Common to have .005" to as much as .010" variance.
Lee Info>
Seating depth variations
There are a number of possible causes for overall length variation. One is the way it is measured. If you measure overall length from the tip of the bullet to the base of the case, remember to subtract the variation due to bullet length tolerance. The bullets will vary in length due to manufacturing tolerances (bullets with exposed lead noses are the worst in this regard) and this will add to the overall cartridge length variation. Remember that the bullet seater plug does not (or shouldn't) contact the tip of the bullet when seating, but contacts farther down the ogive. For a more accurate seating depth measurement, take the seater plug out of the bullet seating die, place it on top of the cartridge and measure from the base of the case to the top of the seater plug.
Another possible cause for bullet seating depth variation is seating and crimping at the same time when trying to apply a firm crimp to untrimmed cases. Variation in case length also causes variation in the amount of crimp applied. Long cases get a heavier crimp than short ones. When seating and crimping at the same time, the crimp is formed as the bullet is seated into the case. The crimp will form sooner on a long case, and therefore the bullet will not be seated as deeply. The solution is to seat and crimp in a separate step (the Lee Factory Crimp die is good for this) and/or trim cases to a uniform length.
The amount of force required to cycle a progressive press varies with the number of cases in the shell plate. When the shell plate is full, it is harder to lower the lever than when there are one or two cases present. This can lead to variation in cartridge overall length because there are different loads placed on the working parts of the press. When the shell plate is full, seating depth will be slightly long, because the load is higher and all of the clearances are taken up. With the shell plate nearly empty, the load is not great enough to squeeze out these clearances, and the seating depth is short.
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From Reddings Tech Line Tips
Seating Depth Variation
There are many factors that can cause bullet seating depth to vary when using our Competition Seating Die. First, make sure you're comparing bullet seating depths correctly. You cannot check bullet seating uniformity by measuring cartridge overall length off the bullet point. You must use a bullet comparator, like our Instant Indicator, to compare bullet seating depths. A comparator contacts the bullet at the bore diameter contact point. This is important, as bullets can vary slightly in overall length.
We have designed the seat stem in our Competition Seating Die to contact the bullet ogive as far down as possible. Our Competition Seating Die features a bullet guide that is only .0005-.001" larger than bullet diameter. This tight fit between the bullet guide and bullet ensures that the bullet is seated straight in the case neck. It also limits how far down the ogive the seat stem can contact the bullet. If the ogive of your bullets aren't uniform, you may notice a slight difference in seating depth. Generally, this isn't a problem as modern bullets are very uniform. In rare instances, when using inexpensive bulk bullets, you may find that the bullets were made on several different machines and then blended.
If your loading press is worn, the ram may not stop in exactly the same spot each time you raise it. Obviously, this will cause variations in bullet seating depth. Although our instructions warn against it, raise the shellholder and adjust the outer, threaded die body to make light contact with the shellholder. (Make sure you keep the contact light, so you don't damage the die.) This creates a "dead length" seating chamber that is unaffected by where the shellholder stops. The only disadvantage to using the die adjusted this way, is that it may be awkward to read the micrometer if it ends up on the back side of the die.
Inadequate or excessive neck tension can also cause bullet seating depth variations. If you're using a bushing style sizing die, make sure you've selected the correct diameter bushing to size the case necks. Our current recommendation, is to select a bushing tha t is .001" smaller than the neck diameter of your loaded cartridges. (See the bushingselection newsletter in the "Tech Line" section of our website for more information.) As cases are fired over and over, their necks become progressively harder. This can cause the necks to "spring-back" excessively when they are sized, which reduces the neck tension on the bullet. Either anneal the case necks after several firings, or discard the cases and start with new, soft ones.
Heavily compressed loads can create problems when seating bullets. Our Competition Seating Die is not a powder compression die. The excessive force required to seat a bullet on a compressed load can damage the die and may cause seating depth variations. Switching to a faster burning or ball powder may eliminate the need to excessively compress the powder charge.
If you have any further questions, please feel free to call our technical support line at 607-753-3331 607-753-3331 .
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