Conversion to the Epps Improved 303 will do absolutely nothing for case life in a Lee Enfield. I don't base this on exhaustive reading of theories put forth by proponents of the concept but on actual testing I have done.
The straighter walled case should, in theory, increase the grip of the case on the walls of the chamber and thus, reduce bolt thrust. The truth is, the standard case already grips the chamber walls with sufficient force to positively hold the forward portion of the case static under full pressure. This is why the case heads separate; the front portion of the case sticks to the chamber wall while the case head is able to stretch rearward as the bolt is deflected by the thrust of the cartridge upon it. The separation occurs just behind that point at which the brass is not adhering to the chamber wall. In a #4 Lee Enfield with zero head clearance on an unfired cartridge, this separation will occur after about five loadings of the case if heavy loads are used. For the sake of this discussion, heavy loads may be considered to be loads which will push 180 grains bullets to about 2500 fps. So, it is plain that the standard case sticks to the chamber wall with sufficient grip to exceed the strength of the brass. If the case is changed to the Epps form, it may indeed grip the chamber wall more effectively but, even if it does, it is of no consequence since the grip was already sufficient to eliminate any rearward movement of the forward portion of the case.
Interestingly enough, in one rifle I re-chambered (a #1 Mklll), the case actually appeared to grip the chamber wall less effectively so that the shoulder was pushed forward with each firing. I attributed this to a smoother finish on the chamber walls than in the original chamber.
During the tests, I first chronographed factory loads to establish a baseline pressure level. If the rifle produced 2400 fps with factory loads, I developed a load which matched that level for subsequent loadings. I used 4064 powder.
In a rifle which produced 2400 fps with the factory load, about 75 fps was lost (firing the same load) when the rifle was rechambered. It was, therefor, necessary to increase the powder charge slightly to recover the pressure which was lost due to the larger chamber.
In a rifle with a more solid lockup, such as a P14 or a Ruger Number One, adheesion of the case to the chamber wall becomes a moot point since the bolt or breech block is not deflected significantly; even at much higher-than-factory pressures.
Ultimately, the Epps offers a means of increasing performance of a 303 with a strong action. It does so by producing a noticable increase in case capacity. If you have a P14 or a Ross and want to increase velocities, re-chambering to the Epps will allow you to do so (as will simply loading to higher pressures than a Lee Enfield will handle). If you want a rifle which is a bit different, the Epps will certainly fulfill that requirement.
A caveat; feeding issues are not infrequent in P14's or Ross rifles which are rechambered to the Epps.
As with all "Improved" or wildcat cartridges, the Epps 303 is capable of performing at a level which one might expect for a cartridge of it's size, loaded to appropriate pressure levels. It also illustrates that there is no magic in the shape of a case although, I must say, the Epps cartridges does have a pleasing profile! Leeper.
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