That's OK. If it bothers you you can have friend with a lathe make you a slightly larger expander plug. It is far better to have it a bit tight than a bit loose! ~Muir
PS: For what it's worth, I often use a Lee Universal Case Expander die to put a slight bell in my case mouths. It aids in bullet seating accuracy and you can get away with not even removing the "bell" if you are careful about how much of a flare you put in the case mouth to begin with. Just a thought...
the small amount of deformation to the head makes very little difference. it is a far worse problem if the base is deformed.
I always chamfer the inside of the neck quite well with my ace rcbs chamfer tool. this does help. The neck tension does need to be tight to hold the bullet in when the round is in the magazine and another is fired. if the bigger chamfer doesn't help maybe a boat tail bullet would be worth a try.
after you have fired the round you should be able to push a bullet into the neck of the fired round manually. if not then the neck might want turning or reaming. this is more of a problem in down sized cases (.308 down to 7mm08 eg).
It's like the expander on a pistol die set, except that it produces a 'step' which is slightly wider than the bullet diameter. Push it in a bit deeper and you get get a chamfer, suitable for cast bullets (so that you don't shave lead). The step gives you a ledge for starting bullets straighter.