In 1914 the chambers of the Enfield rifles were reamed larger in diameter and longer to the case shoulder in the chamber. This was done to make room for the mud of Flanders fields and the terrible muddy conditions of trench warfare.
Then we Americans in 1926 decided to set firearms standards and started the SAAMI and created a problem. Commercial .303 British cartridge cases are not made to British military standards. The cases are smaller in diameter and have thinner rims, when these type cases are chambered the case lays in the bottom of the chamber and when fired it warps (its called a banana shaped case)
Once the base of the case warps and is no longer 90 degrees to the axis of the bore the case is worthless and your group size will take on shotgun patterns. As stated above if a small thin rubber o-ring is slipped around the rim it will hold the case against the bolt face and prevent case stretching. The rubber o-ring also when compressed by the bolt flattens and when compressed centers the case in the rear of the chamber and prevents the case from warping.
In another forum the "brilliant" moderator had the same case warping problem and stated the Enfield chambers were drilled off center. This same problem occurred in the M14 rifle with its larger diameter chamber and commercial cartridges and the American Rifleman wrote an article on the subject. I also had a 1943 03-A3 with a large diameter chamber and my Remington cases warped when fired also.
You have two choices, find military grade cases that are made heavy duty or fireform the lighter and thinner commercial cases and not load them hot.
Don't tell anyone but Prvi Partizan .303 cases are made to military standards,
The Prvi case on the left has a larger base diameter, has over .010 thicker case walls in the base web area and thicker rims. And the Prvi cases don't warp when fired unless your the moderator at surplusrifle.com and think the British couldn't bore their chambers straight.
Below is a photo from surplusrifle.com and the moderator Rapidrob, he tells everyone the rifle has excess headspace. What you can actually see in the photo is the case has a small base diameter and the "hourglass shape" is caused by the case expanding outward to meet the chamber walls.
And now my favorite photo of Rapidrob's, lets blame the British for not being able to drill chambers straight with the axis of the bore. Rapidrob had to blame the British because he doesn't know the difference between heavy duty military milspec cases and lighter and thinner commercial cases. I also don't think Rapidrob ever forgave the British for burning Washington in 1814.
(or else he was using the Mayor of Toronto's crack pipe)
Now if any of you Canadians "get bent" over this problem just blame the British if you don't understand skinny cases and fat chambers.
