Historian...i tried your suggestion and it worked perfectly!
Honestly I was pessimistic that it would work but I wanted to try it for process of elimination.
Thank you very much!

To Canadian; Perhaps youre right, anything is possible I suppose To Historian: Ill try your suggestion. Honestly, this even has the machinist baffled (no pun intended) at one of the suppliers. He was the one that suggested that its the aluminum that may be binding up but it was a shot in the dark.

Thanks for your reply and suggestion. We've already tried that but for some reason it seems to happen on just the aluminum. Same thing happened on another aluminum one that's 5/8X24, hence why Im looking for a stainless or titanium one for that. They thread about halfway down then start getting tigh...

My AR is the common 1/2X28 but aluminum end cap doesn't seem to work. Titanium went right on with no problem. Has anyone ever encountered this before?
Also, and more importantly, is there anyone making a C tube end cap out of either stainless or titanium in 5/8X24?
Thank you