In 1914 the chambers of the Enfield rifles had to be reamed larger in diameter and longer to the chamber shoulder. This was done for two reasons, the dirty and muddy conditions of trench warfare and a ammunition scandal in Britain over poorly made ammunition and who was awarded the contracts to make it. Some of this poorly made wartime .303 ammunition would not even chamber until the chambers were reamed larger.
The tilt in the case you see is caused because of the diameter of the chamber and the diameter of the case. The oversized chamber allowed the case to lay in the bottom of the chamber causing the case to be tilted when fired. These fired cases will not chamber again unless they are indexed and aligned the same way they were chambered. And this is called a warped banana shaped case.
If you wish to reload your cases then fire form the cases with a thin rubber o-ring around the base of the case. The o-ring will hold the case against the bolt face and when the bolt is closed it will compress the o-ring and center the case in the rear of the chamber.
Below the rubber o-ring holds the case against the bolt face and the case can not stretch and thin in the base area. Also compressing the o-ring centers the case in the rear of the chamber and no more "tilted cases" after firing,
After the case has been fire formed to your chamber you neck size only and let the case headspace on its shoulder and not the rim. Meaning the case shoulder will now hold the case against the bolt face and no case stretching.
When .303 British cases are not fire formed properly, the will stretch and warp when fired.
Below the "cheap bastards" way to check headspace, measure a new or full length resized case and write it down.
Next just using your fingers start a fired spent primer into the primer pocket.
Now chamber this case slowly closing the bolt and letting the bolt face seat the primer. Remove the case and measure again from the base of the primer to the case mouth and write it down. Now subtract the first case measurement from the second and this will be your head clearance or the "air space" between the rear of the case and the bolt face. Now if you add your rim thickness to your head clearance you will have your exact rifles headspace.
Minimum headspace is .064 and maximum is .074.
A second method is to fit feeler gauges between the receiver and the right locking lug while pushing forward on the bolt. What you are measuring with the feeler gauges is your head clearance or the air space between the rear of the case and the bolt face. With the average rim thickness of .058 if you can fit a feeler gauge over .017 you are over the maximum headspace of .074.
If you do not want to be a cheap bastard then just buy some Enfield headspace gauges. The .067 gauge is a maximum commercial headspace gauge and if the bolt does not close you have a Enfield with good tight headspace.
I fire form my cases with .312 pistol bullets and Trail Boss powder using the o-ring method. This also make the brass butt plate softer when fire forming large quantities of cases.