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COL Question

1780 Views 13 Replies 6 Participants Last post by  Paul Workman, 
George Foster
I would like to ask if say I am using Sierra 243 bullets in either a 243 or 6mm would bullets of different weights with the same ogives would give the best accuracy with the same COL? If I am shooting a 6mm and using a 2.860COL (Best accuracy) for the 75HP would a 80SPBT and a 85HPBT (They all have the same ogives) give the best accuracy after you found the best powder charge for each bullet give the best accuracy with a 2.860COL also?
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T
George are you talking about cartridge overall length or bullet jump to the rifling?
The 6MM is a longer cartridge than the 243. So I don’t think that the COL could be the same. Now if you are talking about bullet jump ,or seating depth, it may we’ll be a starting point. But it’s been my experience that even the same caliber (22-250) using the same bullet, seating of the bullet always is different in the rifles
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George Foster
Yes I am talking about the bullet jump, I have a 6mm Ruger that likes the Sierra 75HP 0.50 off the lands. I am curious if say the 85HPBT would like the same jump as they have the same ogive. They would end up being the same COL if I am correct as the 85 would be seated deeper in the case. My other 6mm likes the same 0.50 jump but is a different COL.
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Kentucky Fisherman
The answer depends a lot on two things, George:

1) Is this a multi-shot rifle and do you plan to use the magazine? If so, this limitation may limit how far out you can seat some bullets. If you plan to single-feed the rounds, then this doesn't matter.

2) Are you loading for ultimate accuracy or are these hunting rounds? Tweaking seating depth is important for competition shooting, but if the goal is to whack a deer, coyote, or even a prairie dog, it isn't going to matter.

Bullets from the same maker, with the same ogive shape, and only a small difference in weight may shoot well at the same seating depth. It's worth starting there, then maybe testing a few groups that are 5 or 10 thou longer and shorter. Three-shot groups are usually enough for this type testing. Hope that helps.
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George Foster
KF, what I normally do is find the most accurate powder charge for a given bullet (With the powder I am using) and then I work to find the best bullet seating depth. I find the length to the lands and then load three rounds each starting 0.10 off the lands and work deeper 0.10 at a time. Now for whatever reason four rifles that I shoot all the time seem to like bullets seated 0.50 off the lands for best accuracy. Now as bullet weight increases the bullet is longer so what I am wondering if when I seat the 85HPBT deeper to match the 75HP COL would I achieve best accuracy for the 85HPBT. Sierra tech Paul said in theory it would work but with different barrel vibrations for the bullets it might well not.

When I have a chance I am going to try it to satisfy my curiosity. All my loads fit in the magazines on my rifles but the only time I use the magazines is for deer hunting. For target shooting and varmint hunting I keep a singe shot adapter in the magazines.

Thank You for the thoughtful reply and I fully agree with the points you brought up.
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George Foster
KF, I should have stated when I work up the powder charge in the case of a 6mm I use the standard bullet COL to find what powder charge proves to be most accurate. This is what I do for calibers and bullets I use.
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George Foster
I got up to the club this morning to check on my theory. Well as usual it didn't work out, the loads I had with the 85HPBT loaded to 2.860 COL shot worse then the same load at the standard 2.825 COL just the opposite of what I thought would happen. I am at a loss to know why but at least now I know.
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T
George, is it really 0.5 off the lands? That is 1/2 an inch.
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L
Cartridge measurements

George,

I'm not sure you have an understanding of two measurements from the case head:
1) Case head to bullet tip
2) Case head to bullet ogive

The case head to bullet tip is the simplest of the two measurements but produces
less consistent accuracy-----can vary as much as a few thousandths from the same box of bullets. The most common use of this measurement is for cases that are loaded into a magazine.

A consistent Case head to bullet ogive is one major step in accuracy. A comparator device is usually used for this measurement.

Please forgive me if I've underestimated your knowledge of this part of loading.

There'll be more help on the way if you need it.

A. Weldy
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George Foster
Yes I am aware of all you spoke of as I have been reloading since '72. I do appreciate people trying to help though and I am aware of the distance of the lands in this rifle and my other rifles. The reason I tried this is according to a tech at Sierra it might or might not work as the ogive on both the 75 and 85 Sierra are the same. Yes I know both measurements will vary but I have found it doesn't make a big difference in a sporter barrel. All my sporter rifles will shoot five shot groups of 1/2" or less with the loads they like if I do my part. I could tell though from shooting the 85HPBT's yesterday they needed to be seated deeper from the groups shape.

T.D.C. Yes the best seating depth for this rifle with the 75HP Sierra is .050 off the lands which in this rifle the COAL is 2.860.
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T
George, your earlier post (5:42, 7:29) referred to 0.50 a moving at 0.10 at a time. That threw me off. 0.050 makes more sense.
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George Foster
I'm sorry, I get in a hurry sometimes and don't realize when I type something wrong.
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T
George, no apology needed. I was a bit confused. What is a decimal point among friends.
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Paul Workman
George,
I agree w/ Mr. Weldy far as COL goes being unreliable as a reference in many cases - so much so I only note seating depth to ogive when working up a load. And, I usually single-load (for accuracy) in my magazine rifles, and only consider COL if I must feed from the magazine.

Anyway...FWIW, I 2 have found that regardless of the source of a new/different bullet that starting with the previously (accuracy) established lands to Ogive (seating depth) to be the best starting point for the new load.

One of the advantages to specifying a custom bbl is being able to specify the chamber specs so that the chosen bullet weight/type's Ogive will reach the lands when the COL can be fed from a magazine - something rarely possible with a factory chamber. (The exception being the Wylde or Weatherby [Mark] chambers which feature substantial free-bore.)

Your mileage may vary...
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