Looks like to save material KART decided to make the inner barrel smaller. Since making it a tightbore would be more expensive they reduced the wall thickness. I am trying to install a tightbore and nothing I have will fit down the outer barrel. I took my caliper and found the outer diameter of several barrels.
JBU - 8.54mm
Prometheus - 8.64mm
KART EBR stock barrel - 8.21mm
The stock barrel fits really snug and since I can't drill out the outer barrel I now need to buy an aftermarket outer barrel just to install a tightbore.
Just stay away from KART/ECHO1, they are shit.