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Locked Topic The correct way to do full length resizing. (Read 3721 times)
Bryce
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The correct way to do full length resizing.
Jan 24th, 2008 at 11:27am
 
Just thought I’d post this information regarding the proper way to full length size rifle cases, it seems to be an aspect of reloading that seems simple but many people don’t quite grasp fully.

On the face of it you just wind the full length die down until it just kisses the shell holder, lock it up and call it good, right ?  Wrong !!

When the rifle manufacturer chambers the barrel they have a certain set of tolerances to work with, same for the die manufacturers and the same applies to the ammo and brass manufacturers.  Thing is they all need to work within a tolerance range so that the biggest brass possible will work in the smallest chamber possible.  What this means is that if you just size the case the smallest that the die will allow you may very well be making the case a loose sloppy fit in the chamber.

HEADSPACE is the area of main concern.  This is the fit of the case into the chamber along the length of the case.  If a case has been fired in the camber once or twice with just neck sizing and is essentially the same size as the chamber it can be considered to have zero headspace.  Headspace is the gap that exists between the back of the case head and the front of the bolt face when the case is chambered so that the case shoulder is hard up against the shoulder area of the chamber.  Basically it is the area that the case has to move into in a back and forth direction in the chamber.  You need a little gap so that the case isn’t tight to chamber but not to much so that the case is a sloppy fit.  Technically rimmed and belted cases vary in how and where headspace is measured and defined but the principal is the same, the case needs to fit the chamber correctly.

To much headspace and the case will stretch on firing to fit the chamber.  If you size it back and make the headspace gap big again the case will stretch on the next firing and so on for as long as you keep loading the brass.  The stretching will eventually make the brass thin in the area above the solid head and finally the case will split apart there.  This can mean a ruined case, a part case stuck in the chamber or even leaking high pressure gas escaping which is not a good thing !!

OK, so we can see that this headspace thing is important but how do we control it as part of our reloading procedure ?

Well, the easiest way is to neck size the cases only.  That will allow the case to fit the chamber nicely with no headspace gap to allow the brass to stretch into.  That is fine for certain calibres and certain situations but with normal high pressure loads you will eventually end up with tight cases and need to full length size them to allow them to function correctly, that is to chamber and eject freely.  That is obviously important if you are on the hunt of a lifetime after dangerous game but is also desirable even if just shooting paper or tin cans at the range.

There are two things that we need to be aware of when setting up a full length die correctly to provide a proper functioning case without excessive headspace gap.  First is that you can’t just wind the full length die out a few turns thinking that if the die doesn’t hit the shoulder area of the case it can’t push it back to far.  If you do that the diameter reduction that the die does to the case will make the case body grow a smidge longer resulting in the cases being an even tighter fit than they were before, possibly so tight that you can’t shut the bolt on them.  The die needs to be in proper adjustment so that it just nudges the case shoulder back just a little giving a small headspace gap for proper functioning.  This is referred to as “bumping the shoulder”.

The second thing that you need to be aware of is the big one that most people miss.  THE LOADING PRESS STRETCHES under the load applied to it when full length sizing a case.  A lot of guys struggle to believe it because the press looks big and strong, it is true nevertheless, to some extent or other depending on the press and the size of the case being sized.  What this means is that if you just wind the die down to touch the shell holder the press will stretch when sizing a case, lifting the top of the press up a little and taking the die with it.  The end result is the die is higher than it was before relative to the shell holder and as such the shell holder doesn’t meet the die base any more.  Take a look down at press level when sizing a case, you can visibly see the gap that opens up between the shell holder and the die.

OK, so we can see that we need to take that press stretch into account when sizing the case, but how ?  All that we need to do is wind the die down further to counter act the amount of stretch that the press suffers, but how much ?  The best way is to get a bullet comparator a few calibres bigger that the calibre of your cartridge so that it slips over the neck and contacts the case about mid shoulder area.  A 375 calibre one is about right for a 308 head size case, a 30 calibre one would be about right for a 223.

Fit the comparator to a vernier caliper and take a reading from the fired case to the case shoulder.  Knock the primer out of the case manually (without using the full length die) so that the primer doesn’t possibly interfere with the readings.  OK now you have a headspace length of a fired case, you want to set the die up in the press to result in a sized length just a little shorter than that, by say 2 – 3 thou or about 0.05 – 0.08mm.  These are pretty small measurements we are dealing with here and doing it without the proper tools is hopeless, you just can’t fudge this important part of the reloading process, proper and safe function of your loads relies on it.  Everybody reloading should have a vernier caliper and a comparator isn’t expensive.

Now to set up the die.  Wind the die down into the press until it hits the shell holder when the ram is at it upper most position and back it off say a half a turn.  That should get the die at a starting point that is to far away.  Size a case and measure that headspace length again.  After sizing but not bumping the shoulder the length may very well have gotten longer.  Wind the die in a little at a time measuring each time until you have the die positioned so that it bumps the shoulder that small amount needed, 2 – 3 thou or about 0.05 – 0.08mm shorter than the fired case.  The maximum shoulder bump should be about 5 thou or 0.12mm but aim for smaller.  Check the sizing on a few more fired cases and fine tune as needed.  Don’t keep sizing just one case as the fired case will take less force to size and will stretch the press less resulting in slightly different bumping of the shoulder.  Lock the die up at the final setting.

There are a couple of refinements that can help with this process.  Redding make a set of shell holders that let you size the case to a longer headspace measurement by using the different height shell holder.  You wind the die down until it bottoms out firmly on the shell holder and then just swap the shell holders until the sizing is bumping the shoulder the proper amount.  The added bonus with these is that when set up right the shell holder is always jammed firmly on the die throughout the sizing operation which means that the press flex is not effecting the sizing operation.  The press flex is forced to be the same each time by the firm shell holder to die contact.  This will also result in nice consistent sizing since the cases entry into the die is very well controlled by the bottomed out shell holder.  These shell holders are normal looking but the depth from the top edge to the platform the case sits on is incrementally deeper over the 5 shell holders in 2 thou steps.  Once you grasp how they work they make set up easy and they work well.  You can’t size a case shorter than a normal shell holder with these, just longer.

The other option is die shims.  These are a series of shims that can be placed under the dies locking ring between it and the top surface of the press.  In use you place a nominal shim under the lock ring and set the die up close to correct.  Final die adjustment is made easy but swapping over to a thinner or thicker shim thus lifting or lowering the die and altering the shoulder bump.  Just swap shims to set the die up or to adjust it for different rifles.  The down side of this method is that there is not the firm shell holder to die contact to counter the press stretch and if the press stretches different amounts the shoulder bump and thus headspace can also alter a little.  This isn’t really an issue if using say 2 - 3 thou of headspace but if working with really tight tolerances for a precision target rifle it may be a factor.  Variations in press stretch can result from variation in brass hardness, usually not a big factor with decent brass that has been sized the same and fired the same number of times in the same rifle.

There are certain situations where one of these two methods of helping with the set up works a little better than the other but for most situations they work about as well as each other.


Continued .....
  
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Bryce
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Part 2
Reply #1 - Jan 24th, 2008 at 11:30am
 
If loading for a semi auto you will want to loosen the headspace tolerances a little, aim for maybe 4 - 6 thou headspace.  They need looser fitting ammo than a bolt action to operate correctly.  Also make sure that the base and shoulder area of the case is reduced enough to allow a nice free chambering if working with a semi or even a lever or pump for that matter, none of them have the camming powder to chamber a firm fitting round like a bolt action does.  A "Small base" full length die may be needed in some situations with these actions.

On occasion you can get a short chamber and/or a long die such that even with the shell holder bottomed out hard on the die the case will still not size enough to bump the shoulder.  In this situation the best bet is to have the shell holder top surface precision ground down just enough to let the case enter the die a little further so the shoulder reaches the shoulder area in the die and thus bumps the case should as needed.  Set this shell holder aside and use it only with that die for that rifle.

All this may seem a little over the top and technical to some but there is really no other way to get the die set up as precisely as is needed for this important part of reloading.  You can soot or permanent marker the case shoulder and observe that it has been bumped back or you can just size by feel until the brass chambers freely.  The down side of both these methods is that you just can’t tell how much sizing you have done and when the dimensions dealt with are small you need to be accurate to do it right.  I did one by feel and then checked with the above approach and found that I had created about twice the desirable headspace just relying on feel to assess the correct shoulder bump.  By all means confirm by the feel of chambering but actually measure the case at the headspace length and adjust the die based on accurate measurements.

The possibility of a split case after a few loading releasing hot 50,000PSI gas into your face is as good a reason to do it right as any !!

Bryce
  
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