BC numbers before and after

D

Dave Tooley

Active member
I stated a couple of weeks ago that I would report accurate BC numbers in regards to meplat trimming and pointing. Below are numbers for a 30 cal. bullet over 200 grs. with the BC averaged between 500 and 1200 yds.

Stock G7 BC .289
Conservative meplat trim G7 BC .286
Pointed G7 BC .298
Trimmed and pointed G7 BC .297

Pointing increased the BC 3.1%

That's inline with the testing I did for Sierra by installing platic tips in their MK's.

Dave
 
Thanks, Dave, that's most interesting. Did you get E.S./S.D. data for BC, like when you did the tests on the tipped bullets?
 
Charles

No I didn't. It was a long day of testing. The first five hours dealing with a virus in three different computers. When we got rolling it was a couple shots then on to the next load before something bad happened. While not a large sampling the numbers are accurate.

After having identified the importance of meplat uniformity years ago and testing and observing all the goings on with pointing I now believe that pointing greatly reduces the influence of the meplat. I have done things at extreme distance we could only dream of 18 years ago.

Dave
 
Thanks Dave , Question . When shooting 187 BIBs would you repoint as well as trim now you have shot these tests .. While i would not knock back an increase in BC some shoots the only thing i really want is the best accuracy ..JR.. Jeff Rogers
Dave Tooley said:
I stated a couple of weeks ago that I would report accurate BC numbers in regards to meplat trimming and pointing. Below are numbers for a 30 cal. bullet over 200 grs. with the BC averaged between 500 and 1200 yds.

Stock G7 BC .289
Conservative meplat trim G7 BC .286
Pointed G7 BC .298
Trimmed and pointed G7 BC .297

Pointing increased the BC 3.1%

That's inline with the testing I did for Sierra by installing platic tips in their MK's.

Dave
Click to expand...
 
Hi Jeff,

Maybe Dave will reply, but he's been shooting those military chamberings. I've been trimming and repointing the BIBs for a year now. I do this for consistency reasons, but there is a BC increase as well -- As good as Randy is, he has no control over those pinched-off jackets that come from J-4.

Actually, I've been shooting some experimental bullets I'm not suppose to talk about. So I won't. Get up with Randy & see what's what.
 
Jeff

Charles is right. It's all about consistency. For pure accuracy either trimming meplats and/or pointing can improve consistancy. Trimming meplats is hard to screw up. The only thing you could do is trim more than needed and reduce the BC. Pointing on the other hand has some risk attached to it. It is a fact that once the diameter of a meplat reaches about .1 to .15 of the caliber diameter BC starts to go down. Something about additional shock waves that are attached and travel down the sides of the bullet. There needs to be a bit of a bow wake for the bullet to fly, seaching for a word here, I guess unencumbered through the air.

I have to wonder what happens to the shape of the ogive after the bullet is pointed. Jackets are thin and want to spring back after the initial forming. Some have lead to the tip and some don't. Over point them on the nose and I think you can create a condition similar to nose slump that happens during acceleration down the barrel. Different ogive shapes give you different BC's. I think one of the advantages of the BIB 187 is that it's lead line is very low in the ogive, no lead in the nose to slump. God knows they get driven hard at times. Some of you know I've been working with Hornady and others for the past two years developing new heavy bullets in 338 and 30 cal. for military use. One thing Hornady's learned, I'm sure the hard way, is that over reaching with the pointing die can cause problems. I just shot and compared Hornady 338-285's in the stock pointed condition and the same bullet with the meplat closed even more. The BC's were identical. Without changing, lengthening, the ogive we can only do so much, about 3%, to improve the BC.

Dave
 
Did your test indicate BC consistency from stage to stage?
That is;
Did nose trimming improve/hurt BC consistency?
Did pointing alone improve/hurt BC consistency?
And the combination, or doing neither?
 
Mike

We didn't have enough data to look at that in this test. I wanted to but it didn't work out this time. So many questions and so little time/money. From past tests trimming meplats or at that time installing plastic tips greatly improved the BC consistancy. Either method yielded the same reduction in BC spread. Tests at that time were shot at 300 yds and at 600 yds with an Oehler 43 and acoustical screens at the target. The numbers may not have been completely accurate but for comparative purposes they were what we needed.
From what I've seen and what I know about pointed bullets, the effect of the meplat is minimized to a point that it has very little influence on the BC consistency after pointing. With that being said if I was pointing bullets, just to be on the safe side I would trim first for concentricity then point.

Dave
 
Thanks Dave , Exactly what i had years ago in the repointer i made , it distorted the front of the jacket (j4s) enough to make the Vertical dispersion of a group larger than unsorted bullets ..I lose only 1/2 a minute in elevation from selected melpated to selected non meplated with zero pointing .Bullet AOL only changes from 1.325 back to 1.321 to end up with true meplats thanks to Randy's excellent 187s.. I have had a few targets with the Bartlien this year that have the 1st 5 of 10 go into sub 2 inch so the pit crew say ,it's just the fickle conditions jusy won't allow me to find out if the other 5 can follow .. Still more matchs left in the year so there's hope ..Will email you about pointer ..JR..Jeff Rogers
Dave Tooley said:
Jeff

Charles is right. It's all about consistency. For pure accuracy either trimming meplats and/or pointing can improve consistancy. Trimming meplats is hard to screw up. The only thing you could do is trim more than needed and reduce the BC. Pointing on the other hand has some risk attached to it. It is a fact that once the diameter of a meplat reaches about .1 to .15 of the caliber diameter BC starts to go down. Something about additional shock waves that are attached and travel down the sides of the bullet. There needs to be a bit of a bow wake for the bullet to fly, seaching for a word here, I guess unencumbered through the air.

I have to wonder what happens to the shape of the ogive after the bullet is pointed. Jackets are thin and want to spring back after the initial forming. Some have lead to the tip and some don't. Over point them on the nose and I think you can create a condition similar to nose slump that happens during acceleration down the barrel. Different ogive shapes give you different BC's. I think one of the advantages of the BIB 187 is that it's lead line is very low in the ogive, no lead in the nose to slump. God knows they get driven hard at times. Some of you know I've been working with Hornady and others for the past two years developing new heavy bullets in 338 and 30 cal. for military use. One thing Hornady's learned, I'm sure the hard way, is that over reaching with the pointing die can cause problems. I just shot and compared Hornady 338-285's in the stock pointed condition and the same bullet with the meplat closed even more. The BC's were identical. Without changing, lengthening, the ogive we can only do so much, about 3%, to improve the BC.

Dave
Click to expand...
 
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