In my last post I looked at the idea of using 54R brass as an alternative to 8x58RD so lets have a closer look at the two cases and consider if this really is a viable option. Below are the drawings for the two cases with the 7,62x54R at the top and the 8x58RD at the bottom, I have red ringed the key dimensions.
Starting from the head and moving towards the neck lets compare the two.
|Head to shoulder||39.76||40.90||1.14|
|Neck outer diameter||8.53||8.98||0.45|
|Case base diameter||12.37||12.75||0.38|
|Case shoulder diameter||11.61||11.95||0.34|
A closer look at the table shows the rim diameter is smaller on the 54R however not enough to be significant. The head to shoulder is shorter on the 54R so it will fit however the Alpha shoulder angle is very different however again this should not cause issues as only the front of the shoulder will be set back and then only marginally. Luckily the case is rimmed so no worries about building over length to hold the case in the chamber for first firing.
The 54R case finished length is 4,03mm shorter which is a bit of a problem and the cartridge overall length of the 58RD is 4,77mm longer than the 54R – this is an important detail and does mean we will be jumping the bullet quite a bit regardless of if it is cast or jacketed.
What I cannot predict although it can be calculated is the developed length of case after the first firing, it is at times like this it is worth considering fire forming without a bullet, to do this you use a low charge of faster powder, add a filler and then plug the end of the case with something – soap works well for this and it means the associated pressures with firing are drastically reduced, wear your safety/shooting glasses regardless of predicted chamber pressures.
So that is my musings on the use of 7,62x54R brass in the 8x58RD. Looking at the dimensions the case is going to fit quite nicely and should blow out to conform to the chamber without issues. I would suggest using a reasonable make of brass, Lapua springs to mind. Having written this I am tempted to open the 54R brass out to 8,0mm with a mandrel stage followed by passing the case through a full length sizing die and then fireform with a plugged case.
Pictures to follow..