The basic problem you are experiencing is that this was a military action, the military had no expectations for reloading once fired ammunition, and out of all the military actions, the Lee Enfield is the most flexible of them all. Lee Enfields are not as rigid or as strong as the front locking Mauser types of actions, but, as long as they are fired using issue ammunition, they will function just fine.
The cartridge is a 40,000 psia round, so it was never high pressure. The action is rear lugged and given that steel compresses something like 0.001” per inch (don’t quote me on this!), given that you have close to three inches of lug distance, you already have a significant amount of “built in” stretch in this action.
When you fire a dry case in a dry chamber, the thinnest part of the case expands and sticks to the chamber, which for cartridges, is the case mouth. As pressures rise the thicker, back part of the case has to stretch to the bolt face, because the front of the case is stuck to the chamber, but this causes the case to neck somewhere between the shoulder and the base. In the cartridges I have shot, this "necking" is in the bottom quarter of the case, but with my 223 cases, the stretch ring is almost in the middle.
If this was a front locking rifle I would recommend lubing the cases and breaking the friction between the front of the case and chamber. This is something I do, particularly in Garands/M1a’s, and I have taken cases 22 reloads without any case head stretch ring. But, your rifle is not very structurally strong and if you were to lube some hot loads, you will severely strain the action.
R stands for times reloading in my Supermatch M1a
When I loaded cast bullets in my two groove Savage No 4, given the low pressures, I lubed my cases and fired them lubed. This worked out well as these cases had been once fired in a number of different Lee Enfield chambers, all of which had radically different shoulder to base profiles. With these lubed cases the shoulder blew out to exactly the Savage chamber shape, there was no sidewall stretch, because the front of the case could not grip the chamber, and the next time I loaded the things, I carefully adjusted the die just to neck size and not bump the shoulder back.
There are those who advocate installing an "O" ring just in front of the rim, to ensure that all the slack is taken about of the system prior to ignition, I don't do this. About the best you can do in a Lee Enfield is to fire moderate loads and neck size. Case life will never be as good as a more rigid action, such as the P14.
