Thread: How does barrel twist affect ballistics calculations?

Hey All

I just got a set of Sig BDX binos and have been tinkering with the AB set up, I normally use the hornady app to work out dial up but there is a bit of a difference between the 2 calculators when im working out the same distances

My question is

Why does the hornady app require the barrel twist rate and the sig app doesnt and could this be the reason for the difference?

Cheers
Strider

I would recon the twist rate affect the stability of a given weight of bullet. If the bullet is unstable, then it will affect the BC for that bullet. But I am not convinced the ballistic apps are that advanced to consider whether the twist rate for your selected bullet will be optimal or not and therefore the ballistics will be calculated accordingly - but I might underestimate how advanced they are.

I had a big discrepancy between the Shooter app and the 4Dof calculator. Turned out the 4Dof calculator was more correct and that the Shooter app had some crazy factor it applied if you turned spin drift and coriolis compensation on. Nowadays I just use StelokPro - seem to be the most trustworthy for me.

What distance, how much difference, and what drag profiles are you using?

using g7 for hornady app, using bullet library info in the sig app which i assume is correct.

discrepancy starts at 200m .25 moa and increases past that point.

In the real world would you notice 1/4 moa given all the varifles.

Twist rate shouldn’t be giving that much discrepancy at 200m. I would hazard a guess it’s either the drag models or a missed variable somewhere. What projectile are you using?

139 gr sst 7mm
ive changed the bc to g1 and input it manually.

the hornady app and bullet info doesn’t have a G7 bc so the library in the bdx app may have incorrect info.

strelok and hornady app sing the same tune but both are using g1

will have a tinker

I think the BDX use an Applied Ballistics calculator. AB have the G7 at 0.231 and G1 at 0.451. Hornady have a G1 of 0.486. This might be the discrepancy.

I got the data from: https://bisonballistics.com/bullets/...-28202-284-7mm

Hey strider the short answer to your question is that twist rate is unlikely to be the reason for the differences.

I would suspect it’s either environmentals or the fact that Hornady 4Dof apparently use Doppler readings to map the projectiles entire flight rather than just assign an average bc.

The sigs are great in my experience but will only give you a ballistic solution out to 800yards. Sig recognise there are accuracy limitations to its ballistic calculations past this point.

As you get past about 500m with your 139gn projectile it’s factoring in the environmentals and your altitude that will give you accurate dope.

The sigs don’t factor in any of this unless you pair a kestrel.

The Hornady app will factor it in but only if you manually set the correct data or have cell reception to update the weather in the app.

Hopefully some of that made sense.

I probably should have asked, how far do you need to shoot?

Thanks for all of the info. I reckon i’ve got enough to work out what’s going on now. will see how it goes next time i’m out slapping the gongs.

Max for deer would be 400m, then just out as far as i can on the gongs.

Using G1 for hornady app instead of dof

Cheers for the input