Let me ask you a question. Actually 2 questions.
The upper receiver you are using, in reference to the ejection port, is the ejection port the same length and placement front to back as a standard upper?
Is the 9mm buffer you're going to use longer than a standard carbine buffer? I know it should be heavier, but I'm asking about the length from front to back.
When you get it together, pull the charging handle all the way back and see if the bolt goes well past the back of the ejection port into the upper receiver and get back to me.
The upper receiver you are using, in reference to the ejection port, is the ejection port the same length and placement front to back as a standard upper?
Is the 9mm buffer you're going to use longer than a standard carbine buffer? I know it should be heavier, but I'm asking about the length from front to back.
When you get it together, pull the charging handle all the way back and see if the bolt goes well past the back of the ejection port into the upper receiver and get back to me.