Gaps in dope and can't shoot at all ranges?

[Snakum]

Shooting yesterday with more experienced guys I was able to more precisely check my tables from 400 to 600 yards. With a good spotter I was able to nail my dope at 400, 500, and 600 yards and found that I was way high. And when I made the corrections on my spreadsheet I tried to rerun the JBM calcs to make the new data 'fit' and also to fill in the lower ranges with better numbers. But even splitting the distance when running new calcs (400 was high while 600 was low) I was still off enough that I could easily miss a small plate a medium ranges. So ...

If you are unable to shoot all ranges (100 - 400) to perfectly firm up your dope, and the JBM calcs don't come close enough to your real data to extrapolate what you can't shoot ... what do you guys do? Do you SWAG the numbers to sort of plot a line of best fit and just go for it?

 

[sinister]

Re: Gaps in dope and can't shoot at all ranges?

If you can't shoot the ranges you want everything is an estimate.

Ballistic tables will get you close, but as you've dicovered none matches a real, fired zero with your combination of weapon, ammo, glass, and weather.

 

[Lindy]

Re: Gaps in dope and can't shoot at all ranges?

The primary purpose of a ballistic program is to get you close on the first round in an environment that is different from that in which you normally shoot.

At ranges of 500 yards or less, your dope will not vary much irrespective of the atmospheric conditions - except for extreme temperature variations with some ammunition, which can affect muzzle velocity.

What I do is to shoot at the longest possible range at which the bullet will reliably stay supersonic. I shoot enough rounds to have a good idea of the average elevation required to hit that target, while carefully recording the temperature, relative humidity, and barometric pressure.

Then I adjust the BC or muzzle velocity in the ballistic program until the elevation calculation for those conditions matches my shooting results.

If I'm using one of Bryan Litz's tested BCs, I adjust the muzzle velocity, because his are usually more reliable than the manufacturer's.

You might find this note informative: Sources of Ballistic Program Inaccuracies

 

[Snakum]

Re: Gaps in dope and can't shoot at all ranges?

Excellent info. Thanks guys!

 

[dmgreene]

Re: Gaps in dope and can't shoot at all ranges?

Two things come to mind. First are the distances exactly what you are calling them and have you checked your scope adjustments to make sure that they are correct.

If the 600 yard distance is actually 610 yards that is almost a half moa correction difference with typical .308 ammo.

I have seen scope adjustments off by as much as 6% from what they are advertised to be. These differences don't show up so much at shorter distances as they do at long range.

I have run several dopes using JBM and every one has had less than 1/2 moa difference from actual range results when using Berger BC's inside of 600 yards. I used the Bryan's G7 BC for the 95 VLD and believe it not it matched to the exact click from 100 to 900 yards. The results that JBM can give you are amazing if you feed it the correct data.

 

[Snakum]

Re: Gaps in dope and can't shoot at all ranges?

Yeah, checked the scope adjustments Saturday with help from one of the guys here. Almost dead on, which I am told is rare. And the targets were lased - they've been up for competition and practice for (probably) years. I'll just have to find a way to shoot all the ranges.

 

[gugubica]

Re: Gaps in dope and can't shoot at all ranges?

The first thing I thought was the scope accuracy, but you said you have that covvered.

What BC were you using, published or derived. And, have you actually chronoed you load?

 

[Snakum]

Re: Gaps in dope and can't shoot at all ranges?

I have read, and calc'd, BCs of .488, .467, and .501 for 168gr SMK. Dunno which is actually correct.

Don't have access to a chrono. Will buy one as money allows.

 

[Mike]

Re: Gaps in dope and can't shoot at all ranges?

Heed Lindy's advice as it is usually spot on.

 

[Woodlanddude]

Re: Gaps in dope and can't shoot at all ranges?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Don't have access to a chrono </div></div>

Guessing velocity is a likely culprit. You can be way off rifle to rifle w/same ammo.

 

[Snakum]

Re: Gaps in dope and can't shoot at all ranges?

Just got back from shooting at 500 yards (as far as I can go here at home) and recording the numbers. Will plug the data into JBM and go back out this evening with new dope. Thanks everyone.

 

[Greg Langelius *]

Re: Gaps in dope and can't shoot at all ranges?

I've faith in Lindy's comments. I spend most of my time here learning from others, and I learn a lot from Lindy.

 

[BOLTRIPPER]

Re: Gaps in dope and can't shoot at all ranges?

to play this game one must have good ammo and a chrono.....

 

[gugubica]

Re: Gaps in dope and can't shoot at all ranges?

all of your BCs might be just a little optimistic. what have you been using for MV?

What is your barrel length? Standard bore?

Try .462 at 2650 and see how close that gets you to actual.

 

[Snakum]

Re: Gaps in dope and can't shoot at all ranges?

Gug ... 24" hvy bbl 1:10 twist w/brake.

Definitely a chrono is the second thing on the BUY list after an LRF. Any recs on a good one that isn't too expensive?

When I first came here I poured over Lindy's website. Learned a lot, for example I had never thought of checking a scope for turret spacing. Always assumed they were correct. Little did I know.

Ran a modified version of Lindy's dope derivation - I could only shoot to 500 yards today - and got very close on the range. Used this:

168gr HPBT BC= .462 Vel = 2750 200MSL 90 deg 30.00hg 75% hum

Only needed plus or minus a tenth mil here and there, otherwise it was damned close. I also got my hand loads close enough to the HSM to use the same dope tables: trim to 2.007, 41.5gr H4895, OAL = 2.800. Just need to make enough ammo to run thru the dope table at all ranges and I'll be ready for the next shoot.

Thanks again for taking the time to reply, everyone.