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..

Re install cone, see if it goes through both holes and centered.
Check that threading matches and you should be good.

Not an expert, but I did see a red roof in once

Paladin Machine opened up a KX3 and KX5 for use on a couple of .458 SOCOMs.  No problems at all so if you did it right the 9mm won’t be an issue.

You are kinda overthinking it. If the whole cartridge drops through it should be fine.