Hi Colin,
First, let me say that it is great to see a young guy getting involved in the sport. You have a lovely collection of old school wood and blued gun metal guns there!
I am not a wildcat loader by any means - the other guys who have responded are old pros. I grew up with my Dad handloading for several cartridges, and picked up things from him, but have not done any lately as work seems to get in the way of fully enjoying my shooting hobby. I do however have a large library and would like to share a bit of the info I have looked up on this wildcat cartridge.
The manual on cartridge conversions lists both a .25/303 British and .25/303 Epps. The major difference is that the Epps is longer overall and has a sharper shoulder angle. The instructions for forming the cases for both are much the same: start with 303 British brass and neck size in a 7mm/08 full length size die making certain not to set the shoulder back. Trim to length, chamfer and full length size in appropriate die. So, you will need a set of loading dies for the .25/303 Brit. I have never seen such dies. I suggest you not undertake this unless you can find a seasoned wildcat loader. Wish my old gunsmith friend was still alive as I am certain he would know about this cartridge and be keen to assist you.
Ackley lists some info and loading data in his old handbook on the .25/303 Brit. Unfortunately Ackley has been dead for many decades now and his book dates to 1962, so many of the powders he lists are older. Doing some cross referencing to data I have on powder burn rates, Ackley was using faster burning powder than your loads above. The powders used for your loads are some of the slowest burn rate powders. BUT I still think they are on the hot side and likely maximum loads. I would be cautious about these until you get some more confirmed loading info. The barrel heating up on a high speed rifle cartridge like this is not unusual. The thing to look for are signs of high pressure: do the cases extract from the chamber normally, set back of the primer, bright mark left on case head from the extractor, more than one thousands of stretch in overall case length from a single shooting, bright mark around case just ahead of rim from stretching. I would suggest you closely examine those fired cases with this in mind. (You can thank my elderly father whose brain I picked for some of this info.) The other thing Ackley mentions is that his loads are for the P14 and Mauser 98 actions but for the SMLE action he says to reduce his loads by 3 grains.
Just be careful with this lovely gun until you can get some more detailed info form someone more in the know than myself.
If you want copies of the info I have, drop me a pm with your email and I can photograph the pages and send to you.
Best Regards,
CD
