MP5 Assembled - Dry Fire Troubleshooting

Utilize these steps after following all the Lower Troubleshooting steps.

• Issue: The bolt carrier group “binds” or “sticks” about ¾ of the way to the rear, often requiring the removal of the brace/stock/endcap.
  • Solution 1: The TPU buffer isn't installed when using an unrounded hammer OR the hammer is not rounded enough. You may install the round black tube over the recoil spring in your MP5 and this typically prevents the bolt carrier from traveling far enough back and getting caught behind the hammer. You can also round the hammer with the Leber V2 hammer jig available HERE and not use the buffer tube at all. You can also purchase one of our MP5 Hammers HEREwhen using a mil-spec FCG and not use the buffer tube at all on Leber V2 or ARMP5v3 lowers.
    • NOTE: Geissele triggers typically require their hammer to be slightly rounded for this same reason. We have a Geissele hammer jig HERE (essentially just rounding the top tip of the hammer to allow the bolt to travel over it smoothly, but not too much so that it doesn't reset properly). We recommend testing dry fire and live fire before rounding your hammer as it may not be needed and Geissele will not warranty or replace modified parts.
  • Solution 2: The ejector lever may be out of spec in certain ways.
    • On our ARMP5v3 Rev1 lowers (Rev1 has an exposed screw on the outside of the lower for the ejector lever), we recommend verifying the tightness of the ejector lever nut to prevent side-to-side wobble of the ejector lever which can bind up the bolt carrier. 
    • This issue may be due to the exact ejector lever used. Some aftermarket, and even some OEM, ejector levers can be slightly too tall in the rear or have sharp corners in the very rear that can benefit from being rounded (see pic 1 below for before and after rounding). You may want to lightly sand/polish the highest point of the ejector lever to allow smoother travel.
  • Solution 3: This solution should not be needed if the previous solutions are attempted, we do not recommend this solution in 99% of installations. The flat “top” of the lever is too tall and needs to be ground down some to prevent binding between the BCG and cam. If it is still necessary, use a Dremel with a drum sander (40-80 grit) to grind the top of the lever some until it functions smoothly. Note: We will not cover any failures of the lever resulting from this modification.
  • Solution 4: Some stocks/braces/endcaps may be too tight and cause binding at the rear of the firearm, preventing return of the bolt forward (or at a minimum reducing bolt velocity significantly).
  • Solution 5: If the above did not resolve this issue, please contact us.

• Issue: The trigger is “dead” after one round during dry fire or live fire. It acts like it’s in the safe position or hasn't been reset.
  • Solution 1: Verify the hammer spring is not installed backwards, this may be allowing hammer follow.
  • Solution 2: Verify your hammer is resetting properly. If you immediately remove the lower without re-charging the handle, you can check to see if the hammer has dropped or is still in the cocked position. If it's not resetting properly, this can indicate an issue with either the hammer or the disconnector used (replace one at a time).
  • Solution 3: Verify Geissele is not over-rounded. This is done the same way as solution 1. If it is over-rounded, you can still use the hammer in an AR-15 or similar style build, but you will need a new Geissele trigger kit as they do not sell separate hammers.

• Issue: The bolt won’t close fully into battery.
  • Solution 1: Simplest troubleshooting step for this issue is to not ease the charging handle forward and instead "HK slap" it or move it down to the lower track and let it go from the rearmost point.
  • Solution 2: Double check your OEM lower does not cause this same issue. This can be caused by an incorrect bolt gap or improperly installed parts (lower, slip trip, bolt carrier, spring, etc.). In rare instances this could be caused by an out-of-spec ejector lever preventing forward travel.
  • Solution 3: This can also occur when using a fully loaded 30-round magazine in certain cases. You can attempt to underload to 29 rounds and see if it still occurs. This may indicate an issue with the OEM mag catch.
  • Solution 4: Some stocks/braces/endcaps may be too tight and cause binding at the rear of the firearm, preventing return of the bolt forward (or at a minimum reducing bolt velocity significantly).

• Issue: Lower has too much "play" compared to the original housing.
  • Solution: Due to the tolerance stacking present in most firearms (especially in MP5s), our lowers are designed with some slack in the hole sizes/locations to allow fitment on a wide variety of hosts. You can typically eliminate the slack by either adding small foam pads between the upper and lower or similar ways of adding some thickness between the lower and stock/brace/endcap.