Body die and neck concentricity clue me in.

[dgc940]

After about four or five reloads on a factory chamber my groups start opening up badly. I then run them through my redding body die and they tighten back up. What exactly is this doing? To My understanding does the body die bump the shoulder back am I wrong? I also noticed when the groups open up I check the neck concentricity and it runs from .0001 to .0008 and after the body die they are all under .0002 with most.0001. Does the body die also center the neck back with the case? should I also just start dong this every three firings? and last question is the body die just a fl die without a decapping rod that doesnt re size the neck? And while Im asking questions I have a new barrel But im waiting till fall to have it fitted what atvantages would I have by doing .243AI over .243 besides ease on my brass life? I dont want a BR bench gun just a fun gun.

Don

 

[BoydAllen]

Don,
You are correct about the body die. It is like a FL die without the decapping rod and with the neck larger so as not to size. In order to address this issue I need to go into why cases get crooked. Putting it simply, standard dies over size the neck of the case and then when you drag the expander ball back through, the axial pull on the case causes the shoulder to give, usually on one side more than the other, cocking the neck. The secret is to use a die that either does not size the neck so much so that the pull of the expander is reduced to what the case can handle without cocking the neck, or to use a tight necked chamber that requires neck turning, but does not need an expander ball. With this, neck tension is usually a matter of using the right bushing. Are you planning to turn necks? if not, you may find that using a body die in conjunction with a Lee Collet die is a good way to go, if you don't want to FL size every time. Another way to go would be to have a custom FL die made.

 

[dgc940]

I am using bushing neck dies with a .0002 under bushing and turning my necks just enough to clean them up. It also seems to me that this problem is getting worse as the brass gets older.
I'm now at about 25-28 reloads on this brass and has about 4-5 partial fl resizes . and about 6 annealings. I have also removed the expander ball.