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HELP.....Meplat uniforming tools

 
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  #8  
Old 03-17-2005, 11:51 AM
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Re: HELP.....Meplat uniforming tools

If this tool of yours measures bearing surface, how can it measure just the bearing surface if it indexes off the base instead of where the boat tail and the ogive meet the shank? WOuldn't you just be measuring base to one end of the bearing surface or have I missed something? [img]/ubbthreads/images/graemlins/confused.gif[/img]
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  #9  
Old 03-17-2005, 12:04 PM
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Re: HELP.....Meplat uniforming tools

Harv,
Im going to try to take some pictures and do a little writeup later on in the day. My camera was acting up last time I tried to use it, so well see...
My tool is a very simple design, and depending on what you have laying around the house, very cheap to put together.
As to if you will see any improvement at 800 yds, I believe that you absolutely will. Maybe a brief history lesson....

This is how the whole thing started -- Larry Bartholomew & Dr. Oehler set up a model 43 at 1,000 yards, and took a bunch of measurements of different chamberings and bullets. The raw data - about 100 pages worth - was never published, but it was circulated among many industry insiders and top 1000yd benchrest shooters. One thing that was quickly noticed is that the variation in BC of bullets from the same box was significant. On the data from the 43, you get extreme spread (ES) and standard deviation (SD) of ballistic coefficient, just like ES and SD for velocity with any chronograph. It was remarkable the number of bullets which had an ES for ballistic coefficient of .020 or greater - both bullets from a green box and bullets from a yellow box. The bullets which had the lowest ES for ballistic coefficient were the Hornady A-max, in a number of calibers. That doesn’t mean the A-max are better bullets overall, just that they, with their plastic tips, had a low ES for BC variation.

It is very easy to check on whether or not an ES of .020 for ballistic coefficient matters at 1,000 yards. Simply use a ballistics program, and enter all the numbers for a bullet. Let's say the velocity is 3,000 fps and the ballistic coefficient is .540. Run the program, and look at the total drop for the bullet at 1,000 yards-- you'll see a number, something like 240 inches.

Now change only the ballistic coefficient; instead of .540 use .520, and run the program again. You'll get a different number for total drop, something like 246 inches.

That means that for a change in BC of .020, you get six inches difference in drop -- practically speaking, you have 6 inches of vertical when everything else is perfect.
You can also run the comparisons of the variation of wind drift with that same .020 ES. Most match bullets will have about 2.5 inches of wind drift more for the lower bc bullets.
From this data we can determine that it is mechanically impossible to get production bullets to shoot samller groups than 6"x2.5" groups at 1000yds due to these variations.

Dave Tooley was one of the people who saw the Bartholomew data, and it lead to the meplat trimming tool as a part of his study on what makes for good bullets. He eventually got a 43, and initial testing showed that trimming the meplat COULD reduce variations in BC. The tool is not a panacea - if you trim too much of the meplat, you can dramatically reduce BC. And not every bullet shows a significant reduction in ES when the meplats are trimmed - there are other factors that lead to variations in drag, but generally good bullets did show a reduction in ES for BC when the meplats were trimmed slightly & uniformly; not quite as good as using plastic tips, but good enough to remove about half that 6-inches of vertical, and half of the 2.5-inches of horizontal. Futhermore tests have shown the uniforming the meplats on match bullets and weighing them virtually eliminates the usual weight variance encountered in non-uniformed bullets.
Most tool designers and shooters today recomend removing somewhere between .003, and .010 from the meplat, with a small number going as far as .012. Bullets trimed to these dimentions typically loose 2%-3% of their rated BC in trimming, but often show extremly low BC variation. It should be noted that the more material that is removed, the more the BC will be reduced, and perhaps the best option is to remove only enough to produce the same measurement.on a batch of bullets. Many top shooters including G.David Tubb have reported identical retained velocity readings for strings of bullets.

My groups at 1350yds shrank from 16-21 inches of vertical to between 8.5-11 inches of vertical when sorting bullets by bearing surface length and uniforming meplats on my 220SMKs. My individual groups from the last 3 sessions at 1350 will comfortabely fit on a sheet of paper. I removed (depending on the specific bullet) about .006 from the meplat, and I notticed that the bullets were hitting slightly lower than usual, but im not convinced that it was entirely due to the BC being reduced, we had a high pressure weather condition for those last 3 outings.
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  #10  
Old 03-17-2005, 12:11 PM
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Re: HELP.....Meplat uniforming tools

Sorry for the confusion GG,
I sort by BS first, then trim the meplat.
2 different tools.
I am of the opinion that if you have batches with the same BS length and base to ogive length, trimming from the base will give a more reliable outcome.
I went back and sorted the BS sorted bullets from a couple of weeks back, for base to ogive as well. In most cases, the groups only lost a bullet or two. What im trying to say is that in these bullets, most of the base to ogive variation was in the BS length.
Make any sense?
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  #11  
Old 03-17-2005, 12:16 PM
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Re: HELP.....Meplat uniforming tools

Ok. Got it now.
Yes it makes sense to me. I spent several hours yesterday doing the same thing.
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  #12  
Old 03-18-2005, 04:22 PM
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Re: HELP.....Meplat uniforming tools

To those interested in whipping up a tool of their own, ill try to get some pics and a description up by sunday. The girlfriend comes into town this weekend along with her better digital camera (I think mine is dead). Won't have time for shooting, but maybe enough to sneak on here and make a post.
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  #13  
Old 03-19-2005, 06:45 AM
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Join Date: Dec 2004
Location: Memphis,Tn
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Re: HELP.....Meplat uniforming tools

I also have a homemade one,with some new tooling comeing.I'm gonna re-swag to start with,then on to my length. [img]/ubbthreads/images/graemlins/tongue.gif[/img]
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