Why inflate BCs? What might impact BC other than velocity?

[Iamero]

I was reading some other threads on here and have read on here in the past that sometimes the BCs are inflated by the manufacturer. I was curious if that was intentional strictly as a selling point, or if the BC is tested in a lab rather than real world conditions.

I've been pretty happy with the Hornady ELD-M performances across the calibers, but haven't had a chance to shoot over 800 in quite a while so I might not be shooting far enough to notice an inaccurate BC make a substantial difference.

What made me ask this is that I have some Nosler 105 RDFs coming and I noticed that they boast a .571 G1 BC, while the Hornady 108 ELD-M is posted at .536 G1 BC. Typically a heavier bullet and ballistic tip versus HPBT would bring in a higher BC. Did Nosler find the sweet sauce in their bullet designs, or are they posted high?

Lastly, what else effects BC besides velocity? Does altitude, pressure, and humidity playing any factor or what do people mean when they suggest "calculating your true BC"?

Thanks ahead of time guys. I know a crap ton about components and accessories, but still pretty green when it comes to long range external ballistics side of shooting.

 

[stello1001]

I think weight, speed, and ogive profile are the biggest factors which dictate BC butI really don't know anything so I'm also curios to see what the pros have to say...

 

[EXTREMEPREJUDICE]

http://www.dtic.mil/dtic/tr/fulltext/u2/a554683.pdf I don't remember who originally posted this whitepaper here but I kept the link, it addresses most of what you are asking.

 

[Deleted member 113831]

I was reading some other threads on here and have read on here in the past that sometimes the BCs are inflated by the manufacturer. I was curious if that was intentional strictly as a selling point, or if the BC is tested in a lab rather than real world conditions.

I've been pretty happy with the Hornady ELD-M performances across the calibers, but haven't had a chance to shoot over 800 in quite a while so I might not be shooting far enough to notice an inaccurate BC make a substantial difference.

What made me ask this is that I have some Nosler 105 RDFs coming and I noticed that they boast a .571 G1 BC, while the Hornady 108 ELD-M is posted at .536 G1 BC. Typically a heavier bullet and ballistic tip versus HPBT would bring in a higher BC. Did Nosler find the sweet sauce in their bullet designs, or are they posted high?

Lastly, what else effects BC besides velocity? Does altitude, pressure, and humidity playing any factor or what do people mean when they suggest "calculating your true BC"?

Thanks ahead of time guys. I know a crap ton about components and accessories, but still pretty green when it comes to long range external ballistics side of shooting.

The more you get into this sport, the more you will realize that advertised BC's are a mere suggestion. Even if carefully and conscientiously obtained and reported, there is no guarantee they will be correct for your (rifle / load / shooter) system.

Manufacturers use different methods to calculate the average BC's to begin with. Additionally, the manufacturer has no control over YOUR barrel length, bore diameter, groove diameter, chamber or throat dimensions, your altitude, your temperature....it goes on and on.

All the things you mentioned affect BC and more. Published BC's are an average. They are an average obtained at a specific distance, altitude, speed and temperature. From the muzzle to the target, a bullets BC is a continuously changing value.

 

[Bradu]

The more you get into this sport, the more you will realize that advertised BC's are a mere suggestion. Even if carefully and conscientiously obtained and reported, there is no guarantee they will be correct for your (rifle / load / shooter) system.

Manufacturers use different methods to calculate the average BC's to begin with. Additionally, the manufacturer has no control over YOUR barrel length, bore diameter, groove diameter, chamber or throat dimensions, your altitude, your temperature....it goes on and on.

What does bore diameter, groove diameter, chamber or throat dimensions have to do with b.c.?

 

[Iamero]

This is probably why I need to stop building so many guns and just shoot them already. At the rate I change, I'll never figure out a dope for any of my rifles, haha.

 

[THEIS]

What does bore diameter, groove diameter, chamber or throat dimensions have to do with b.c.?

Hi,
Those can have affects on how fast the projectile becomes stabilized and the faster you can stabilize the projectile than what the projectile manufacturer did during their testing then you will increase your "BC".
The more or less friction you put onto the projectile compared to what the manufacturer did...you will decrease or increase your "BC".
The list goes on and on.

Another note to mention----some of the new modern projectiles really do not fit into any of the current "G" Series BC Formulas so the manufactures inflate the advertised BC based on a projectile design that does fit into one of the "formulas". I think we are in the last generation of shooters to use "BC" numbers for anything. It is like using a sundial to tell the time when you have the atomic clock sitting right next to you.

Sincerely,
Theis

 

[Greg Langelius *]

If you use the Pejsa software, as I do, and/or the Hornady 4DOF calculator, then you're going to see some 'funny things' happen when you input your environmental factors.

The Pejsa calc will tell me that I have a 'corrected' BC.

When I input the local station pressure for my altitude (4350ft ASL/25.52in-hg) into the Hornady 4DOF, that bullet starts behaving (on paper) sorta like an E-E-ELD.

Those published BC numbers are reference numbers for comparing between different projectiles (and I would use them only between bullets of a single brand), and are not cold and hard constants that never change.

A matter for consideration is that BC is not constant over the entire trajectory, but varies according to velocity, making it a sliding range of BC values.

I don't use calculators to predict POI's beyond a general reference. It will usually get me on paper; and then, I shoot sighters to refine a valid zero for that specific occasion. So you are not losing your mind when your rifle shoots to a different POI on a different day. Expect it to change some.

Beginners don't do this and many believe that a calculator will predict DOPE for a clean/cold bore first round hit. Close, probably; perfect, maybe once in a blue moon.

Greg

 

[Lowlight]

When I switched my Tikka to the Left Hand Gain Twist Barrel, the factory Prime I use went from having an SD of 18fps to an SD of 7fps, magic, or does the barrel matter?

BCs are fluid in much the same way, tune the BC for your system because you are not shooting the manufacturer’s. That is the number they got using their rifle under those conditions. Not to mention other than Hornady now, all use a 300 yard average.