Partial Sizing Fired Brass
- Thread starter Yosemite Steve
- Start date
I think the apparent mystery can be explained by understanding what happens in the resizing process. Chambers are wider than cases to ensure smooth, fast fit. If a chamber is very tight and the load pressure not very high, a case may come out the same size it went in, due to spring-back, but that is unusual and is not considered here. Here I refer only to cases that come out wider than they went in.
Most modern cases have some degree of body taper. As they enter a resizing die there comes a point where the sides of the case body contact the sides of the die. If there is no taper, as with a long straight-wall case, that contact occurs as soon as the case enters the mouth of the die. If there is a large angle of taper, contact with the sides won't occur until the case shoulder is almost to the die shoulder and the neck is almost fully resized. But with an Ackley-improved case or other cases with only slight taper, side contact with the die is very early. With the .30-06 or other in-between tapers and depending on how fat the case got in the chamber, it occurs somewhere in between.
So, why care about how far into the sizing stroke die body wall contact? Because once that contact occurs, the case is beginning to be narrowed and lengthened by the die. At that point, unless the shoulder of the case contacts the die shoulder, it won't go back to being short. The squeezed brass then has to go somewhere and where it goes into growing the length of the die. The only way to avoid that is not to insert the case as deeply into the die. This means the neck is resized only for a shorter length.
That said, a small amount of growth may not impede your shooting. The case will have had some amount of spring-back before leaving the chamber, so there may be enough room for you to jam the shoulder a little during chambering and the rest of the case will just fatten out into the chamber diameter if it isn't too little. Indeed, if you look at SAAMI chamber drawings you see the case maximum lengths often exceed minimum chamber length. SAAMI dimensions are critical dimensions and that overlap allows for the fact you can still close the bolt on that extra length because minimum chamber diameter has room for the case to bulge outward even in a minimum chamber, as long as the maximum head-to-shoulder length of the case is within their limits. Any longer and feeding into a minimum chamber is impossible to guaranty.
The idea behind partial resizing is to leave enough neck at its original expanded diameter to help center the bullet in the neck portion of the chamber. With some bullets, the cost is bullet pull and could affect the load level and pressurizing timing a little bit. It can make it easier to have a tipped bullet. So it's a balancing act. Also, how much that extra centering support helps is a little unclear. If you narrow a case and fully resize the neck and bump the shoulder back just a little, the shoulder will tend to center the case body when the firing pin and primer-backup push it forward, which happens before pressure builds to a level that sticks the brass to the chamber.
My guess is the answer to the above will change with the chamber and the bullet choice, so it becomes yet another thing one can opt to "try" in the quest to make bugholes. If you partially resize in a neck sizing die you can avoid narrowing the body at all. If you partially resize but meet resistance chambering, you can run the die into a Redding body die which will push the shoulder back and leave the fat part of the neck alone.
So many toys, so little time.
Most modern cases have some degree of body taper. As they enter a resizing die there comes a point where the sides of the case body contact the sides of the die. If there is no taper, as with a long straight-wall case, that contact occurs as soon as the case enters the mouth of the die. If there is a large angle of taper, contact with the sides won't occur until the case shoulder is almost to the die shoulder and the neck is almost fully resized. But with an Ackley-improved case or other cases with only slight taper, side contact with the die is very early. With the .30-06 or other in-between tapers and depending on how fat the case got in the chamber, it occurs somewhere in between.
So, why care about how far into the sizing stroke die body wall contact? Because once that contact occurs, the case is beginning to be narrowed and lengthened by the die. At that point, unless the shoulder of the case contacts the die shoulder, it won't go back to being short. The squeezed brass then has to go somewhere and where it goes into growing the length of the die. The only way to avoid that is not to insert the case as deeply into the die. This means the neck is resized only for a shorter length.
That said, a small amount of growth may not impede your shooting. The case will have had some amount of spring-back before leaving the chamber, so there may be enough room for you to jam the shoulder a little during chambering and the rest of the case will just fatten out into the chamber diameter if it isn't too little. Indeed, if you look at SAAMI chamber drawings you see the case maximum lengths often exceed minimum chamber length. SAAMI dimensions are critical dimensions and that overlap allows for the fact you can still close the bolt on that extra length because minimum chamber diameter has room for the case to bulge outward even in a minimum chamber, as long as the maximum head-to-shoulder length of the case is within their limits. Any longer and feeding into a minimum chamber is impossible to guaranty.
The idea behind partial resizing is to leave enough neck at its original expanded diameter to help center the bullet in the neck portion of the chamber. With some bullets, the cost is bullet pull and could affect the load level and pressurizing timing a little bit. It can make it easier to have a tipped bullet. So it's a balancing act. Also, how much that extra centering support helps is a little unclear. If you narrow a case and fully resize the neck and bump the shoulder back just a little, the shoulder will tend to center the case body when the firing pin and primer-backup push it forward, which happens before pressure builds to a level that sticks the brass to the chamber.
My guess is the answer to the above will change with the chamber and the bullet choice, so it becomes yet another thing one can opt to "try" in the quest to make bugholes. If you partially resize in a neck sizing die you can avoid narrowing the body at all. If you partially resize but meet resistance chambering, you can run the die into a Redding body die which will push the shoulder back and leave the fat part of the neck alone.
So many toys, so little time.