That's not good. What is the inside diameter of the case neck, after it has been resized/expanded?
Usually a .30 cal expander ball is about 0.306" and this leaves the inside of the case mouth at about 0.304". With a little chamfer on the case mouth, this normall works OK for 0.308" bullets, even flat base (which have a slight radius at the base anyway). Boattails are hardly ever a problem.
What bullets are you trying to seat.... did you check to ensure that they are in fact .308, and not something else that got mismarked? If you are using cast bullets that are sized 0.309"-0.310" then you will probably need to open the case mouth a little more with an M die or something, per Caster's remarks.