Help me build up my range card... BC confusion...

[srv656s]

I put together a range card a while back for a load I cooked up using www.jbmballistics.com. I was using 155 grain scenars and they were going about 2850 fps. When I took this load to a competition, it was a bit off. As I got out to extended ranges, the rounds were impacting lower than they should have been. I was talking with a fellow Snipershide member at the range this past weekend about G7 BC’s and decided to try using that ballistic coefficient instead. I’ve since changed my load slightly so my rounds are going 2875 fps, but other than that, the data’s exactly the same except for G7 vs G1 BC.

Here’s the calculated data: G1 @ 2850 is on the left and G7 @ 2875 is on the right:

That’s a huge difference. I don’t think the G7 can be right, based on my experience. Can someone clue me in on the right info to punch in for the 155 grain Lapua?

Here’s what I used for the table above (on the right):

Shawn

 

[memilanuk]

Re: Help me build up my range card... BC confusion...

You do realize that when you change forms from G1 to G7, you don't re-use the same G1 BC number? Try something more along the lines of .236 for your G7 BC instead of .508.

 

[Short-bus]

Re: Help me build up my range card... BC confusion...

http://02b0516.netsolhost.com/blog1/?p=62

There's some good reading on the G7 Ballistic Coefficients that most of us should be using (IMO) which will give us better calculated data.

I was comparing numbers between G1 and G7 BC's on my load and at the closer ranges the difference in calculated drop was just a few inches until the ranges got way out there. The biggest differences was in the maximum range until the round went subsonic. With my load data there was a almost 150yard difference between the two BC's.

Seems that we need to head out to my long range spot and get some targets out at 500+ meters and do some real world comparison between the 2 types of BC's.

Branden

 

[Rob01]

Re: Help me build up my range card... BC confusion...

Use .508 as the G1 and you'll be fine.

 

[srv656s]

Re: Help me build up my range card... BC confusion...

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: memilanuk</div><div class="ubbcode-body">You do realize that when you change forms from G1 to G7, you don't re-use the same G1 BC number? Try something more along the lines of .236 for your G7 BC instead of .508. </div></div>

Didn't know that. I've never used anything but G1.

 

[MitchAlsup]

Re: Help me build up my range card... BC confusion...

http://www.appliedballisticsllc.com/index_files/Lapua_Scenar.pdf

 

[srv656s]

Re: Help me build up my range card... BC confusion...

I re-ran the data using .236 for the G7 BC and also re-ran the G1 data using 2875 fps so all things were equal otherwise:

 

[MitchAlsup]

Re: Help me build up my range card... BC confusion...

At this point, I strongly suspect the G7 data is a better indicator of what will happen in the field (under those conditions).

For this level of accuracy, using a single G1 number is not sufficiently accurate (except to get 'on paper' at 500 yards).

 

[Lindy]

Re: Help me build up my range card... BC confusion...

Well, I run Lex Talus Field Firing Solutions, tailored to my load, with data verified at 1000 yards, i.e., the program's output is tailored so that it matches my shooting data at 1000 yards.

I ran JBM using the average G7 BC for my bullet using Bryan's data, to see if it would produce results close to what FFS does.

It doesn't.

Nor does Bryan's program.

By modifying the G7 BCs, I can get the Bryan's programs to match the FFS output. The BC I get from doing that also makes JBM produce the same program output as FFS.

Since my FFS is "tuned" to match my shooting data at 1000 yards, where accuracy is most essential, I see little reason to use a G7 B.C. In addition, the intermediate distances between 100 yards and 1000 yards are essentially identical for all three programs, to within 0.1 mils at all distances.

And my tuned FFS data also agrees with Exbal using the 3 Sierra-supplied B.C.s in velocity ranges, to within 0.1 mils at all distances out to 1000 yards.

It's possible that there might be an advantage to using the G7 BC at ELR.

We all remember the old saw about, "A man with one watch knows what time it is. A man with two watches is never sure." It would appear to be the same using ballistics programs.

Once you "tune" the program to match your shooting data, there isn't much difference between the four programs I just mentioned.

Therefore, my choice of ballistics programs comes down to the feature set of the program. Since FFS was designed with a feature set for military sniper teams, that will continue to be my choice.

 

[Rob01]

Re: Help me build up my range card... BC confusion...

I have always used the G1 BC and never have been off more than .2 mils at 1000 yards and have also shot the numbers to 1650 yards and was within an MOA. I am going to keep using the G1 BCs because they work for me even though it doesn't seem to be cool anymore

 

[Lindy]

Re: Help me build up my range card... BC confusion...

Roger that, Rob.

I'm way too old to worry about cool.

I'm perfectly willing, though, to use a better way, when I find one.