MP5 Super Safety/ARC-Fire Troubleshooting

This guide is oriented specifically toward MP5 builds in 9mm. If you are using a G3/91, 33/53/93, 32, or similar, this guide may not be perfectly applicable as the exact troubleshooting steps may vary. Note: All products are recommended to be installed by a qualified gunsmith.

 

MP5 Disassembled - Lower Troubleshooting

Utilize these steps after ensuring you are using mil-spec AR-15 parts in your lower AND that you are using 124gr NATO 9mm ammo (no 115, no 147, no +P, etc.). DO NOT use M16 FCG parts or “fancy” parts for initial testing (you can use upgraded hammers/springs/etc after you have it functioning, but we will not help troubleshoot those), the more generic, the better. The only exceptions to these rules are Geissele triggers provided by ASD or other SS sellers pre-cut AND/OR the ASD MP5 Hammer, we will help troubleshoot these.

• Issue: The trigger pull is very heavy in super safe/ARC.

  • Solution: Lightly sand the rounded corner of the trigger with 220-grit sandpaper. Only take off a little material, it shouldn’t take a lot to get it running smoothly. MAKE SURE you keep the round shape on the corner. After function tests pass, we suggest using 320-grit, 400-grit, and possibly one more higher grit to polish the corner without removing more than a surface amount of material.

• Issue: The trigger does not function in super safe/ARC. It either doesn’t allow you to pull it or it won’t release the hammer when the lever is fully forward.

  • Solution 1: Attempt to jiggle the super safety around the middle position slightly back and forth and see if the trigger is still prevented from being pulled. This can indicate the width of the tail of the trigger is a bit much (you can narrow this down some with a dremel/file) or that the safety detent hole is slightly out of spec (which aligns the super safety too far to one side).

  • Solution 2: Sand the rounded corner of the trigger with 220-grit sandpaper. Only take off a little material, it shouldn’t take a lot to get it running smoothly, but may take a few tries. MAKE SURE you keep the round shape on the corner. After function tests pass, we suggest using 320-grit, 400-grit, and possibly one more higher grit to polish the corner without removing more than a surface amount of material.

• Issue: The super safety cam is jumping positions during function checks or live testing without you pushing it.

  • Solution 1: Verify you are using a rounded safety detent and the round surface hasn’t flattened out too much.

  • Solution 2: Verify the safety detent spring is not crooked/bent/pinched and appropriate pressure is applied to the detent/cam (you should feel distinct tactile clicks when you move between positions). If the pressure isn’t sufficient, you can add some material under the detent or spring to compress it further. In rare cases, you may need to reduce the pressure by removing a coil or coils from the spring.

  • Solution 3: In extremely rare cases, the cam may be out of spec, please contact us.
    
    

• Issue: The trigger does not reset when you move the lever as far back as it can.

  • Solution 1: Verify the safety cam is in the middle position or the ARC-Fire selector is in the third ARC position, this is “super safe.”

  • Solution 2: Trigger may be overcut, please contact us if you purchased the trigger from us.

• Issue: The trigger and hammer pins are “walking out” and you need to push them back in sometimes.

  • Solution: We recommend always installing Anti-Walk Pins on Super Safety kits. DO NOT USE ANTI-ROTATION PINS!!! Anti-Walk Pins are available from a variety of vendors.

• Issue: The safe position is not “safe.”

  • Solution: The trigger is most likely very incorrectly cut or something is not installed correctly, please restart installation from scratch and if it continues, please contact us

 

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 HERE when 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: 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.

• 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.

 

MP5 Disassembled - Upper Troubleshooting

The majority of upper-related issues are either due to an OEM issue or the denial blocks. If it is not functioning well from the factory, we can not assist with that and it should be sent for RMA repairs.

• Issue: The slip trip doesn't slide past the rear denial block smoothly.

  • Solution: Simply Dremel/file the slip trip until it clears the denial block fully. We suggest pushing the slip trip in each direction (side to side and up and down) as you slide it past the denial block to ensure there is zero chance of it catching/rubbing against the denial block.

    • DO NOT MODIFY THE DENIAL BLOCK! EVER!

• Issue: The lower does not fully mount up on to the upper for a Century AP5-P made after early-2025.

• • Solution: Verify you are using the latest revision of the lower you purchased. All of our lowers following batch 4 were modified for this change to the denial block in the underside of the AP5-P. Some Leber V2s have been updated for this change, verify with the seller.

 

MP5 Assembled - Live Fire Troubleshooting

Utilize these steps after following all the Lower and Upper Troubleshooting steps above.

• Issue: <5 rounds are fired before a LPS (Light Primer Strike) or hammer follow occurs and the charging handle needs to be pulled again.

  • Solution 1: Test different ammo. Some ammo has very hard primers or is too weak to properly reset the hammer.

  • Solution 2: Replace the hammer spring with an "extra power" hammer spring from Wolff (30095).

  • Solution 3: If these steps do not resolve the issue, please contact us.

• Issue: Failure to extract or eject. Can result in issues such as one live round and a spent casing in front of the bolt carrier at the same time OR casings getting caught in the ejection port or in the cocking lever joints.

  • Solution 1: The most common issue here is underpowered ammo that will not adequately cycle the ejector lever with enough force to kick the spent casings out of the gun. We STRONGLY recommend 124GR NATO 9MM ammo for all initial testing/cylcing and troubleshooting (we use Remington 23718 for our in-house testing, but we see plenty of success with other brands too). DO NOT USE SYNTECH 150GR (OR OTHER UNDERPOWERED SUBSONIC ROUNDS) WHEN TROUBLESHOOTING.

  • Solution 2: Check ejector lever function and for any signs of damage (chipping or breaks). Ensure there is not too much side-to-side wobble and that it can move freely up and down.

  • Solution 3: If you are using a K model (especially when suppressed), you may need to replace your locking piece with an 80-degree or 90-degree to slow the rate of fire. If you are running underpowered ammo (especially 147gr+ subsonic ammo), you may need to go to a higher degree locking piece. All of this varies based on the exact host, bolt gap, ammo, suppressor, etc. used and may take some trial and error.

• • Solution 4: As a last resort, we would replace the following parts in order: Extractor spring, Extractor, Ejector Lever. We recommend using H&K branded parts despite the steep additional costs. After replacing these,  If it continues, test other ammo, suppressors, etc. as it may simply be too much or not enough bolt velocity.

• Issue: The lever snapped in the first 100 rounds.

  • Solution 1: Please review the Dry Fire troubleshooting steps again.

  • Solution 2: The hardening of the lever may have been incomplete, we will replace the lever, please contact us.

• Issue: The rate of fire is far too high! Full size exceeding 950RPM or K size exceeding 1200RPM.

  • Solution 1: Verify you are not using overpowered ammo (i.e. +P or high velocity). You should be utilizing 124gr NATO 9mm with no suppressor for all baseline troubleshooting. Suppressing will increase the rate of fire, so we recommend verifying good function without the suppressor first, then move to suppressed and adjust as needed.

• • Solution 2: Replace the locking piece to a lower degree version.

    • Full Size Models - Use the 100-degree locking piece in 99% of applications. Most OEMs use a 100-degree, but if you cannot verify, you may order one from RCM, PTR, etc. they are interchangeable between brands.
    • K Size Models - Use the 80-degree for fulltime suppression with high powered 147gr loads. If you shoot both suppressed and unsuppressed, you should use the 90-degree locking piece (we generally recommend this overall for K models). If you shoot solely unsuppressed, you should be fine to use the 100-degree locking piece (various OEMs use 100-120 degree locking pieces for the K models).
    • SD Models - Use 115 or 120-degree locking pieces, either should be fine.

• Issue: Failure to feed.

  • Solution 1: This can indicate an issue with your mags. Genuine HK mags are the best starting point for troubleshooting the mags.

  • Solution 2: Test a different kind of ammo. Flat nose ammo is notoriously difficult to feed in MP5s, you should utilize round nose ammo only (hollow point is hit or miss).

• • Solution 3: This can also indicate an issue with the mag catch position resulting in the mags sitting too high or too low or at an angle. Replace the mag catch with an HK brand mag catch.

• Issue: The slip trip broke within X amount of rounds (X < 5,000).

  • Solution 1: This can indicate an issue with bolt velocity being far too high. We strongly recommend reading the above troubleshooting step discussing rate of fire being too high.

  • Solution 2: This can also result with certain braces/stocks/endcaps. Look at the inside of your stock/brace/endcap and see if there are any marks/indents/damage from the slip trip slamming against it. If this is occurring, you may need to clearance that area to allow a little more travel or install a buffer tube to prevent full travel.

• • Solution 3: Contact us, we will replace the slip trip the first time it fails prematurely.

• Issue: I've followed the whole guide and it still doesn't work!

  • Solution: Buy a HK SP5. (J/K, contact us)