What causes overall length inconsistency?


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edpm3
January 6, 2009, 09:14 PM
Seemingly at random, the overall length of cartridges varies after the bullet seating step. For example, loading 9mm Luger with 115gr Win FMJ into once-fired brass, and adjusting the seating die to give an overall length of 1.120 with no crimp. Two out of three end up at 1.120 or 1.121, but the third may come in at 1.126 or 1.124 and occasionally one may push to 1.116. I'm using an RCBS Rockchucker and a Hornady Precision Seating Die. I've measured with two different calipers, one digital, one dial. I have a similar problem with .380 ACP but not with .45 ACP. Could the actual nose shape of the smaller bullets be inconsistent enough to cause a deeper or shallower push into the casing? Am I doing something wrong? Any advice?

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Ol` Joe
January 6, 2009, 09:28 PM
Different nose shapes require different seater geometry. RCBS and a couple other die makers offer a seater for RN and another for truncated nosed bullets such as LSWC, or most HPs.

Redneck with a 40
January 6, 2009, 10:36 PM
If I set my seating die to 1.125" on my Ranier 40 cal bullets, it will vary from 1.125 up to 1.132, obviously not precision made bullets.

joneb
January 7, 2009, 01:34 AM
Does your seater plug leave a impression on the bullet ?

DssG19
January 7, 2009, 02:41 AM
Mixed brass doesn't seem to help OAL. If you sort headstamps and use all one brand for a batch can tighten up the OAL. I really don't care about .004 +/-, for the volume shooting I do.

1858
January 7, 2009, 02:50 AM
Accurate seating depth is determined by a number of factors assuming that you're using the correct seater die:

1. The geometric consistency of the ogive of the bullet.
2. The consistency of the inside diameter of neck of the case.
3. The consistency of the force/speed used to seat the bullet.
4. The method used to measure the OAL of the loaded round.

When I seat my rifle bullets, the OAL varies by +/- 0.001" at the most and usually less than that.

:)

TimRB
January 7, 2009, 11:24 AM
Get a comparator and measure the length of the cartridge to the bullet ogive rather than meplat (tip). Bullet lengths vary quite a bit (at least in the rifle world) so COL variation of several mils is not uncommon. You can convince yourself of this by measuring the length of a few bullets.

Tim

rfwobbly
January 7, 2009, 01:00 PM
There are several issues, most of which 1858 touched on....

Using the wrong seating die. A RN seating die pusher block will push on the corners of a HP or FP round, rather than the central flat part of the nose. (See the attached graphic.) Therefore the OAL will vary with any variation in width of the flat area on the bullet's nose, rather than the height of the bullet itself as one would expect. If using FP or HP bullets, you have to use a flat faced bullet seater.

Compressed powder loads will have a tendency to push the bullet back out of the case.

Poor press design/ function

ranger335v
January 7, 2009, 01:54 PM
"1. The geometric consistency of the ogive of the bullet.
2. The consistency of the inside diameter of neck of the case.
3. The consistency of the force/speed used to seat the bullet.
4. The method used to measure the OAL of the loaded round."

Excellant summation 1858. May I expand on your #2 and #3?

Matching the expander to the bullet OD to the case ID vs. the press applies here. We can't change that much but if we understand it, we can avoid some of the problem.

A tight bullet requires more force to seat (your #2). Using more force to seat places more stress on the ram and the toggle linkages, and vice versa. Varyiing seating forces will push out more or less oil/grease from the several pivot points. That will make tiny differences in how high the ram actually lifts for each cartridge. Using light oil to lube the pivot linkages is better than thick oils and greases.

Lube push-out (and press spring) makes only a little difference, but it's there. Raising the ram smoothly and holding it there a couple of seconds when seating can improve things (your #3).

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