Polish (sand with high grit) the barrel extension until it fits. This is what Dlask will tell you to do and it is very common.
ive had this happen 3 times.
1st time i broke the barrel - dont ask!
2nd time i lightly touched up the barrel and heat/cold until fit
3rd time i just sent it to Dlask and had him do it for me
with how much barrels cost im always reluctant to sand them, if i had this happen again id probably just send it to Dlask again, was much easier.... trying to think if he even charged me to do it?
How much was it for them to install it??