I have the same problem with my Browning Pump Rifle in .300 Winchester Magnum.
The base expands about .0015
The factory stock rounds goes in and out with no issues, the reloads will not chamber.
I went to two gun shops and took the die's back one time and traded them for used ones - didn't know that they were used until I brought them home. Anywho - none of my reloads will chamber and I am going to have to order a second set of dies.
One other trick a old timer showed me is to put soot on the bullet and then chamber it and look to see if there is any marks on the bullet.
Check the head space by putting the bullet into the barrel with your finger and push it forwards as far as it will go and then close the bolt and take a measurement and then push the round back with a ram rod and take another measurement from the top of the barrel down to the shell. It takes a depth mic to do it and a precision rod of the correct length and size not to damage the bore.
That is how a real gunsmith checks it.