Ballistic app.

[DLT]

I run the bullet drop app on an iPhone. Most of the time it’s spot on or very close. I only have the opportunity to shoot 625yds no further. I recently bought a new rifle, chronographed the load and stretched it’s legs. Shooting the 69gr smk which Sierra claims the bc is .301. At 3275fps the app claimed 12 minutes. I was way high. The end result was 10.25minutes at 625 and once on target I was drilling it. I had to change the bullet bc to .425 to get it right on the app. With my other rifles I do have to play with the bc a little bit but nowhere near that big of a difference. I only shot it at that distance to so I don’t know if it would have been spot on at other ranges. Anybody else ran across the bc being off for that bullet ? Thanks in advance

 

[Coyotefurharvester]

Center of bore to reticle height off, chronograph reading off. Wind value. Don't think Sierra would under value the bc performance.

 

[DLT]

Center of bore to reticle height off, chronograph reading off. Wind value. Don't think Sierra would under value the bc performance.

Thanks for the reply ! I do need to measure the center of bore to center of sight and get it accurate. There was no wind, and I did chrono 5 rounds with a es of 18fps and an average of 3275 with my magneto speed. I guess it could be off but I wouldn’t think so. Your right I will agree about sierras bc I’m pretty sure they are higher than it really is.

 

[DLT]

Center of bore to scope center is tricky trick to get. I came up with 1.75”. If it’s not that it’s very close. That’s what I had figured for in the app

 

[Coyotefurharvester]

Elevation, inclination, actual flight distance are also drop parameters. G1, G7 tables for data error entry issues. Otherwise you don't have magic anti gravity bullets.

 

[22vartarg]

since it’s a Sierra bullet, then the BC actually increases the slower it goes. Not too common at all. Could explain you hitting high.

 

[stevenpoplin]

The ballistic coefficient of a bullet is not static. The number you get from a manufacture is an estimate average but even that's not absolutely correct. A G1 BC is not for a long boatail bullet either. You'll want to go with G7 BCs in which you'll usually get 3 for different velocities. But the BC doesn't change at only three specific velocities. It's dynamic and constantly changing throughout the velocity and stability of your bullet's travel to its destination. Some apps have a feature that allows you to adjust your BC at several different velocities along the way. Some like the ones I use you just get one G7 number to play with to tune your app.
I've been using a couple for years along with a Kestral weather unit for shooting out to 2,640 yards, and I always refer to my DOPE chart for a better probability of hitting target. Apps, Kestrals, and chronographs are all electronic devices made to be affordable and by no means absolutely accurate.

 

[DLT]

Thanks guys. I have talked to Sierra because if you look at their listed bc it goes up the slower the slower the fps which did not make sense to me. They have it listed on the web site, on printed posters and catalogs like that I have also seen it on other bullets they offer it’s like it’s backwards. The guy I talked to looked and said it should not be backwards, he said that all they have listed is .301. Anyways the app I use all in all it’s good for what I do and spot on at different ranges I usually just have to fudge the bc a little one way or the other just not as big as .301 to .425. It only goes g1