new Hornady dies, now getting COL variation


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Halo
August 24, 2008, 05:45 PM
I just got a set of Hornady New Dimension dies for 9mm and I'm pretty impressed with them except I'm now getting up to 0.009" variation in my COL. I loaded up a test batch today and was getting COL anywhere from 1.154 to 1.163. I was aiming for a nominal COL of 1.158.

In contrast my old RCBS seating die hardly ever produced any variation at all, so I'm trying to figure out what's different with this new die. Maybe the seating plug in this die engages the bullets further down on the ogive, and the seating depth variation is due to ogive variation on the bullets themselves? I'm loading 124 grain round nose FMJ bullets. The RCBS plug looks like it might engage closer to the tip of the bullet.

I like the alignment sleeve the Hornady seating die uses and would like to stick with it. I bought it so I could use my RCBS seat/crimp die for crimping only. Any ideas on what might be causing this COL variation?

TIA

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Halo
August 25, 2008, 10:44 AM
In case anyone else runs into this, my initial suspicion seems correct. The tapered seating plug engages the bullet at whatever point that bullet's ogive fits the width of the plug, which varies enough from bullet to bullet to cause these variations in COL. By switching to the flat seating plug, which engages the bullet at the very tip, I was able to get consistent COL.

Are flat seating plugs meant exclusively for FP and JHP? This plug does create a very small impression on the nose of my FMJ-RN bullets.

edit: should I even be concerned about variability in the COL? It was never an issue with my old dies, so I never even gave it thought. It seems that if bullets are seated based on ogive contacting the seating plug, even though the COL may be different they would all basically have the same amount of "jump" to the rifling. Is this actually better than aiming for consistent COL? I just hate seeing all those different numbers on the caliper! :)

mkl
August 25, 2008, 02:47 PM
if bullets are seated based on ogive contacting the seating plug, even though the COL may be different they would all basically have the same amount of "jump" to the rifling. Is this actually better than aiming for consistent COL?

I would bet your Hornady dies are giving you a more consistent internal case capacity/volume than the RCBS, even though the OAL of the cartridge is slightly different. Theoretically, that would mean more uniform pressures shot to shot.

Also, since the first point of bullet contact is ogive-to-leades (lands) and not bullet tip, as you suspect, the "jump" would be more consistent. Again, theoretically, this would mean better accuracy.

I've used the word "theoretically" in the above since I have doubts that in a 9mm pistol that is hand held, you would be able to notice the difference that a few thousands of an inch in OAL would make. Perhaps in a machine rest, but at least for me, I could not hold steady enough to see the difference.

Hope this helps.

Halo
August 25, 2008, 03:34 PM
I think you're right, the difference would probably be next to impossible to notice in typical pistol shooting. I'm going to try loading two test batches identical in every respect except for seating technique. One, I will use the ogive seating plug which should yield a consistent bullet jump; with the other batch I will use the flat seating plug to push bullets in by the nose, which should yield a consistent COL.

I'll post the results back here in case anyone's interested.

mkl
August 25, 2008, 03:47 PM
If you are going to try a test, be sure to weigh your powder charge as opposed to throwing it from a powder measure. That will eliminate the +/- one or two tenths grain variation that most powder measures exhibit.

Yes, please post your results. We would be interested in what you find.

Halo
August 25, 2008, 04:45 PM
Yep I always work up my test loads with a trickler to the exact charge for each round.

Shoney
August 25, 2008, 05:56 PM
Halo:
Bullets vary in length quite a bit when measured from tip/hollowpoint to base.
The most accurate way to measure COL is with a comparator. It measures strictly on the ojive. You don't really need to run out and buy one, as they are spendy. They are very handy when measuring and setting the distance to the lands (Freebore) with different shape bullets.

cpttango30
August 25, 2008, 06:13 PM
.009" is nothing to worry about. Plus like Shoney said you really need a comparator to measure OAL correctly. With pistol ammo if it works and does not give you pressure signs go with it. worrying about .009" difference is like worrying about weather the sun will come up or not. You can do it but it ain't doing anything for you to worry.

rcmodel
August 25, 2008, 06:24 PM
Measure a box of factory ammo sometime when you are bored!

It's a real eye-opener with some brands!

rcmodel

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