I may have figured it out. First I took my 308 modified case and measured the inside, .309" Next I took a resized/formed case and measured it, .306".
With nothing too lose but a case, I dremeled the inside case neck with a stone to about .308, then rolled up a piece of emery cloth and spun the case around it for a fine finish.
I ended up with .310", which a bullet slides nicely in. Now I just need to repeat the process on a modified case.

Thanks for the ideas guys.