OK need a sanity check.. 

I am going to run an extended rail on a SBR..
I need to get out a couple of inches.. 

I already own a KX5 that I don't use.. 
The inside was really eroded from being on a 7.5in 5.56 AR15
I cut the cone down a bit to open up the hole.. a full 9mm casing drops freely through it now.. so the bullet should easily pass through it. if it dealt with that insane abuse of 5.56 out of a 7.5 I see no issue with it handling 9mm..

I believe that with the cone cut open.. it will have no problem sitting on the end of a 9mm SBR.. is there something I am not thinking of?

really poor quality picture taken in the dark.. but you get the idea.. 

Personally I'd put it on and shoot it but I'm more comfortable with a higher level of risk than the majority of people here. 

Even without the cone cut open... you'd probably be ok - it would open up the cone eventually