Reloading .303 British : Case stretch

You can see that the shoulder is blown forward on the fired case. As Shamu said, the throat has nothing to do with this, it is the chamber dimensions, particularly the depth to the shoulder from the back face of the barrel. These chambers are all reamed “long” to accommodate a certain amount of dirt from the battlefield. The answer is not to throw dirt in the chamber, but to neck size the brass. 

After 12 to 15 neck sizing cycles, you will begin to feel some resistance in fully locking the bolt.  At that point, the firing and reloading cycles have stretched the brass enough that its headspace to the shoulder is longer than the chamber and locking the bolt has to squeeze the case down.  The answer here is to partial length resize. Back out the die such that the shoulder is just barely bumped back, I go for a 0.001 - 0.002 inch setback. That will get you another 10 to 12 neck size reloads. 

Use a Lee Collet neck sizer if you have one. 

If the rifle headspace is on the tight side, that is just over 0.064 inches, you will get 40 plus reloads from your brass until you begin to get case web thinning and the first signs of circumferential cracks, about 1/2 inch above the case head. 

None of this really matters unless you are a reloader who shoots a lot and would otherwise quickly run out of brass.