Below is a photo of a
"recessed" Remington 700 bolt face, this type of recessed bolt face holds and centers the rear of the case in the chamber.
Below is a Enfield bolt head, it is not recessed, nor does it hold and center the cartridge in the rear of the chamber. When the bolt is closed the extractor moves to the right and away from the rim of the case allowing the case to lay in the bottom of the chamber. When fired the case will expand more on the thin side of the case and push downward on that side of the case. This is what causes the case to lean to one side.
Because the design of the Enfield bolt head it allows the cartridge to lay in the bottom of the chamber.
Commercial American SAAMI .303 British cases run on the small side of allowable base diameter.
Enfield chambers in 1914 were reamed larger in diameter and longer to the chambers shoulder. This was done because of a ammunition scandal in England over who was awarded contracts to produce ammunition for the military. Some of the ammunition was so poorly made it would not chamber in the standard Enfield chamber. And this the main reason the chambers were reamed larger. When you combined the poorly made ammunition and the dirty, muddy conditions of WWI trench warfare you can see why the chambers were reamed larger.
If your cases are warped and appear to be leaning then throw them in the trash, if you full length resize these type cases the base of the case will no longer be 90 degrees to the axis of the bore. (warped banana shaped case) Meaning the reloaded cases will cause the bullet to be out of alignment with the bore and your group size will increase.
This is where correctly fire forming your cases comes into play and centering the rear of the case in the chamber. When using the rubber o-ring method it important to use the correct size/diameter o-ring for the cartridge to chamber. The o-ring will hold the case against the bolt face and prevent the case from stretching. The second benefit of using a rubber o-ring is when the o-ring is compressed and becomes oval shaped it "centers" the case in the rear of the chamber. This allows the case to expand equally around the circumference of the base of the case.
When resizing your .303 British cases you should neck size only and let the case headspace on its shoulder and not the rim.
Below the o-ring holds the case against the bolt face and centers the case in the rear of the chamber.
Below once the case has been fire formed to the chamber you should only neck size, the cases shoulder will then hold the case against the bolt face.
I found the photo below in Google Images and can't remember who or what forum it was posted in. (sorry)
The case below is the worst I have ever seen and I think the case is a Prvi Partizan which is one of the best cases for reloading the .303 British.
The point being the case below was fired in a very oversized chamber and the rifle should be a wall hanger. I say this not because this rifle is dangerous to shoot, I say this because I reload and any cartridge case fired in this rifle is now junk and not worth reloading.