The goal is to adjust the die to set the case shoulder back just enough to let the cartridge chamber freely. Normally, this is envisioned to be a reduction in shoulder dimensions of about 0.001” to 0.002" from fired to sized shape. Exactly where you reach the desired die adjustment totally depends on the dimensions of the die vs. the dimensions of the chamber vs. the brass you use (brass spring back is a factor here). The weakness of this method is that you simply cannot know exactly how much your shoulder has been set back, and it is hard to be sure it was not another case dimension that needed reduction before your case chambered freely. You can try to assess this by measuring case diameter at the shoulder and in front of the case head (fired vs. sized) while you adjust your die. If the case diameters are reduced before you reach your ‘sweet spot’, you are probably ok. To be one hundred percent sure, you need to use a headspace gauge and actually measure the difference in head to shoulder dimension on a fired vs. a sized case. Now that said, anal agonizing over the above is an exercise in futility because once you have determined where the die needs to be set to allow the cartridge to chamber freely it is what it is, the firearm has told you in no uncertain terms what it will or will not accept!In the absence of custom fitted dies, adjustment to the die body is all we have for achieving optimum case sizing, especially when PFL sizing to match the dimensions of the chamber. Adjusting the die to touch the shell holder with the ram up is the starting point. Further adjustment is accomplished by screwing the die down either 1/4 or 1/2 turn, which is accomplished by lowering the ram and then giving the die the additional 1/4 turn or 1/2 turn. The additional turn increases the ability of the press, die and shell holder to overcome any spring in the press caused by the resistance of the case to be sized. The problem with the advice to make sizer adjustments in ‘small changes like 1/4 or 1/2 turn’ until they get the cases sized right, results in changes to the shoulders in steps from about 18 to nearly 36 thousands. Given that most bottleneck cartridges only have a minimum to maximum range of about 6 thousands for headspace; those so called 'small changes', when put into context, are massive! The amount of additional sizing equates to; 1/4 turn is equal to 0.0178”, 1/2 turn is equal to 0.0357”, and anything close to (or over) 0.035” is a work out for a press. A careful review of your reloading procedure will often bring to light bad habits that necessitate the additional 1/2 turn.
The thread pitch on all standard dies sets is 14 turns per inch. 1" = 1.000", 14 TPI = threads per inch, or 1000/14 = 0.0714" per full turn. 1/8 turn = about 0.009". 1000/14 = 71.428, so the exact answer is (71.4 thousandths per turn of the die), but for practical purposes use 72. An added bonus is that 72 is divisible by just about everything (2, 3, 4, 6, 8, 9, 12) so you can usually get within a couple of thousandths of the adjustment you need to make. All threads have pitch errors, let alone 'running' clearance between the male and female threads, so, depending on the particular die, there may only be 13.9 threads per inch, or 14.1. One complete revolution of the die is equal to 0.072”. Masking tape measures on average 0.08mm to 0.1mm in thickness. It is better to measure and remove the variables. In any event, you will only get ‘close’ to an exact measurement but if you only partially loosen the die lock ring enough to allow turning the die body, there will be little to no ‘slop’ in the threads. It is nearly as accurate as a micrometer die unless you loosen the lock ring completely and let the die threads slop around in the press threads.
Unless you are setting the shoulder back significantly below SAAMI minimum, there should be no danger in firing that round! The reason there is a range of allowable measurement is because each firearm is different and many are the re-loaders that fire form cases to fit chambers that are over SAAMI maximum chamber space. If you really want to get anal about this, do a chamber cast, or pounded impression of your chamber and size your cases based on the measurements from that process.
The thread pitch on all standard dies sets is 14 turns per inch. 1" = 1.000", 14 TPI = threads per inch, or 1000/14 = 0.0714" per full turn. 1/8 turn = about 0.009". 1000/14 = 71.428, so the exact answer is (71.4 thousandths per turn of the die), but for practical purposes use 72. An added bonus is that 72 is divisible by just about everything (2, 3, 4, 6, 8, 9, 12) so you can usually get within a couple of thousandths of the adjustment you need to make. All threads have pitch errors, let alone 'running' clearance between the male and female threads, so, depending on the particular die, there may only be 13.9 threads per inch, or 14.1. One complete revolution of the die is equal to 0.072”. Masking tape measures on average 0.08mm to 0.1mm in thickness. It is better to measure and remove the variables. In any event, you will only get ‘close’ to an exact measurement but if you only partially loosen the die lock ring enough to allow turning the die body, there will be little to no ‘slop’ in the threads. It is nearly as accurate as a micrometer die unless you loosen the lock ring completely and let the die threads slop around in the press threads.
Unless you are setting the shoulder back significantly below SAAMI minimum, there should be no danger in firing that round! The reason there is a range of allowable measurement is because each firearm is different and many are the re-loaders that fire form cases to fit chambers that are over SAAMI maximum chamber space. If you really want to get anal about this, do a chamber cast, or pounded impression of your chamber and size your cases based on the measurements from that process.
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