The Firing Line Forums - View Single Post - How do I neck-size only?

HiBC · September 20, 2010, 12:59 AM

This is one of the terminology debates.The short answer:you cannot neck size only with a full length resize die.
Lets use a .308 for example.Suppose you back the die up .050 from the shellholder.That is .050 of neck tension,so I don't think we need more.
Case dia at the head is .470.At the shoulder,454.We have .016 taper over1.560 case body length.May we fudge a bit and say it is about .001 thousandth on the dia per .100 thousandths length?So you are making a .0005,or 1/2 of one thousanth change on the dia of the case body.In a short necked 308,you have lost .050 of effective neck.
Now,take a half a bananna,hold it like a handgun in a two handed grip,supported with the weak hand from beneath.Squeeze the bananna.Note the bananna squeezing out the top of your hand.this is what is happening to your headspace at the shoulder.The material has to go somewhere.If the die is backed off,this extrusion is happening in an uncontrolled manner.That introduces variability.The bolt may not close.
I have heard the Lee collet necksizer is very good.The other die mfg's also make neck size only dies.
I do advocate having the means to measure the amount of bump back on the shoulder,and I do advocate setting back a known amount,such as .002 on a bolt gun.
Just thumping over center with the shellholder against the die,too much is unknown,we likely over shorten the brass,and shorten the life of the brass.

September 20, 2010, 12:59 AM	#15
HiBC Senior Member Join Date: November 13, 2006 Posts: 8,286	This is one of the terminology debates.The short answer:you cannot neck size only with a full length resize die. Lets use a .308 for example.Suppose you back the die up .050 from the shellholder.That is .050 of neck tension,so I don't think we need more. Case dia at the head is .470.At the shoulder,454.We have .016 taper over1.560 case body length.May we fudge a bit and say it is about .001 thousandth on the dia per .100 thousandths length?So you are making a .0005,or 1/2 of one thousanth change on the dia of the case body.In a short necked 308,you have lost .050 of effective neck. Now,take a half a bananna,hold it like a handgun in a two handed grip,supported with the weak hand from beneath.Squeeze the bananna.Note the bananna squeezing out the top of your hand.this is what is happening to your headspace at the shoulder.The material has to go somewhere.If the die is backed off,this extrusion is happening in an uncontrolled manner.That introduces variability.The bolt may not close. I have heard the Lee collet necksizer is very good.The other die mfg's also make neck size only dies. I do advocate having the means to measure the amount of bump back on the shoulder,and I do advocate setting back a known amount,such as .002 on a bolt gun. Just thumping over center with the shellholder against the die,too much is unknown,we likely over shorten the brass,and shorten the life of the brass.