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Setting up Full Length Sizing Dies
Setting up full-length sizing dies can be a recipe for short brass life and case head separations; or it can even end up with cartridges that will not chamber in the gun, depending on whose instructions are followed. There are many suggestions bandied about for "partial full length resizing" that can be confusing to the new or even experienced reloader. Usually these instructions are in the form of "back the sizing die off half a turn (or some other vague amount) from touching the shellholder when the ram is fully up." Theseinstructions, however well-meaning, simply cannot account for all of the manufacturing tolerances at play.
How can sizing a case less than the full amount end up with a case that will not chamber? If the die tolerances for diameter of the case are on the small size, and the chamber diameter is on the large side, the sides of the case will get squeezed in before the shoulder is touched by the die. This can lead to the shoulder being pushed forward slightly (the brass has to go somewhere). In addition, it is possible to have a chamber slightly out of round. If so, the case may not go back into the chamber unless it is oriented exactly the same, or the case body is sized down slightly. While bolt action rifles can have enough camming leverage to force a slightly overlong cartridge into the chamber, this is not desireable for hunting reliability and may not be possible with other action types.
This graphic by Shooter's Forum member UncleNick shows how a case can be lengthened when being squeezed down:
Another picture, by Shooter's Forum member opus, shows how the sizing die can contact the body of the case well before the neck. The case has a layer of soot before sizing and the soot has been wiped off the body just as the neck starts to touch the sizing die.
How do we set back a case shoulder only "slightly?" Good question. Common reloading dies have a thread pitch of 14 threads to the inch. This means that one full turn of the locking ring is 0.0714" (rounded) or not quite 0.072." Considering that headspace in excess of 0.010" (the usual "field" gage) is considered dangerous, there is a lot of play in one full turn of the lock ring! To make matters worse, rimmed or belted cartridges may have the shoulder placed anywhere the manufacturer deems appropriate. This article will show how a common .30-30 cartridge with a random set of dies can have considerable variation in shoulder setback.
So, what is the reloader to do? First, it is vital to have a way to measure the shoulder position of the case. There are commercial products that do this, or simple gages can be made. This article, Precision Reloading on the Cheap, shows how to quickly make a simple gage. In order to make setting up the sizing die easier, though, we can use the six-sided lockring. First, make a mark across each of the points of the ring. One sixth of a turn of the ring comes out to very close to 0.012" of travel and the small amount of error can be ignored.
Next, using a ruler or just eyeballing it, make marks halfway between each corner of the ring. This gets us down to 0.006" or so.
Last, I just eyeballed each remaing increment into thirds, making each mark represent about 0.002" or so.
With a mark on the die body (use a piece of tape over knurled or irregular sections of the die body), we can now change our die settings in quite small increments with a fair amount of precision! At the very least, this will make setting up dies much less tedious.
How much difference can the die adjustment make? To illustrate, my homemade gage was used on a random once-fired .30-30 case. The initial measurement is about 2.405" - the exact number is not meaningful except in relation to die adjustments.
The resizing die was then installed in the press. With the press ram raised to the top, the die was screwed down until the bottom of the die just touched the shellholder. However, with the additional resistance of a case going into the die, a gap of about 0.013" was created at the top of the ram stroke. This is illustrated by inserting an automotive feeler gage into the gap between the shellholder and the bottom of the die.
Even with this gap, the case shoulder is already pushed back about 0.008" - far more than it needs to be for easy chambering. Case life won't be long.
Next, the die was screwed into the press an additional half-turn or so to take up all of the slack in the linkage, etc., and the case was sized again. With no daylight showing between the shellholder and bottom of the die at the top of the ram travel, the case shoulder was pushed back an additional to 0.016!" This is clearly excessive.
Last, to illustrate a technique that might be useful in certain limited circumstances, a feeler gage was inserted between the shellholder and the case head to increase the amount of shoulder setback beyond what the die would ordinarily allow. I have only had to do this once, when setting case shoulders back reforming .300 Win Mag brass to .257 Weatherby. My .257 Weatherby sizing die, which works perfectly on cases fired in my gun, needed an additional 0.010" of sizing for the forming task. Be sure to remove the decapping pin from the die first!
The shoulder was bumped back yet another 0.003" or so by doing this. Note, this .30-30 case is probably not safe to use in this condition and will be discarded.
Not all combinations of sizing dies and rifle chambers may exhibit this range of adjustment, but it is possible. My own preference is to resize all cases for hunting approximately 0.002" shorter (as measured to the shoulder) than the as-fired length. Either home made gages or purpose made tools such as the RCBS Precision Mic can make this a simple task. By making the markings on the die lock ring and die body, the process can be made a little less tedious. The markings can also help return to known settings quickly if the dies are used for multiple guns or multiple presses that require different settings. Hopefully, this article takes some of the mystery out of the confusing and somewhat contradictory term, "partial full length resizing."
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