Hi guys, I've been reloading for several years now and have used standard RCBS sizing and seating dies. I have always just set them up (adjusted the body jam nut) according to the instructions and just went with it, never had any problems or accuracy issues. My question is can I adjust the large jam nut on the sizing die base to allow me to bump the shoulder .002 and neck size rather than completely sizing the brass? Also, what tool will I need to measure the shoulder to monitor my progress? This is 300RUM N/C brass and 210VLD's.
Yes, you can bump with the sizer die. Tools for measuring bump are the Hornady L&L headspace guage or the Redding Instant Indicator (my preference). You must be careful to not bump it too far. On the RCBS die, loosen the lock ring and adjust it down so that the die base does not quite touch the shell holder. Check the shoulder bump both before and after you do this. If your have not moved the shoulder, turn the die down about 1/8" at a time until you do. When the guage shows the shoulder has moved about 1 or 2 thous, stop and place the empty case in your rifle. You should feel just slight resistance when the bolt is about half way down, meaning the rifle shoulder is beginning to touch the chamber shoulder. Stop there, lock the die nut in place, and you are done. (Note): It is difficult to feel the fit unless you have a bolt dissasembly tool, to remove the firing pin and spring. You can find all of these tools in the Sinclair International catalog, which is available online.
I can't think of any reason why you would choose to bump the shoulder back without taking the rest of the brass with it. Factory dies are purposely made to yield smaller dimensions than necessary. The die was probably not adjusted after fire-forming in which event it is incorrectly adjusted. Bump dies have a limited use and generally used only in an intermediate brass forming process. I think you're interests might be headed towards Neck Sizing Dies. Unless the load is so hot that it is difficult to extract the fired brass, we have a correct question but the wrong idea behind it!
Thanks guys. I guess my question should have been how to correctly set up the die in general as I think the instructions are pretty generic. Not trying to make this a neck sizer only but the more reading I'm doing on here makes me wonder if I may be bumping the shoulder a little too far. I just want to size to a minimum. I guess I will pick up the guage and see where we're at.
Back your sizing die off a turn or two. Size a case and see if it fits. If it does, back it off some more. If it still fits, you will have to make sure the neck is gettring sized enough. If it will not hold a bullet properly, take the die back down until it does. Then you are sizing enough for your rifle. If it does not fit, then take the die down until it does. Again, you are now sizing properly. You have to fit to your chamber and neck.
The instructions are to make sure every round will fit every chamber just like factory ammo. After the handloader gets some experience, they will find, like you, there is room for adjustment. This method also works for belted cases. Just ignore the belt and size to fit. The belt was originally used for headspace on cases with little or no neck. With normal bottlemeck cartridges, it is not necessary.
A well known problem with using full length sizing dies to size only part of a bottleneck case's neck is at issue here. Such techniques usually end up pushing the fired case shoulder forward a bit. If there's any binding when closing the bolt on a case partially sized in a full length die, that will surely cause the bolt head to seat in a different place for each shot. And that's a sure cause of poor accuracy. Sierra Bullets tried this partial sizing method back in the early 1950's for their bottleneck cases used to test bullet for accuracy. Didn't work very well. They got best accuracy then and still do today with proper full length sized bottleneck cases. Don't believe me? Ask them via their web site.
What's important is that the fired case shoulder has to be set back a couple thousandths. This lets the bolt close home on a chambered case exactly the same way for each shot. The most accurate rifles tested at ranges further than 200 yards I know of have all done as well as they do with such fired case sizing. But you'll need a case gage to measure how much the shoulder gets set back.