Range Report Calculating velocity from drop data

[jeffersonv]

I may be able to use some help from some of you ballistics ahh gentlemen. I used JBM to get close to the numbers that indicate a velocity that I am a little skeptical of.

6.5 Creedmoor shooting 140 AMAX. 4350 powder (42.3). 24" barrel!

Data collected at the Pala range on a pleasant and sunny 80 degree day. 500' elevation.

Yardage and drop in mils
220 – .6
315 – 1.4
415 – 2.2
490 – 2.7
545 – 3.1
600 – 3.6
650 – 4.1
770 – 5.1
850 – 6.2
885 – 6.8

To match the curve I had to give myself about 2890 fps which I would consider a just a tad hot and not really needed. If this velocity proves true I might want to look for the next lower node.

Oh yeah, I also chronographed this day and got the same numbers I ended up with using the JBM, which is strange because the last time I chronographed this load I got 2750 fps. Much skepticism also comes from using a Shooting Chrony. Sometimes I feel like a large question mark is hovering over me on days like this.

Oh yeah, I input a BC of 590(G1) to try to match the curve. My results say the BC is 550 however. Just a JBM glitch I suppose?

I tried one round of RL17 this day and stopped right there. I was well into 6.5x284 territory according to my chrono. 41.5 grains. Will have to start lower, though the brass looked OK.

Any input is appreciated.

Thanks.

 

[jbmillard]

Re: Calculating velocity from drop data

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jeffersonv</div><div class="ubbcode-body">
Oh yeah, I input a BC of 590(G1) to try to match the curve. My results say the BC is 550 however. Just a JBM glitch I suppose?
</div></div>

This is a very inaccurate way to calculate velocity. The errors in drop measurement pretty much make it worthless.

Part of the problem is that you don't have one curve, unless you shot ONE bullet and measured the drops at those ranges (if you did this, disregard this post). You really have a bunch of curves and you're taking part of each and making one curve, but there's no guarantee that the all the bullets you fired have the same velocity, point of aim, etc.

If you read my FAQ, you'll see I don't recommend doing this.

Brad

 

[jeffersonv]

Re: Calculating velocity from drop data

OK! FAQ's here I come.

Thank you for the response JBM. I was thinking I might not get one!

Shooting ONE bullet and measuring the drop at all ranges....
I guess that would entail shooting through multiple cardboard targets?

I was hoping to confirm my chronograph data this way. No such luck I suppose.

One question though should you get back to this thread -
If I input a BC of 590 in JBM, why do my results give me a BC of 550? Just curious.

Thanks again!

 

[MALLARD]

Re: Calculating velocity from drop data

you should try using the g7 and not the g1

your method is flawed for many reasons ,

1 your not measuring baro pressure
2 range uncertainty
3 scope tracking error
4 bullet weight, mv, bc variations
5 sun spots

 

[wadebrown]

Re: Calculating velocity from drop data

A couple of questions:
1) Is this an accurate load for your gun?
2) Is this an accurate load for your gun?
3) Is this an accurate load for your gun?
4) Are there pressure signs on the brass?
if the answers for 1-3 are yes and the answer to 4 is no then it seems like you have a keeper load.

Since you have established a drop table why does the velocity really matter? In the past 4 years I have developed loads for shooting F-class for 4 different calibers and about 8 barrels and I have not worried about velocity. If someone asks me what velocity am I looking for I say has to be supersonic at least 50-100 yards on the other side of the longest range I want to hit a target.

All of the above applies to target shooting on a KD.

wade

 

[jeffersonv]

Re: Calculating velocity from drop data

It's hard to find one load that is really stands out in my rifle. They all shoot tight (about .5 MOA), at least all the ones from 41.3 to 42.7 which is all I'm concerned about.

the yardages are KD and are pretty accurate at least as far as I know. The scope (PST) tracks well. At least it is repeatable.

Baro pressure is a biggie and I wish I had that one. I tried to shoot between those damn sun spots but you never know...

It would be nice to know my MV and I was trying to get a ballpark figure to try to validate my chronograph. The reason being is I want to get a FDAC from Adaptive and a 6.5 Creedmoor card from Impact Data Books. It kind of hard to know which one to get without knowing your MV, and I'm skeptical of what I am showing.

 

[gstaylorg]

Re: Calculating velocity from drop data

jeffersonv,
Next time you go out to Pala, ping me before hand, and I can bring my Chrony and/or MagnetoSpeed if you want to compare values to yours. I just got back from the short range there this afternoon and was using the MagnetoSpeed to get some MV data for a re-barreled rifle. Although I can't make it out for every single open shoot, I'm there a lot, so I'd be happy to help out.

 

[jeffersonv]

Re: Calculating velocity from drop data

That's a really nice offer gstaylorg.

Are you free on mondays or fridays?

Thanks.

 

[gstaylorg]

Re: Calculating velocity from drop data

Not normally, but I thought I might go out this coming Friday as soon as they 're open, from 8:00 until maybe 9:30 or so. I have a match coming up Sunday and I need to get some dope on a re-barreled GAP I just got back so I can use it in the comp. If your schedule is open then, just let me know and I can bring either the MagnetoSpeed, the Chrony, or both.

 

[jeffersonv]

Re: Calculating velocity from drop data

That sounds like a possibility.
I could be mistaken but the range doesn't open till 10:AM, unless I'm being too formal?

 

[gstaylorg]

Re: Calculating velocity from drop data

Rec shooting 8/24: 8:00 AM - 2:00 PM per the NCSA Pala Scedule. It better open at 8:00 because I plan to be there at 7:45 to get my targets and other stuff set up. It'll be a long wait if they don't start until 10:00 (LOL). Anyhow, I'll be ther from around 8-9:30 or so. If you don't catch me then, there will be other opportunities as I'm out pretty regularly. I don't go as often Mon/Fri due to work, but I'll be doing some load testing in the next couple weeks and I'd rather not fight the afternoon wind if I don't have to. So I'll probably be back for the either Mon or Fri time slot in the near future because it's relatively calm in the AM.

 

[jeffersonv]

Re: Calculating velocity from drop data

OK. Is your real name Taylor?