Chronos wrote:Out of curiosity if you don't size new cases but you do trim to length
a) how do you find case length after firing? I've found even Lapua brass varies in base to shoulder length by a couple of thou, hence why I full length size before doing anything else with new brass.
and
b) how can you neck turn and trim with partial neck sizing? How is the consistence of fit for the pilot on your neck turner and case trimmer? Guess it depends in what gear you use but for my K&M neck turner I need to run a expander tool in the neck before I can turn.
No need to find case length after firing . I just trim all new cases to the shortest one in the batch . Then after a few firings , trim again at the same length setting to keep the case mouth square and neat , some trim extra some don't . A slight difference in case neck length between batch's is of no consequence to accuracy . What is more important is squareness and neat even undamaged edges / chamfers .
Neck sizing and neck expanding is part of neck turning but it is not full length sizing . Full length sizing a new case that already fits the chamber is just over working the brass. I never full length size . I do separate body and neck sizing as required. You are correct generally to get the correct fit of the turner mandrel an expander mandrel of the right size is required and sizing the neck down first may also be required to then expand up to best fit .
Once the case is fired the first time it is best to try an preserve the fire formed fit with minimal sizing techniques . A traditional full length sizing die tends to oversize and loose best fit . Fine for autos and pumps etc. but not the best for bolt actions trying for top accuracy .
Partial neck sizing is a technique used after the first firing . You size the full neck for turning a case neck but after the first shot is fired from then on you only size part of the neck length usually about 2/3 depending on chambering ease and bullet grip etc. This system is achieved by using separate body and neck sizing dies . A traditional full length die is not very good at partial sizing the neck . Some people get away with it but it lacks precision and there is no guarantee that the base of the case will be sized enough for easy fit requiring the die to be adjusted down more and more until the partial neck size changes , possibly changing tension and chamber fit . The body die never sizes any of the neck ever so it can be used to get good base to shoulder fit without ever changing anything at the neck and because no neck friction is involved like old expander ball dies have , very small sizing amounts can be achieved easily .
Partial neck sizing is a way of using skim neck turning in a factory chamber . The partial size regains the lost neck wall thickness by supporting the case neck better with the slight second shoulder that is formed after the first partial neck length size is applied. Then if you use the right dies the slight second shoulder stays there for the life of the case . BR shooters with tight neck chambers can also use short neck sizing but you can't see any second shoulder on the case neck because there is only a very small diameter difference so it's not obvious to the casual observer .
If you hold your base to main shoulder fit to a close chamber fit then the base to where the bullet ogive sits in the chamber as set by the seater die will stay consistent also . Keep the second shoulder short as it can sometimes give chambering resistance if it's too long and then bullet grip also suffers .
