Higher BC or lower BC with the extra velocities

[No i deer]

I'e heard these these things said so many of times.i real terms does a slightly lower BC bullet travelling faster than the higher BC bullet work as a trade off.eg 6.5 cal  139gr scenar versus 123gr scenar or is it an old wives tale.

Thoughts please

No I deer.

[gbal]

There is effectively a trade off.Just what tade off does depend on distance-very approximately,velocity is rarher more important than BC to about 250/300y (varies with the cartridge/calibre) but as distance increases,BC comes more into play.

   You might notice some hunting bullets are flat base -does nothing for BC,but these are short range designs. AS intended range increases,the  boat tail,and then different ogive (bullet tapers)  have  more effects. Bench rest precision shooters (that's 5x5 shots at 100y into aggregate  sub .25 inch,shoot flat base- fairly fast,but not at the cost of any precision.!000y needs all the BC and velocity that you can handle -trade off with recoil then etc.

SInce you mention 6.5 calibre,here are data from a few SAAMI 260 Rem commercial loads-comparable since pressures etc controlled and known-hot loading simple means comparisons are not valid-though the individual 'improvements' might indeed be.(drop/drift"/energy ft lb    200y zero,10 mph wind):

Nosler Partition 125g BC .449    2950         300y 7/6.5/1533     600y 70/29/926

Rem    Nos Part  125g BC .45      2875         300  7.4/6.7/1449   600 75/30/872

Here the same bullet/BC at 75fps slower has minimal d/d diff at 300,and not much at 600,though energy is less (as velocity is less).

Nosler Partition 140g BC.490   2800           300y 7.7/6.4/1594       600y  76/28/999

Remington 140 Core LOck BC .457  2750     300 8/7/1484    600 82/32/ 891

In these two 140g,lowering BC and MV a bit has under 1" diff at 300, but about 5 inches more d/d at 600,and -of course- less energy at both distance (less MV)

 

THeseareessentially hunting loads,but the same pattern holds across loads/calibres (not of course the same D?D/energy!)...and of course these are not widely different loads- there really are very very few comercial loads that deviate much-performance sells!. In general,within the  cartridges' performance window,and perhaps the application (LR competitors will  be attravted to every bit of improvemnt they can get-thoufgh the wind error will be their big problem)...there  isn't a lot in it at least to 300y or so:longer distances will need a bit more dialed in correction.BUt that said,100fos or .1 better  BC is not usually going to transform field performance,though of course,bettr to hhave than not have-accuracy being preserved (not a given-varies with rifle).

gbal

 

 

 

 

[VarmLR]

The way to think of it is that for any given bullet weight, a higher bC bullet will shed velocity more slowly than a lower BC one and be less prone to wind drift (ie depending on bullet, BC and MV, for any given range it may drift less for a given 10mph full wind than a lower BC bullet).  Drift is unlikely to be much different until out past 200 yards.  Beyond that it can become more significant between bullets and loads so choosing the appropriate BC bullet helps or driving the bullet a lot faster.

Target shooters generally look for as much velocity as they can for longer ranges to minimise things like wind drift and vertical spread but the price paid may be shorter barrel life.  Hunters and plinkers are more concerned with precision for a given kill/target zone for a given distance so along with required terminal ballistics you choose the bullet and load accordingly.

Whether you can compensate sufficiently using a lighter, lower BC bullet at higher MV effectively depends on the range  (hence the shed velocity of the lower BC bullet), and the rate of drop-off of velocity.  

You can compare these things using any on line ballistic calculator.

 

[No i deer]

I was thinking on the lines of long-ange.i generally go for the heavier option.i very much like the 139gr scenars for long and short range for target shooting.ive I good 6 or 700 of the 6.5 cal 140gr amaxs in stock that I wanna try them at 1000yds just out of intrest.

[Laurie]

You can find out easily enough using a ballistic app or program using comparable muzzle energy (ME) values. For example:

139gn 6.5 Scenar @ 2,800 fps = 2,420 ft/lb ME

the equivalent ballistic performance for the 123 (that also produces 2,420 ft/lb) is 2,977 fps MV.

(As a general rule, a given rifle with a given barrel length will see optimal loads produce similar MEs for different bullets assuming their weights suit the cartridge / barrel length and an appropriate burning rate powder is used.)

The Litz G7 values for the pair are 123 - 0.265; 139 - 0.290

Run the pair through a ballistic program with these MVs and BCs and you get at 1,000 yards in standard conditions:

123 @ 2,977 fps MV ........ 1,439 fps retained velocity; 7.86 inches drift per 1 mph wind

139 @ 2,800 fps MV ........1,424 fps retained velocity; 7.66 inches drift per 1 mph wind

 

So, on the main criterion for target shooters - wind drift - the 139 is marginally superior, but there is so little in it that a slightly tighter grouping 123gn load with lower MV spreads and SDs could in practice prove superior.

This is despite the 123 having an aerodynamically better shape than the 139, its G7 'form factor' which compares drag to that of the (G7) reference projectile being listed as 0.950 (produces 5% less drag) against the 139's 0.983 (produces 1.7% less drag than the reference whose value is 1.000).

If you compare two different weight bullets in a calibre with the same 'form factor' at  ME equivalent MVs, you will find that the heavier bullet always outperforms the lighter in wind drift terms - eg comparing the figures for the 175gn .308 Berger 175gn LRBT (form factor 0.999) against the 210gn Sierra MK (form factor 1.000).

However, there are other potential criteria to judge different weight bullets against even when they offer equal precision - felt recoil and trajectory shape being the main ones. A rapid fire target discipline  or varminting at unknown / estimated distance targets may suit a lower recoiling / flatter trajectory light bullet even if it moves a little more in the wind.

 

[No i deer]

Thanks Laurie.

Top knowledge as usual.cheers

[Cienfuegos]

Bravo!!!