Magazine will not seat
Warning: ensure the firearm is unloaded before performing any steps in this guide.
1: Ensure a magazine release is installed properly.
1A // Inspect the magazine release to make sure that it is a) present, and b) installed properly and functioning.
2: Inspect the FCU to make sure the trigger bar is not blocking the action.
2A // The images below show the trigger bar in incorrect & proper alignments respectively: