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How To: Get Correct Seating Depth

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DairyFarmer View Drop Down
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    Posted: August 11 2014 at 12:40pm
Did you know that you are seating your bullets at different lengths every time you reload a round? And did you know that your bullets may be jumping the gap every time you fire?

Let me explain. Bullets are not all created equal. They are not the same length, weight or diameter. Diameter and weight are a manufacturing symptom and, if you buy in bulk, can be graded into batches. The more worrying thing is length.

When we talk about length we need to talk about the tail to ogive length, not the nose to tail length. Nose to tail length can vary depending on the type of bullet. Soft nose bullets are very seldom the same length, for example.

The tail to ogive length is more important as this is what dictates the amount of gap the bullet has to jump. The greater the gap the less accurate the bullet will be.

The gap is the, well, gap between the ogive and the start of the rifling. The closer the ogive is to the rifling the better the bullet will preform.

The ogive is the point at which the bullet reaches its final diameter as measured from the tip backwards. If you take a handful of bullets and measure the length from the tail to the ogive you will notice a difference. How do you measure the length? The easiest is to purchase a gauge. It is a simple tube with a cone shape inside. It clamps onto the callipers. As the bullet enters the cone it will stop at its widest part or ogive. Well come back to this part later.




All very technical and boring so far. There are two very simple methods of measuring the correct length of a round for you specific firearm. There is the blackening method and the split case method.

The blackening method requires you to seat the bullet a lot longer than needed. Soot the bullet using a flame. Chamber the round. There should be rifling marks on the bullet where the ogive touches the rifling. Reseat the bullet until it is just short of touching the rifling.

The second method is the one I prefer. The split case method. Take a case that has been fire formed for you firearm. Make sure the case is trimmed to the correct length. Remove the primer. Using a fine saw or a Dremel cut a slot or two down the side of the case, from neck to shoulder. The case should grip a bullet just tight enough that you need to pull hard to remove it.



I drill out the primer pocket too. This allows me to use a rod to push out the bullet.



Now you have a reusable gauge for your firearm. Simple insert a bullet, by hand into the neck of the case.




Put the round into your firearm and close the bolt or slide. The bullet will be pushed back into the case by the ogive pressing on the rifling. Gently remove the round and measure the distance from the ogive to the head of the case. BTW the head is the bottom of the case. You can now set your seating die to this depth plus a bit. Maybe 1mm or so. Measure all the rounds in this batch to this length (ogive to head).

You have now, easily, set the bullet to a minimal gap jump. Remember to check that the rounds are short enough to fit the magazine and cycle correctly! Vary the length to best suite your firearm.
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Gun Nut 4 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gun Nut 4 Quote  Post ReplyReply Direct Link To This Post Posted: December 18 2015 at 3:56am
Interesting, although I load cast, this looks like an idea I can use. Thanks
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Zed View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Zed Quote  Post ReplyReply Direct Link To This Post Posted: December 18 2015 at 4:13am
thanks DairyFarmer, thats definately an improvement on my home brewed method. I will have to get a gauge adaptor made.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DairyFarmer Quote  Post ReplyReply Direct Link To This Post Posted: December 18 2015 at 6:00am
It's a lot cheaper than the commercial ones. It costs one case only.
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