Instructions and Troubleshooting
Instruction Documents
Troubleshooting
Failure to eject (FTE) is the most common issue that arises when using AR-15 FCG lowers. This is due to the added pressure of the AR-15 hammer and the force required to reset the super safety cam. Both of these require significant bolt carrier energy.
That said, this is an easy issue to fix as long as you know what to look for and are willing to spend a small amount of money on quality parts.
MP5 extractor spring: The MP5 extractor spring is the weakest part of the MP5 design and can be easily damaged, resulting in FTE. I recommend replacing your bolt face with an RCM MP5-E bolt face or regularly checking your extractor spring stiffness.
Locking Piece: The MP5 locking piece is the most common cause of FTEs. The lower the degree of the locking piece, the less bolt carrier energy is available, increasing the likelihood of FTEs. This issue can be resolved by using a locking piece with a higher degree (no higher than 100 degrees unless using 115-grain ammo). RCM locking pieces are recommended.
Ammo: Using the wrong locking piece in combination with certain types of ammunition can result in FTE.
Examples
115-grain 9mm with a 90-degree locking piece and no suppressor attached will likely result in FTE on every shot. (Switch to a 100-degree locking piece.)
124-grain 9mm with an 80-degree locking piece and no suppressor attached will likely result in occasional FTEs. (Switch to a 90-degree locking piece.)
AR-15 Hammer: The AR-15 hammer requires more force to reset than the factory MP5 hammer. While this is perfectly functional, if you want to improve reliability with weaker ammo types, the Geissele SSA-E FCG is compatible with the Super Safe Leber V2 and is much easier on the bolt carrier. It also requires less force on the trigger, making resetting the cam easier as well. A reduced AR-15 hammer spring might also help; however, if you encounter light primer strikes, it may be too weak.
Other Issues
Light primer strikes: Verify that your hammer spring is correctly installed and not backward. If that's correct, you can switch out your hammer spring for a new one or upgrade to the heavy hammer spring.
Bolt carrier stuck in the back of the upper: Verify that your 48mm TPU buffer is installed over the recoil spring. Also, avoid using non-standard AR15 hammers, as this will require a different buffer size or modification to the hammer itself.
Feeding Issues: If you encounter feeding issues, I recommend using high-quality magazines such as genuine HK magazines or KCI Gen 2s. If failures persist, try replacing your mag catch with a genuine HK mag catch.
Lower retainer pins are tight: This can happen when the back plate is not aligned correctly before being tightened down. The back plate is designed to be adjustable, so make sure it is lined up with your upper's holes before tightening the screw.