FL bushing dies, will I get a doughnut?

[PatMiles]

Me no expander ballz.

You sure do talk funny!

 

[bozo699]

It's been mentioned here many times, but I'll say it again. Try a standard FL die, I prefer Forester, without the expander ball. They will size the full neck, usually bringing it down .003" to .005" under caliber. Then run an expander mandrel to get the tension you desire.

The mandrel will push differences to outside of case. Mandrels and die holders are readily available and the mandrels come in .0005" increments, so you can fine tune your tension.

Although I turn all my brass, I've migrated from my bushing dies over to FL dies and mandrels. No doughnuts and improved ES. It's an extra step ( and the necks need lube, I prefer Dry Imperial) but it's worth it to me.

Spot On!
Wayne

 

[AckleymanII]

I have a 30-28 with 5 firings on my brass. I went with the Redding bushing dies. I did a light trim on the necks and light cut into the shoulder initially. So far so good with no donuts detected.

I don't recall what bushing I am using, but would go at least 0.002 under. The brass should spring back and your neck "tension" should come out a little less than the bushing reduction.

Feman, turn your bushing around, the number on the Redding die goes down or else you get this "light cut into shoulder".

 

[BoydAllen]

Reading the posts in this thread, it seems to me that many do not really understand much about what causes doughnuts. There is a cycle that takes place that produces them, and a couple of factors that can be used to minimize them.

First of all lets take a look at what happens when a round, built on a rimless case, is fired. The first things that happen are that the force of the firing pin against the primer and the force of the primer compound exploding pushing against the bottom of the primer pocket, push the case forward until contact between the case shoulder and the chamber shoulder stop it. Actually, if the shoulder angle is relatively small, this can bump the shoulder back a bit, which is one of the advantages of sharper shoulders. They resist this much better.

Next, as pressure builds the body of the case expands to meet the chamber and friction, assuming a dry chamber and brass, locks it in its forward position, leaving a gap between the case head and the bolt face, except for the primer, which slides out to touch the bolt face. This all assumes that the loaded round had some shoulder to head clearance in the chamber.

If the peak pressure of the load exceeds the yield strength of the brass, the case stretches just above the solid head, where the side wall first starts to thin out from the radius at the bottom interior of the case. If the FL die was properly adjusted this is minimal, but it still takes place.

Some time after the bullet leaves the barrel, the case is removed from the chamber and is sized and reloaded. It is this sizing that causes the beginning of what is commonly referred to as a doughnut. To understand what takes place we first need to examine the brass.

Generally brass is thicker just below the neck shoulder junction than it is in the neck itself. When the case is FL sized or a body die is used with either set to bump the shoulder, thicker brass from the top of the shoulder is forced into the neck part of the die. The more the shoulder is bumped, the more of the shoulder is pushed into the neck.

The other thing that pushes thick shoulder brass up into the neck is the reduction of the diameter of the body of the case. This brass has to go somewhere, and since the shoulder of the die prevents it from lengthening the body it is pushed up into the base of the neck. The extent to which this happens depends on how much the diameter of the case body is reduced during sizing.

The degree to which the die matches the chamber, and how much its setting pushes back the shoulder are the primary factors controlling the rate at which a doughnuts are formed.

If the clearance between the loaded round neck and the chamber neck is small, and the full diameter of the bullet is seated into the area where a doughnut has formed, an interference fit can occur when the round is chambered that will cause an increase in peak pressure that is undesirable. both in terms of accuracy , and also safety. Even if the chamber neck is large enough so that this does not happen, the doughnut will increase bullet pull when the round is fired, and since this will vary from round to round, and assuming pressure relates to tune, accuracy will be degraded, if the bullet shank is in the area of the doughnut.

For this reason various schemes have been devised to either remove the doughnut, or allow room so that the bullet shank does not contact it.

You can get a rough idea of the rate at which brass is being pushed forward during sizing by measuring the length of cases before and after they are FL sized, be sure to either reseat or remove the fired primer so that its protrusion does not have an effect on your as fired measurement.

Years ago, I had a factory .220 Swift, and an RCBS one piece FL die. Noting that the brass was growing a lot each time that it was fired, around .007, I sent the die back to RCBS and had the body honed out for minimum sizing. Their idea of minimum and mine are different, but even so, case growth was halved.

With a case that has a sharper shoulder angle paired with a die that hardly sizes its diameter, set to bump .001 or less, this growth is very small, and it corresponds to the rate at which a doughnut is formed.

Interestingly (at least to me), the best situation to minimize this is something like when a Dasher case is made from BR brass. If the entire neck of the BR case is turned, the neck then expanded up a caliber or two, and the case sized to form a false shoulder for fire forming, the top of the fire formed Dasher case's shoulder will be the same or less thickness than the neck, with the result that when top of shoulder brass is forced into the bottom of the neck there should not be an doughnut, or if there is one, it should be much less prominent. I should point out that this is conjecture, since I do not own a Dasher. Perhaps someone who does can tell us their experience.

 

[bozo699]

Reading the posts in this thread, it seems to me that many do not really understand much about what causes doughnuts. There is a cycle that takes place that produces them, and a couple of factors that can be used to minimize them.

First of all lets take a look at what happens when a round, built on a rimless case, is fired. The first things that happen are that the force of the firing pin against the primer and the force of the primer compound exploding pushing against the bottom of the primer pocket, push the case forward until contact between the case shoulder and the chamber shoulder stop it. Actually, if the shoulder angle is relatively small, this can bump the shoulder back a bit, which is one of the advantages of sharper shoulders. They resist this much better.

Next, as pressure builds the body of the case expands to meet the chamber and friction, assuming a dry chamber and brass, locks it in its forward position, leaving a gap between the case head and the bolt face, except for the primer, which slides out to touch the bolt face. This all assumes that the loaded round had some shoulder to head clearance in the chamber.

If the peak pressure of the load exceeds the yield strength of the brass, the case stretches just above the solid head, where the side wall first starts to thin out from the radius at the bottom interior of the case. If the FL die was properly adjusted this is minimal, but it still takes place.

Some time after the bullet leaves the barrel, the case is removed from the chamber and is sized and reloaded. It is this sizing that causes the beginning of what is commonly referred to as a doughnut. To understand what takes place we first need to examine the brass.

Generally brass is thicker just below the neck shoulder junction than it is in the neck itself. When the case is FL sized or a body die is used with either set to bump the shoulder, thicker brass from the top of the shoulder is forced into the neck part of the die. The more the shoulder is bumped, the more of the shoulder is pushed into the neck.

The other thing that pushes thick shoulder brass up into the neck is the reduction of the diameter of the body of the case. This brass has to go somewhere, and since the shoulder of the die prevents it from lengthening the body it is pushed up into the base of the neck. The extent to which this happens depends on how much the diameter of the case body is reduced during sizing.

The degree to which the die matches the chamber, and how much its setting pushes back the shoulder are the primary factors controlling the rate at which a doughnuts are formed.

If the clearance between the loaded round neck and the chamber neck is small, and the full diameter of the bullet is seated into the area where a doughnut has formed, an interference fit can occur when the round is chambered that will cause an increase in peak pressure that is undesirable. both in terms of accuracy , and also safety. Even if the chamber neck is large enough so that this does not happen, the doughnut will increase bullet pull when the round is fired, and since this will vary from round to round, and assuming pressure relates to tune, accuracy will be degraded, if the bullet shank is in the area of the doughnut.

For this reason various schemes have been devised to either remove the doughnut, or allow room so that the bullet shank does not contact it.

You can get a rough idea of the rate at which brass is being pushed forward during sizing by measuring the length of cases before and after they are FL sized, be sure to either reseat or remove the fired primer so that its protrusion does not have an effect on your as fired measurement.

Years ago, I had a factory .220 Swift, and an RCBS one piece FL die. Noting that the brass was growing a lot each time that it was fired, around .007, I sent the die back to RCBS and had the body honed out for minimum sizing. Their idea of minimum and mine are different, but even so, case growth was halved.

With a case that has a sharper shoulder angle paired with a die that hardly sizes its diameter, set to bump .001 or less, this growth is very small, and it corresponds to the rate at which a doughnut is formed.

Interestingly (at least to me), the best situation to minimize this is something like when a Dasher case is made from BR brass. If the entire neck of the BR case is turned, the neck then expanded up a caliber or two, and the case sized to form a false shoulder for fire forming, the top of the fire formed Dasher case's shoulder will be the same or less thickness than the neck, with the result that when top of shoulder brass is forced into the bottom of the neck there should not be an doughnut, or if there is one, it should be much less prominent. I should point out that this is conjecture, since I do not own a Dasher. Perhaps someone who does can tell us their experience.

Excellent explanation Boyd.
Wayne

 

[ShooterB]

Thanks Boyd, that is a great explanation. Does an expander button or if using an expander die, remove the possibility of a doughnut forming? I’ve always thought that the button would in a sense swag the brass atleast reducing the doughnut effect.

 

[BoydAllen]

Brass is springy, so while expander button will tend to move some of the doughnut to the outside of the neck, it will not remove it, and the thickness at that point will not be changed, so if you have a chamber clearance issue, with the bullet shank in that area, your problem is definitely not fixed. Where you notice this is when you re-turn necks. The ones that have doughnuts will cause the turning mandrel to stop on them when you try to insert it, even if you use the appropriate expander before turning. You can reduce this somewhat by leaving the mandrel in the neck for a while before pulling the case off of it... but you can still feel it. Some believe that the constriction of a doughnut has an effect on accuracy, even if it does not touch the bullet. I have no data on this. Ultimately the easiest way to avoid most of the issues is for the chamber freebore to be long enough so that bullets do not have to be seated into the doughnut. Of course, for hunting rifles with magazines this may not be possible.

 

[Brians356]

Brass is springy, so while expander button will tend to move some of the doughnut to the outside of the neck, it will not remove it, and the thickness at that point will not be changed, so if you have a chamber clearance issue, with the bullet shank in that area, your problem is definitely not fixed. Where you notice this is when you re-turn necks. The ones that have doughnuts will cause the turning mandrel to stop on them when you try to insert it, even if you use the appropriate expander before turning. You can reduce this somewhat by leaving the mandrel in the neck for a while before pulling the case off of it... but you can still feel it. Some believe that the constriction of a doughnut has an effect on accuracy, even if it does not touch the bullet. I have no data on this. Ultimately the easiest way to avoid most of the issues is for the chamber freebore to be long enough so that bullets do not have to be seated into the doughnut. Of course, for hunting rifles with magazines this may not be possible.

FWIW:
http://forum.accurateshooter.com/threads/gage-pins.3995943/#post-37677235
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