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Benefits of Adjusting COAL during Load Development

#1: Benefits of Adjusting COAL during Load Development Author: slimjim, Location: Fort Worth TX

Posted: Mon Dec 22, 2014 10:30 am
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I've been playing around with different COALs during load development to quantify the benefits in my 6.8mm SPC II. Here are the results of 3-shot groups at different COALs with Hornady's 110 BTHP on top of 28.6gr of AA2200 in resized Hornady brass and 450 CCI primers. The only change beteen groups was the COAL which was stepped in 0.025" increments: 2.270", 2.295", 2.320", and 2.345" (which was 0.025" off the lands for this rifle). The longer two COALs are achievable with just a magazine modification (cutting out the front to let the bullets be seated longer). Interesting to see that a COAL shorter than 2.295" (standard 6.8 mag length) resulted in improved accuracy. I expected some variation but the average velocity was very consistent between the groups: 2715, 2719, 2697, and 2715 fps respectively. I've never shot a group that small with any of my rifles.

#2: Re: Benefits of Adjusting COAL during Load Development Author: chambered221, Location: Lost for good !!!

Posted: Mon Dec 22, 2014 11:28 am
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slimjim wrote:

Interesting to see that a COAL shorter than 2.295" (standard 6.8 mag length) resulted in improved accuracy.

Not interesting at all....it's what I'd expect !!!
Sweet spots come and go in OAL as they do in powder charges.
Did you shoot any at 2.245 (the book OAL for that bullet)
It would be my prediction you can duplicate the 2.345 results or come pretty close with a shorter OAL.

#3: Re: Benefits of Adjusting COAL during Load Development Author: Elvis, Location: south island New Zealand

Posted: Mon Dec 22, 2014 12:05 pm
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hmmmmm maybe why I didnt get hornady 110hpfb to group any good for me....

#4: Re: Benefits of Adjusting COAL during Load Development Author: Aloysius, Location: B., Belgium

Posted: Mon Dec 22, 2014 1:13 pm
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SJ, are you sure you hit that last target more than once? Smile

As so often you again are amazing me. That kind of group I can only get with my airrifles...

#5: Re: Benefits of Adjusting COAL during Load Development Author: gelandangan, Location: Sydney Australia

Posted: Mon Dec 22, 2014 3:09 pm
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That group would make you smile all day !

#6: Re: Benefits of Adjusting COAL during Load Development Author: slimjim, Location: Fort Worth TX

Posted: Mon Dec 22, 2014 3:50 pm
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gelandangan wrote:

That group would make you smile all day !

it was hard to get to sleep that night!

#7: Re: Benefits of Adjusting COAL during Load Development Author: dhc4ever, Location: Ipswich, Queensland Australia

Posted: Mon Dec 22, 2014 4:00 pm
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Nice shot.
Good write up as well.

#8: Re: Benefits of Adjusting COAL during Load Development Author: Vince, Location: Brisbane AUSTRALIA

Posted: Mon Dec 22, 2014 6:58 pm
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You have definitely got me thinking about what I can achieve with my .223 and .243 Slim. A little tweaking of the COAL just might produce some interesting results.

Would you mind terribly if I borrowed your ideas and adjustments, well they are your intellectual property mate? I'll return them, honest injun.

Now all I gotta do is somehow work out how long the throat is in my rifles.

#9: Re: Benefits of Adjusting COAL during Load Development Author: slimjim, Location: Fort Worth TX

Posted: Mon Dec 22, 2014 7:02 pm
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Vince wrote:

Would you mind terribly if I borrowed your ideas and adjustments, well they are your intellectual property mate?

LOL! Vince, this isn't a new idea. I think reloaders just don't bother to try it. I do small adjustments with Barnes bullets to fine tune but not such a wide spread. I need to try this with some other bullets. Let me know what you find.

#10: Re: Benefits of Adjusting COAL during Load Development Author: Vince, Location: Brisbane AUSTRALIA

Posted: Mon Dec 22, 2014 7:08 pm
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Will do mate, just a matter of fitting some range time into the "honey do" list.

#11: Re: Benefits of Adjusting COAL during Load Development Author: MacD, Location: Canada

Posted: Tue Dec 23, 2014 7:48 am
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Thanks again Slim and a group to make anyone proud.

With average FPS being so close the COAL effects stand out well. When fine tuning my. 223 I found I had to first find the right powder and charge before I was able to squeeze out better accuracy with changes in the COAL. I believe the smaller the case the more difficult it is to fine tune as small changes tend to have more general effects. For example I found that one powder with a consistant charge weight would give me significant FPS increases when I decreased COAL compared to another. Near full case charges seemed to help reduce varations but I haven't done the analysis to say this for sure.

Keep the reports coming. Nothing beats the benefit of good information from a credible source.

#12: Re: Benefits of Adjusting COAL during Load Development Author: Azar, Location: Utah

Posted: Tue Dec 23, 2014 10:48 am
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Slim,

Thanks for sharing your results. One thing I have always been curious about, but have never spent the time and resources to resolve for myself has been the question of an "optimal COAL". Mainly, is there an "optimal COAL" for a particular bullet and chamber/barrel regardless of the powder or powder charge?

If you find that a max charge of 50g of X powder is an accuracy sweet spot and then tweak the COAL to 3.000" to fine tune it even further, is 3.000" going to be the optimal COAL even if the powder charge was reduced to a start load of 45.0g of X powder? Is the optimal COAL independant of the charge weight (or even of the powder at all)? Or is it correlated to the velocity & barrel harmonics with the current load?

Could one find an optimal COAL with a start charge of powder and then use that COAL in finding the sweet spot powder charge wise? If one did it this way (often backwards from how most handloaders do it) would this "optimal COAL" intersect with the "optimal COAL" found after determining the most accurate powder charge?

I personally think it would make for a highly interesting experiment and article, but may be beyond the means of my own abilities...

#13: Re: Benefits of Adjusting COAL during Load Development Author: Dawgdad, Location: On the Prairie

Posted: Tue Dec 23, 2014 12:56 pm
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I have copied this before and it comes from to Berger specifically for VLD bullets but the principle is valid with other bullets too.

Getting the Best Precision and Accuracy from VLD bullets in Your Rifle

Background

VLD bullets are designed with a secant ogive. This ogive shape allows bullets to be more efficient in flight (retain more velocity = less drop and wind deflection). While this result is desirable for many rifle shooters the secant ogive on the VLD bullets produces another result in many rifles. It can be difficult to get the VLD to group well (poor accuracy).

For years we encouraged shooters to use a base of cartridge to end of bearing surface OAL (I will use the term COAL to represent this dimension) which allows the VLD to touch the rifling or to be jammed in the rifling. This provided excellent results for many shooters but there were others who did not achieve top performance with the VLD jammed in their rifling. These shooters were left with the belief that the VLD bullets just won’t shoot in their rifle.

Other groups of shooters were discouraged by our recommendation to touch the rifling. Some of these shooters knew that at some point during a target competition they will be asked to remove a live round. With the bullet jammed in the rifling there was a good chance the bullet will stick in the barrel which could result in an action full of powder. This is hard on a shooter during a match.

Yet another group of shooters who were discouraged by our recommendation to touch the rifling are those who feed through magazines or have long throats. Magazine length rounds loaded with VLDs could not touch the lands in most rifles (this is the specific reason that for years we said VLD bullets do not work well in a magazine). When a rifle could be single fed but was chambered with a long throat a loaded round that was as long as possible still would not touch the rifling.

Until recently, shooters who suffered from these realities were believed to be unable to achieve success with VLD bullets. Admittedly, we would receive the occasional report that a rifle shot very well when jumping the VLD bullets but we discounted these reports as anomalies. It was not until the VLD became very popular as a game hunting bullet that we were then able to learn the truth about getting the VLD bullets to shoot well in a large majority of rifles.

After we proved that the Berger VLD bullets are consistently and exceptionally capable of putting game down quickly we started promoting the VLD to hunters. We were nervous at first as we believe the VLD needed to be in the rifling to shoot well and we also knew that most hunters use a magazine and SAMMI chambers. Our ears were wide open as the feedback was received. It was surprising to hear that most shooters described precision results by saying “this is the best my rifle has ever shot.”

We scratched our heads about this for awhile until we started getting feedback from hunters who were competition shooters as well. Many were the same guys who were telling us for years that the VLDs shoot great when jumped. Since a much larger number of shooters were using the VLD bullets with a jump we started comparing all the feedback and have discovered the common characteristics in successful reports which gave us the information needed to get VLD working in your rifle. We were able to relay these characteristics to several shooters who were struggling with VLD bullets. Each shooter reported success after applying our recommendation.

Getting the Best Precision and Accuracy from VLD bullets in Your Rifle

Solution

The following has been verified by numerous shooters in many rifles using bullets of different calibers and weights. It is consistent for all VLD bullets. What has been discovered is that VLD bullets shoot best when loaded to a COAL that puts the bullet in a “sweet spot”. This sweet spot is a band .030 to .040 wide and is located anywhere between jamming the bullets into the lands and .150 jump off the lands.

Note: When discussing jam and jump I am referring to the distance from the area of the bearing surface that engages the rifling and the rifling itself. There are many products that allow you to measure these critical dimensions. Some are better than others. I won’t be going into the methods of measuring jam and jump. If you are not familiar with this aspect of reloading it is critically important that you understand this concept before you attempt this test.

Many reloaders feel (and I tend to agree) that meaningful COAL adjustments are .002 to .005. Every once in a while I might adjust the COAL by .010 but this seems like I am moving the bullet the length of a football field. The only way a shooter will be able to benefit from this situation is to let go of this opinion that more than .010 change is too much (me included).

Trying to find the COAL that puts you in the sweet spot by moving .002 to .010 will take so long the barrel may be worn out by the time you sort it out if you don’t give up first. Since the sweet spot is .030 to .040 wide we recommend that you conduct the following test to find your rifles VLD sweet spot.

Load 24 rounds at the following COAL if you are a target competition shooter who does not worry about jamming a bullet:
1. .010 into (touching) the lands (jam) 6 rounds
2. .040 off the lands (jump) 6 rounds
3. .080 off the lands (jump) 6 rounds
4. .120 off the lands (jump) 6 rounds

Load 24 rounds at the following COAL if you are a hunter (pulling a bullet out of the case with your rifling while in the field can be a hunt ending event which must be avoided) or a competition shooter who worries about pulling a bullet during a match:
1. .010 off the lands (jump) 6 rounds
2. .050 off the lands (jump) 6 rounds
3. .090 off the lands (jump) 6 rounds
4. .130 off the lands (jump) 6 rounds

Shoot 2 (separate) 3 shot groups in fair conditions to see how they group. The remarkable reality of this test is that one of these 4 COALs will outperform the other three by a considerable margin. Once you know which one of these 4 COAL shoots best then you can tweak the COAL +/- .002 or .005. Taking the time to set this test up will pay off when you find that your rifle is capable of shooting the VLD bullets very well (even at 100 yards).

Regards,
Eric Stecker
Master Bulletsmith
Berger Bullets

#14: Re: Benefits of Adjusting COAL during Load Development Author: Dawgdad, Location: On the Prairie

Posted: Tue Dec 23, 2014 1:13 pm
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FWIW I saw a similar sweet spot with .223 bullets seated shorter than mag length in some of my AR's. Like Slims photos not that the other groups are that horrible but one is just consistently better. Sub half MOA compared to just under 1 MOA for mag length.

Azar - I have not tested your theory but am doubtful there is an always magic sweet spot OAL for a cartridge. The time of flight in the barrel and barrel harmonics are optimized for a given pressure and velocity so that the bullet exits the barrel at a very consistent node. Changing the bullet weight or the powder charge would change this sweet spot some what by altering the barrel time and the standing wave that is generated by the discharge of the case.

This is why the conventional way to get a good shooting load is to run the OCW or ladder lest to find a powder charge that gives the least vertical dispersion and then tweak the OAL to get the smallest group using that load.

#15: Re: Benefits of Adjusting COAL during Load Development Author: chambered221, Location: Lost for good !!!

Posted: Tue Dec 23, 2014 2:53 pm
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Dawgdad, I too have posted that article some time ago in this forum. I've referenced it many times while being involved in such discussions at clubs I frequent. It amazes me how many hand-loaders will stick to the in/near the lands theory even when the bullet manufacture is saying it.

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