Nosler ballistic coefficients accurate?

[mcseal2]

I had used Nosler ballistic tip bullets for many years, and now love the terminal performance of the accubonds. I especially like that my rifles shoot the same with both bullets so I can practice & coyote hunt with the cheaper BT bullets and load the AB's when heading out for bigger game.

I have always noticed that the ballistics programs show that my trajectory should be flatter than it actually is. I chronograph all my loads so the BC should be the only variable. I purchased "Applied ballistics for Long Range Shooting" the book last winter, and found that their BC tests had the 7mm bullets nearly .05 lower than Noslers posted BC. Other calibers varied in their results but all were lower than Nosler's version. Anyone else seen this or have a system for finding a more accurate BC to predict trajectory/wind drift with?

 

[royinidaho]

develop a drop chart by shooting at several distances out to the limit of your rifle.

Record the velocity of each shot.

Measure the drop at each distance or count clicks and measure if adjusting for each distance.

Feed this data into something like JBM after entering the proper and accurate set up data.

Four items will be critical.

1. line of sight above line of bore.
2. Accurate muzzle velocity.
3. Accurate drop measurements.
4. Accurate atmospheric conditions

If the initial drop data doesn't fit then, adjust MV to see if it can be made to fit. If not the go back to the original measured velocity and change bc to make it fit.

If it doesn't fit with this, adjust MV and bc to make it fit.

If it still can't be made to fit redetermine the line of sight about line of bore.

There ain't no free lunch.

Once you get it to fit your MV may not be as measured and your BC may not be as published but you will hit what your shootin' at more times than not.

I've been told that different barrels cause a difference in BC which is a consideration also.

Also I find it difficult to notice any difference in the third decimal point value. Thus my BCs are logged as 0.XX instead of 0.XXX.

 

[coyotezapper]

Royinidaho, excellent explanation. The only thing I would expand on is barrel condition. Some barrels are smooth and some are rough. A rough surface can scuff up the bullet causing it to drag a little more. This has been my experience with factory barrels. Computer programs and bc's are only a starting point and you should always shoot the bullet and back into your drops and adjust as Roy suggested.

 

[mcseal2]

Thanks for the replys. I will begin doing the tests myself once I have my loads perfected. I want to get my data all right before having Leupold build turrets for a couple of my rifles.

 

[Pdvdh]

I purchased "Applied ballistics for Long Range Shooting" the book last winter, and found that their BC tests had the 7mm bullets nearly .05 lower than Noslers posted BC. Other calibers varied in their results but all were lower than Nosler's version. Anyone else seen this or have a system for finding a more accurate BC to predict trajectory/wind drift with?

I would use the BC values from the "Applied Ballistics for Long Range Shooting", before I would use the Nosler factory BC values.

I determine bullet BCs by collecting (chronographing) MV data, and collecting bullet velocity downrange @~ 1000 yds. BC can then be determined based on the difference of these two velocities, and the known atmospheric conditions (station pressure, temperature, and humidity) at the time the 1000 yd velocity data were collected. Just don't shoot your chronograph or chronograph skyscreens.

I only go through this process with bullets that have already proven accurate enough for hunting at 1000 yd distances, after I've developed a load with sufficiently low ES and SD. I don't bother with this process for bullets I only intend to take game with at ranges closer than 500 yards. The three bullets I've collected 1000 yd velocities for to date all matched the BC values tested and provided in Bryan Litz's book. So I have high confidence in those bullet BC values from Mr. Litz's book.

The method the others have provided can generate a drop chart that should be pretty accurate for the location where the bullet drop information was collected, but as noted, it can result in the use of inaccurate BC and/or MV data. Change location from 1000' elevation to 7000' elevation and use those potentially errant BC and MV data in a ballistics program, and the quality of the predicted dope could be compromised. Since the BC and MV data input into the ballistics program could be incorrect values.

 

[mcseal2]

That is what concerns me, my load testing will be done at 1200ft elevation at home and my rifles will go with me this fall to hunt mule deer at 7-9000ft in very different conditions. I don't plan to do any 1000yd shooting at game, but I'd like to have a turret built for my Leupold to shoot up to 500 there. I practice 300-600 regularly at home and have taken game up to 478yds and varmints farther. If I read their website correctly Leupold doesn't actually build turrets to match your measured drop, just use your BC, MV, and their ballistics program.

I found a spot last trip in the foothills I can shoot before hunting. I always check my zero when arriving for a hunt on a improvised target (old metal real-estate sign & spraypaint) and shoot 300-500yds to check my point of impact before heading out to hunt.

 

[coyotezapper]

That is what concerns me, my load testing will be done at 1200ft elevation at home and my rifles will go with me this fall to hunt mule deer at 7-9000ft in very different conditions. I don't plan to do any 1000yd shooting at game, but I'd like to have a turret built for my Leupold to shoot up to 500 there. I practice 300-600 regularly at home and have taken game up to 478yds and varmints farther. If I read their website correctly Leupold doesn't actually build turrets to match your measured drop, just use your BC, MV, and their ballistics program.

I found a spot last trip in the foothills I can shoot before hunting. I always check my zero when arriving for a hunt on a improvised target (old metal real-estate sign & spraypaint) and shoot 300-500yds to check my point of impact before heading out to hunt.

Try these guys they are great.

http://www.kentonindustries.com/