Seatin length

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ultratec1

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Ok guys I have a question and its frustrating the crap out of me.


I have been loading 223 with 68gr Hornady Match BTHP, I set the seater die to 2.260 and starting loading my rounds.

After I ran 3 rounds I noticed the OAL is longer, 2.265 then it would go up to 2.268. I thought maybe my die was loose so I checked it and everything was tight. I then started loading some more and they would go to 2.257. Nothing was consistent, its not like it progressively got longer or shorter it was just random lengths.

I checked all my case lengths and they are the exact same. Im using the RCBS short base AR dies. Anyone have any idea why I keep getting varying length???
 
Case length has no bearing on OAL.

Case neck hardness could.

Mixed brass can have different case neck thickness, as well as hardness.

Failing to chamfer the inside case neck can cause hard seating if a sharp edge is cutting into the bullet jacket.

As could measuring off the tip of the bullet, rather then off the ogive of the bullet where the dies seating stem pushes it in the case.

http://www.sinclairintl.com/.aspx/pid=34262/Product/Sinclair-Hex-Style-Bullet-Comparators

rc
 
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The legnth to the ogive of the bullet is the same but the bullets will vary in overall legnth. Take a few bullets out of the box and measure their legnth dimes to dollars they wont be the same. This is where a bullet comparator comes into play it measures from the ogive of the bullet to the case head which is the only important legnth measurement. Just be sure they will feed from your mag.
T

RC Beat me to it and is far more elequent in the wording as well.
 
From Lee FAQ

I am lazy, so i paste what Mr Lee has said. :Dhttp://leeprecision.net/support/index.php?/Knowledgebase/Article/View/143/0/seating-depth-variation
Seating depth variation

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.
http://leeprecision.net/support/index.php?/Knowledgebase/List/Index/13/product-support
 
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