I decided (in retrospect I'm not sure it was a great idea) to neck down some lapua .308 brass to 7-08 instead of buying Norma 7-08. I ran the lapua through a non bushing 7mm-08 full length die. I noticed that post-sizing the shoulder was not particularly sharp given that I'd used a nonbushing die.
I looked at cartridge case diagrams for the two calibres and noted the base to neck-shoulder junction dimension was slightly longer for the 7-08 (40-50 thou depending on the diagrams viewed). I already knew the necks would be a bit shorter.
Unperturbed at this stage I carried on and ran a few through an expander mandrel and proceeded to neck turn them. I was deliberately running the cutter into the shoulder slightly to try and avoid donuts. I was using a cutter with a 35 degree cutter angle. I noticed that the cut around the shoulder at the base of the neck was not concentric (uniform width) around the diameter of the case for some of the cases.
I took the turned cases and checked the necks on a sinclair concentricity gauge and while runout is not necessarily fantastic (say a max of 4 thou (1/2X total reading of 8thou), most around 2 thou), I wasn't sure that it was enough enough to make me think that bent necks were the cause especially given some of the cases with uneven cuts had no more runout than those that were even. The cases were a good fit on the turner mandrel and were expanded with a matching expander mandrel.
I haven't done a huge amount of neck turning but I don't recall seeing this issue before. Previously I used a neck turner (K&M) with a cutter that was less of a match to the angle of the neck than this one (PMA).
Is this likely to cause any problems further down the track and what is the likely cause of the issue?
I looked at cartridge case diagrams for the two calibres and noted the base to neck-shoulder junction dimension was slightly longer for the 7-08 (40-50 thou depending on the diagrams viewed). I already knew the necks would be a bit shorter.
Unperturbed at this stage I carried on and ran a few through an expander mandrel and proceeded to neck turn them. I was deliberately running the cutter into the shoulder slightly to try and avoid donuts. I was using a cutter with a 35 degree cutter angle. I noticed that the cut around the shoulder at the base of the neck was not concentric (uniform width) around the diameter of the case for some of the cases.
I took the turned cases and checked the necks on a sinclair concentricity gauge and while runout is not necessarily fantastic (say a max of 4 thou (1/2X total reading of 8thou), most around 2 thou), I wasn't sure that it was enough enough to make me think that bent necks were the cause especially given some of the cases with uneven cuts had no more runout than those that were even. The cases were a good fit on the turner mandrel and were expanded with a matching expander mandrel.
I haven't done a huge amount of neck turning but I don't recall seeing this issue before. Previously I used a neck turner (K&M) with a cutter that was less of a match to the angle of the neck than this one (PMA).
Is this likely to cause any problems further down the track and what is the likely cause of the issue?
