I recently changed barrels on my P239 and at first, the slide would not work but it now seems to be doing okay so maybe it wasn't seated quite right. I did notice however that the end of the guide rod seems to be "riding up" a bit from the lug when I have taken the slide off to check the fit. I have not seen that occur with the previous .40 barrel so two questions...is this even a problem at all, and if so what's the best coarse? Maybe use a fine or ultra fine arkansas stone to the lug to make a better angle for the guide to fit into?

The flange at the rear of the recoil spring guide makes primary contact with the takedown lever when the slide is assembled on the frame. Its contact with the unlocking lug on the barrel is mostly to keep everything together when the slide is removed from the frame. I believe, therefore, that how the flange fits against the lug isn’t very important, if at all, as long as the rod doesn’t slip off during disassembly and assembly.

In my experience trying to fix something that’s not broken or doesn’t need improving is only likely to cause unintended, and often undesirable, consequences.

I certainly don't want to mess with it unless it is needed so I'll just see if anything occurs. It did occur to me to wonder if the barrel needed a small bit of fitting but i'll see how it goes before anything else. Thanks!

Their is a video from BarSto and a video from Chris Orndorff (moderator for this page) that both had good advice to use when trying to fit a barrel.

I used those two videos and fitted a BarSto 9mm barrel so it works very well in my .40 239DAK SAS.