I've replaced the recoil spring and added a stronger magazine tube to help with feeding/ jam issues.
One other problem is the magazine cap retaining shoulder pin comes out every time I clean/ disassemble the gun. Real M4's have a shoulder pin inserted from behind with a true shoulder to keep it in.
When I took apart the pistons, the O-rings were disintegrated.... not even 100 rounds. I bought replacements online.
The gun just seems cheap. It functions, but may jam every 10-15 rounds, then I'll need to clear it... not good for a self defense shotgun... I need more reliability.