Truing for BC - experience with accuracy of Barnes stated BC

[med358-boise]

Was just curious what other experiences might be, I am reasonably confident in the velocity of 2698 fps and the measured drops and the calculations are running a difference of 20% lower between stated and calculated BC.

It is possible, I was screwing things up but just curious of others experiences.

 

[Wachsmann]

So it's been my experience that when using a box velocity of store bought ammo you will need to get a velocity out of you own rifle. My 7mm-08 runs 20 fps slower than what's on the box generally. So I record my velocity's and I also check my rifle at distance usually I try for 800 to 1000 yards and then I see how the loads are flying to that distance and I may need to adjust velocity a little or true the velocity a little. But I try and get multiple shot over multiple outings so I know where my bullet will land. I do reload a lot and shoot a lot. I'm always tinkering with my loads and bullet combos. Sometimes a little too much. But short answer is you need to shoot you box ammo and get the velocity from your gun. I try not to adjust BC that much unless something just is not working out in my drop data. I shoot for about 1/4 minute of error inside 800yrds. With all the other factors 1/4 minute of error is hard to hold. In a perfect world we would use lasers and not have to worry about drop and wind.

 

[med358-boise]

I am not shooting factory ammo and am using a chronorgraph, not sure what in my post would have implied that. I am shooting handloads and chronographing with a Labradar; hence, the comment "I am reasonably confident in the velocity of 2698 fps."

I have validated drops on 4 or 5 different rifles. I normally don't worry about a difference in drop of 1-2 inches inside of 500 yards from expected. In this case, I was 4 1/2 low from expected at 400 yards and when I derived a BC from the actual drops, the calculated BC was over 20% below the stated BC.

 

[ARlife4me]

didn't state you weren't either, only about bc.

bc is measured at their altitude and atmospheric conditions (most likely) as that will change bc.

 

[Idaho Lefty]

Some Bullet Factories "Fudge" the BC Numbers, More than, others.
Sierra and Berger Bullets, ARE usually,.. "Close" to,.. "Spot On".
Hornady's New,. ELD-M's and ELD- X's, are also,.. very "Close" !
I won't Name,.. Who'se,. AREN'T !
Seeing 10-20 Percent difference in BC's can easily be SEEN, with some Bullets !

 

[ARlife4me]

Some Bullet Factories "Fudge" the BC Numbers, More than, others.
Sierra and Berger Bullets, ARE usually,.. "Close" to,.. "Spot On".
Hornady's New,. ELD-M's and ELD- X's, are also,.. very "Close" !
I won't Name,.. Who'se,. AREN'T !
Seeing 10-20 Percent difference in BC's can easily be SEEN, with some Bullets !

i will! NOSLER!

 

[slv hunter]

The difference can easily be 20% at your location. Adjust it and leave your velocity alone.

 

[Tiny Tim]

Are you certain its not in your turret. I have a scope that is spot on for drops at 1,2, & 300 yards, 1/2 MOA high at 400 yards and then back on till I hit that same spot on the next revolution.

I may have this reversed, but generally speaking, if drop error is consistent (say 1 moa) adjust BC (especially inside 500 yards). If drop error increases with distance, adjust MV. Someone correct this if I am wrong.

 

[med358-boise]

I wasn't dialing, I have fixed hash marks with known subtensions.

 

[Wachsmann]

Sorry I guess I read it as if you were questioning you velocity vs BC and I assumed you were shooting factory box ammo using the listed velocity vs what you were seeing and then VS stated BC. It's interesting my 7-08 with the Nosler 168 ABLR have done ok with the stated BC and drop data. I don't have them fully stabilized according to applied ballistic app but I haven't had an issue out to around 800 to 850. Let me correct that statement. They are not fully stabilized at 2000 to 2800 feet where I shoot for fun mainly but at hunting altitude they are just above the 1.5 mark for stabilization. I read or seen on YouTube where people were talking about the ABLR could use a faster twist to reach the stated BC so maybe there is some truth to that. For me I have never used hold over marks except for windage unless they were just MOA marks and then I still dial my turret. 2 of my scopes have BDC marks for xxx velocity of gun x then they equal xxx but I never have used them on those scopes. One has a MOA scope reticle and I used it once or twice just to check the dial up on the scope to see if it was the same as the reticle and those did match. For me I always want a scope that will dial correctly. I've have 2 vortex HSLR with the 1/2 minute elevation clicks and have not had an issues it 10 years. One is on a 300RUM. So it had seen some recoil but it probably only has about 600 shots on it and 2 tumbles when in the backpack during hunting. The other is on a 308 that has probably about 1000+ rounds threw it. 7-08 has the one with MOA reticle. It's some what of a more affordable scope but as for the dialing the turrets, it works well.

 

[Wachsmann]

Bryan,

FWIW, here are the ABLR twist recommendations from Nosler.

JD338

I found this chart on a really old post but it talked about the BC being off as much as 10%.

 

[dfanonymous]

I am not shooting factory ammo and am using a chronorgraph, not sure what in my post would have implied that. I am shooting handloads and chronographing with a Labradar; hence, the comment "I am reasonably confident in the velocity of 2698 fps."

I have validated drops on 4 or 5 different rifles. I normally don't worry about a difference in drop of 1-2 inches inside of 500 yards from expected. In this case, I was 4 1/2 low from expected at 400 yards and when I derived a BC from the actual drops, the calculated BC was over 20% below the stated BC.

BC is pretty negligible inside 500 for centerfire rifles, and drop is almost always relative to MV. Not saying that's what is happening, but that's a pretty sure rule of thumb.

Calculating the true BC correction if you're trying to reverse calculate the number is best done before transonic.

 

[MagnumManiac]

Because you don't know which velocity window the BC was created from, BC can easily be low in some cartridges.
Most bullets are quoted with the BC being in the middle of the window, Sierra list 3 windows because MV plays a huge part in calculating true BC.
BC is not constant either, it changes as bullets lose velocity…

Anyway, you may be onto the correct BC for YOUR rifles velocity.

Cheers.

 

[Wachsmann]

Was just looking over the BC in my app and the 210 G7 BC is .332 and Nosler list a G1 of .661. Will check my app and see what it list on g1 BC which is……. .648. I think this was greatest on the 6.5mm. But if your using one of those lab radars or Garmin units and it calculates a BC then I would test that out over distances and see how well it preforms given you a BC number to use. That would be interesting to see if your drops are spot on using the calculated BC.