it apart so you're starting from scratch. Complete disassembly is easy and good practice.

  1. Insert the front rail (not mounted in the frame) into the slide and check that it moves freely all the way to the front. Remove it from the slide.
  2. Insert the rear rail (not mounted in the frame) into the slide and check that it moves freely up to the ejection port. Remove it from the slide.
  3. Install (only) front and rear rails and locking block into the frame. Mount the slide to the rails and verify that you can move it all the way to the rear and back to the front with almost no resistance. Remove the slide.
  4. Install the slide lock (bar in front of the locking block) with its spring. Mount the slide to the rails and verify that it still moves without resistance. Remove the slide.
  5. Install the barrel and recoil spring assembly (recoil spring and guide rod) in the slide. Mount the slide to the rails and verify that it "catches" on the slide lock bar and will not move forward beyond that point.
  6. Pull the slide to the rear and verify the only resistance is the force of the recoil spring. Remove the slide.
  7. Remove the rear rails and the two locking block pins. Install the complete trigger assembly with rear rail, slide release, and locking block pins.
  8. Mount the slide to the rails and verify that it moves as it did in the previous check. Make sure it does not hit the ejector or trigger block. Remove the slide.
  9. Install the firing pin assembly (firing pin, firing pin spring, spring cups, spacer) in the slide and install the backplate to hold it in place. Do not install the firing pin safety, extractor, or extractor spring assembly yet.
  10. Mount the slide to the rails. Pull the slide all the way to the rear, then ease it forward into battery. Verify that the trigger "resets" and moves all the way forward. Partly retract the slide and verify the chamber is empty (no dummy round and definitely no live round). Ease the slide back into battery.
  11. Pressing only on the edge of the trigger (to avoid depressing the trigger safety lever) gently depress the trigger. You should feel and see the trigger safety lever catch on the frame and prevent any further rearward trigger movement. Repeat this test pressing from different angles and with different amounts of force until you are confident that the trigger will not move fully rearward unless the trigger safety lever is also depressed.
  12. Pressing on the trigger such that the trigger safety lever is depressed, gently depress the trigger until you feel it meet resistance. Release the trigger and verify it moves freely back to the forward position.
  13. Pressing on the trigger such that the trigger safety lever is depressed, gently depress the trigger until you feel it meet resistance. Gradually apply additional force until the trigger "breaks" and releases the firing pin. You will hear and feel the firing pin release. Release the trigger and verify it remains in the rearward position.
  14. Pressing on the trigger such that the trigger safety lever is depressed, hold the trigger in the rearward position while you pull the slide all the way to the rear. Ease the slide all the way forward while still holding the trigger to the rear. Gradually ease the trigger forward. Part way through it's travel you should feel (and hear) a "click" as the trigger resets.
  15. Pull the trigger until the firing pin releases. Release the trigger and cycle the slide. Verify that the trigger resets and moves all the way forward. Pull the trigger again and verify the firing pin releases. Repeat this test 5 - 10 times.
  16. Remove the slide. Remove the slide backplate. Install the firing pin safety plunger and spring, ejector, and ejector spring assembly. Reinstall the slide backplate.
  17. Mount the slide and repeat the trigger functional tests. Remove the slide.
  18. Install the magazine release and magazine release spring in the frame. Without mounting the slide, insert an empty magazine into the magwell. Verify the magazine locks in place. Pull firmly on the magazine and verify that the magazine release holds it firmly in place. If you can pull out the magazine without depressing the magazine release, it will likely release when firing the gun and you'll experience miss-feeds or a complete drop.
  19. Press the magazine release and verify that the magazine can be easily removed. It need not drop free (especially empty), but it should not take much force to remove it.
  20. Mount the slide and repeat the magazine retention and release tests.
  21. Pull the slide all the way to the rear and verify that the empty magazine causes the slide release lever to engage and lock the slide back.
  22. Press the slide release lever and verify that the slide snaps forward quickly and firmly under the force of the recoil spring.
  23. Pull the slide all the way to the rear so it locks back on the empty magazine. Press the magazine release and remove the magazine. Verify the slide remains locked back.
  24. Load the magazine with two or more dummy rounds. Insert the loaded (dummy rounds only) magazine and verify that it locks in place and the slide does not release.
  25. Release the slide by pulling back until the slide release disengages and then let go. Verify the slide fully returns to battery. Verify the position of the extractor indicates a dummy round in the chamber. Pull the slide rearward 1/8 inch (3 mm) and visually verify a dummy round is in the chamber.
  26. Remove the magazine. Forcefully pull the slide all the way to the rear and verify the dummy round is extracted and ejected.
  27. Insert the magazine (with at least two remaining dummy rounds). Pull the slide all the way to the rear and release. Verify the slide fully returns to battery and that a dummy round is chambered.
  28. Forcefully pull the slide all the way to the rear and verify the dummy round is extracted and ejected. Release the slide and verify the slide fully returns to batter and that another dummy round is chambered. Repeat until all dummy rounds in the magazine have been cycled and the slide locks back on the empty magazine.

Troubleshooting Failures

  • Watch this video to better understand how the parts in the gun are supposed to function: https://youtu.be/y2fMRqFEEZI
  • Print an Armorer's Backplate (https://www.thingiverse.com/thing:3591065) so you can actually see what's going on with the trigger mechanism. This is particularly useful for troubleshooting firing pin engagement and trigger reset issues. WARNING: NEVER FIRE LIVE AMMUNITION WITH AN ARMORERS BACKPLATE INSTALLED!

Live-Fire Testing

WARNING: DO NOT LIVE-FIRE A GUN THAT FAILS ONE OR MORE FUNCTIONAL TESTS! Live ammo will not fix functional problems and shooting live ammo in a gun with known issues can put yourself and others at risk of serious injury or death.

WARNING: WEAR EYE PROTECTION AND EAR PROTECTION AND KEEP THE GUN POINTED IN A SAFE DIRECTION AT ALL TIMES! Accidental discharge is always possible with an unproven build. Additional Personal Protective Equipment (PPE) such as protective gloves, heavy long-sleeve shirts/jackets, etc. is always recommended. Use of a mounting system that allows the shooter to fire the gun without any physical contact is ideal, however untested make-shift mounts may actually be more dangerous than firing by hand.

  1. Manually lock the slide in the rearward position using the slide release. Remove the magazine (if inserted).
  2. Load one round only into the magazine. Insert the magazine and verify it locks in place. Release the slide and verify the slide fully returns to battery and that the live round is chambered.
  3. If not using a mount, hold the gun firmly in your dominant hand, finger off the trigger, waist high and slightly canted so no body part is directly behind the slide.
  4. Verify that the general direction of aim will not overshoot your backstop, nor impact the ground close to you. You do not need to try to aim with the sights to hit a specific target, just verify a safe trajectory.
  5. Squeeze the trigger until the firing pin releases and fires the round. Verify the slide locks back on the empty magazine.
  6. Remove the magazine. Release and remove the slide. Inspect the frame carefully (magnification is recommended) and verify there are no cracks or other damage. Pay particular attention to the areas around the locking block and the magazine release.
  7. Repeat the single round test 10 - 20 times, inspecting after each shot. If any frame damage is observed, stop immediately. If there are any failures (failure to fire, failure to feed, failure to eject, etc.) stop immediately and troubleshoot using dummy rounds. If testing is successful to this point, it should be safe to fire aimed shots from a normal stance in subsequent tests.

Common Problems

Light Primer Strikes

Symptom: Firing pin releases when trigger is pulled but round does not fire. Inspecting the round shows no mark or a shallow dent in the primer.

Common Cause: Firing pin is hitting the firing pin safety plunger. The plunger should be depressed by the "shark fin" extension on the trigger bar. Poor slide fit can cause this. If the slide is sitting too high due to rails being too high in the frame or rails being too loose in the slide or frame, the plunger may not be depressed far enough to clear the firing pin. Out-of-spec non-OEM parts found in cheap parts kits can also contribute to this problem.

Failure to Return to Battery

Symptom: After firing a round, the slide stops short of the full forward position. Trigger is reset, but pulling it either doesn't release the firing pin, or firing pin is released but round does not fire. Inspection of the round may show an off-center light primer strike mark.

Common Cause: Too much resistance in the slide movement. Recoil spring does not have enough energy to overcome the resistance and complete the last part of travel to lock the barrel in place. Perform the steps in the Functional Testing section to determine and correct the source of the resistance.

Trigger Won't Release

Symptom: Pulling trigger all the way to the rear doesn't release the firing pin.

Common Cause: Trigger housing is loose in the frame. If the trigger housing can rock forward and back, it can prevent the cruciform from moving down far enough to release the firing pin. It's often possible to use a thin piece of plastic as a shim between the front of the trigger housing block and the printed frame. Create and print a shim in your slicer or simply cut a small strip from an old credit card, plastic blister packaging, etc. You can dry-fire test with the shim just slid in place to verify it corrects the problem. If it does, use some "super glue" to permanently fix the shim to the frame.

Possible Cause: Cheap Lower Parts Kits (LPKs) may have poorly made and/or out-of-spec parts. This is particularly common with trigger parts since most parts are stamped instead of machined. Use an Armorer's Backplate to observe the cruciform engagement of the firing pin. If there is too much engagement, it may be possible to correct by bending the cruciform. Do this only as a last resort when all other probable causes have been ruled out. If you do this, you must use an Armorer's Backplate to verify that you still have sufficient firing pin engagement. Insufficient cruciform to firing pin engagement is unsafe and can lead to uncommanded discharges when the gun is dropped or even roughly handled.

Trigger Won't Reset

Symptom: Dead trigger (trigger remains in the rearward position) after firing a round.

Common Cause: When the slide returns forward it pushes the trigger connector to the left allowing the trigger bar to move upward under spring tension. Interference between the trigger bar and the frame can prevent this upward movement (which will keep the the firing pin from catching on the cruiciform). Verify the trigger bar can move freely without interference from the frame.

Possible Cause: Cheap LPKs may have poorly made and/or out-of-spec parts. This is particularly common with trigger parts since most parts are stamped instead of machined. Use an Armorer's Backplate to observe the trigger reset in dry-fire. If connector does not move far enough to release the trigger bar, consider replacing the trigger parts with genuine Glock parts.

Frame Cracked or Broke

Symptom: Cracked or broken frame.

Common Cause: Incorrect print settings and/or material. See general printing guidelines in the Troubleshooting Your Print section.

Asking For Help

The amount of help you can expect depends on how much information you provide. If you're going to ask for help, provide as much of the following info as you can:

Build Info

  • Type of frame you printed (i.e. which rails does it use: FMDA, PY2A, P80, Dagger, etc.)
  • Specific design you printed (e.g. Chairman Won, Unseen Killer, Broken Bullets, etc.)
  • Type of Glock slide in use (17L, 17, 19, 26, 43/48, etc.)
  • Brand/model of slide in use
  • Rail supplier
  • OEM (Glock) or aftermarket Lower Parts Kit (LPK)
  • OEM (Glock) or aftermarket Upper Parts Kit (UPK) if you used a bare slide
  • Filament used (type and brand)
  • Nozzle temperature
  • Number of walls
  • Type and percentage infill
  • Chamber? (none, open, closed)
  • Filament drying before print
  • Any post-processing (annealing, moisture-conditioning, etc.)
  • Print orientation (rails-up or rails-down)
  • Print angle (relative to horizontal)

Function Info

  • Which tests in the Function (dummy round) Test section passed?
  • Which tests in the Live-Fire Test section passed?
  • What brand and bullet weight ammo did you live-fire test with?
  • What behavior do you see on the failing test(s)?

Photos/Video

  • Detailed photos showing the isolated problem area
  • Photos of the inside of the frame with the slide removed
  • Video of the failure if it shows something happening
  • Not usually useful: photos of the fully assembled gun, photos without explanation/context, video of you racking the slide and/or pulling the trigger a dozen times in rapid succession (save the rapid hand movements for other things)
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