Radial expansion at the base of the cartridge is a normal function of cartridge brass. Exacerbating any NORMAL expansion here are the larger chambers preferred by the British, differences between SAAMI specs and MilSpec specs for the cartridge and the differences in manufacturing tolerances between manufacturers.
That actually is quite a low-pressure load you are using, friend: very close to my own pet load, which also is very low-pressure.
Primers are coming back because the case ISN'T coming back; it is adhering to the walls of the chamber, balanced on the tip of the firing-pin, until pressures have built. But the rear end of the cartridge comes back at peak pressure anyway, pushing the primer ONTO the firing-pin and deepening the original dimple from when it fired. As it does this, the case STRETCHES, right in the Web (the junction between the Head and the Tube of the casing). This is where it separates.
Actually, I rather think that your rifle doesn't have much of a headspace problem at all, if any. Under these conditions, I have seen brass fail on the original firing, given that the rifle has enough headspace.... and the brass is hard enough.
Brass further lengthens when it is reloaded. The sizing die squishes it back to something close to original size, pushing any excess FORWARD. The die then brings the SHOULDER back to its original point (or reasonably close to it) and any brass from the lengthening process is pushed farther forward...... into the NECK. Further, some makes of dies can lengthen brass most alarmingly, just through expander-ball drag through the case-neck. A certain amount occurs with most dies, but, once in a while, you will come across a set with a badly-undersized neck and, with these dies, your brass lengthens very quickly and actually lengthens MORE in the sizing process than it does in the firing. I have a set of dies such as this, and my solution to the problem is to lube the inside of the case-neck on each reloading BEFORE sizing the cases. It is an extra step, but I am loading for accuracy. If I were loading for a Vickers or a Bren, I REALLY think I would avoid the step!
If I were you, I would solve the problem thoroughly, all at once. Do ALL of your initial firings with Ed's Famous O-Rings (or pony-tail ties) to reduce any stretch from ACTUAL headspace. Done with a NORMAL load, this will give you brass which headspaces on the SHOULDER rather than on the rim, doing away with ANY stretch from Rifle headspace. With LIGHT loads, use the O-rings 2 or 3 times with each casing to assure that it is stopping on the Shoulder rather than on the Rim. Following this, you will have to keep your brass segregated by Rifle or you could have feed/extraction problems if your brass gets mixed up. Then, when reloading, use the Lee COLLET DIE. This die does NOT size the case-body, nor does it stretch the necks. This will reduce neck stretch to an absolute minimum, this being the amount which it ACTUALLY stretches as a result of firing. Again, using the Collet Die, you MUST segregate your brass by Rifle.
Being that brass fired in a generous military chamber has a tendency to REDUCE LENGTH on first firing and not get it all back until third or fourth firing, sometimes not even then. I have a box of brass here for my HMS CANADA Ross, brass which is treated gently and fired with a load a grain UNDER yours (the 30-1/2 barrel of the Ross makes up the velocity). This brass has fifteen firings on it and still does not gauge long enough to require trimming; in fact, 19 of the 20 cases still are UNDER the 2.222" military Max length. This is 1942 Defence Industries brass, BTW.
So: lube the brass case-neck on the inside if using a regular die, a Collet Die is VASTLY preferable..... and ALWAYS use Ed's Famous O-Rings until your brass is thoroughly formed to YOUR CHAMBER and stopping on the SHOULDER rather than on the rim.
Give it a try.
One thing for sure, your wallet will notice because you will STOP buying brass!
Hope this helps.
