Why heavy bullets in 308?

[trg42wraglefragle]

It seems as though a lot of people are using very heavy bullets in their 308 for long range shooting, generally 166-175gr to get the highest BC possible.

Now Berger's website claims a BC of .515 for their 175gr target bullets.
Sierra's website claims a BC of .504 for their 155gr Palma matchking bullets.

Now I know that Palma rifles have very long barrels to push the lighter bullets much faster, but with such close BC's of the bullets and the ability to push the 155gr bullets faster would it not be better to use the lighter bullets in any barrel length?
Hornadys website claims aBC under .500 for their 178gr A-Max, so wouldn't you be much better off with the lighter faster bullet?

I know that real life can be a lot different than numbers on a computer, but using a JBM ballistics calculator the 155gr doing 2800fps compared to the Berger 175gr doing 2600fps, the 155gr had less wind drift, less drop and retained more velocity.

Am I missing something important, or is it one of those cases when real life is a lot different than on paper?

 

[SauerJackson]

Well, this is just my understanding of it....

But That depends on how far are you really trying to go with it. The lightweight bullets will spin a little faster, and hold a little tighter for a while. But when you really start pushing it out there, say 4, 5,6 hundred yards, it's really going to start getting pushed around in the wind.
A heavier bullet is more resistant to those downrange factors.

 

[jmr40]

Some bullet makers use different methods for measuring BC. If both of those bullets were tested in the same way I'd bet you will see a bigger difference. Compare bullets from the same manufacturer who used the same testing methods and I bet you always see a much higher BC on comparable heavier bullets.

 

[Old Grump]

Depends on your gun too, mine has a fast twist and 185's are more accurate than the 150's. I like accurate and I happily sacrifice fps for retained energy and penetration.

 

[btmj]

It also depends on the composition of the bullet. A copper bullet can be the same length and shape as a jacketed lead bullet, but it will be lighter. I don't know if that is what is going on in this case, ..... just say'n....

 

[Art Eatman]

I'd load up a box of each and compare the results on Palma-distance targets. That lets me think about pretty girls instead of why manufacturers claim what they do.

 

[trg42wraglefragle]

Even on Sierras 175gr .30cal bullets have a BC of .505.

I know bullet shape, materials and construction all come into how a bullet will fly, but it would seem all of these things are summed up with one value which is the BC.

Yes barrel twist would come into it, but for some buying a new rifle, would it not be worthwhile getting one to shoot the lighter bullets?

Yes in general a heavier bullet will be better long distance, but its not the bullets weight alone, its that a heavier bullet generally will have a higher BC than a lighter one, but it doesn't automatically mean it'll have a higher BC.

I also plugged all the info into a JBM calculator and using the .505 BC Sierra 175gr compared to .504 BC Sierra 155gr even out to 2000yards the lighter bullet was better.

 

[Jim Watson]

The heavier bullet might be more accurate than the lighter. So if you can read the wind, the higher velocity is no advantage.

Most bullet makers advertise the G1 BC. It is not a great fit for boat tail spitzers but is a larger number than other charts give. Sierra uses the kluge of a "variable" BC by velocity range. Berger also shows the G7 which is a better fit even though a smaller number.
Calculations must be verified by shooting.

Most proficient LR shooters gravitate to the heavier bullets, there must be SOME advantage not apparent from calculations. I have been shooting Scenar 155s and have a hard time telling them from Sierra 175s on the range. But then I am a scratch Sharpshooter and pretty casual competitor.

Of course you are limited to a 156 gr maximum for actual Palma competition.

 

[Brian Pfleuger]

I'm a big fan of light, fast bullets. I almost always use the lightest bullet that will do the job.

Folks talk about 55+ grain 22-250 bullet being "flat shooting". You ain't seen flat until you've seen a 35gr Nosler LF at 4,435fps.

There's another consideration too. Twist rate effects velocity. It's harder to push a bullet in tight rifling. The difference between really tight, like 1:8, versus 1:12 could be a hundred fps.

 

[JimPage]

Generally speaking, the higher the BC the better. But you also have to figure the muzzle velocity. If (exaggerated here) you had a BC of 600 it would be better than the BC 500. But if you can't get the BC 600 going fast enough to start with, the lower BC 500 may be better. So each comparison is different.

Personally for the 30-06 and .308 rifle I have found the BC477 165 grain spire point boat tail bullet to be the most effecient out to about 400 yards, much further than the average hunting shot; but adequate for a long (400yd) shot. That's the max I have ever shot at a deer. Other than a few jump shots close in I have never missed a deer.

Shooter discipline is much more important than BC!!!!!

 

[Bart B.]

OK, folks go do some math.

Use ballistic software to compare a Sierra 30 caliber 190 HPMK leaving at 2550 fps (typical NRA match rifle use) against a Sierra 155 Palma HPMK going out at 3000 fps (typical Palma rifle use) then look at the wind drift difference at 1000 yards.

Use some high school trigonometry to figure out the rifling angle different twists present to bullets in different calibers. Most folks are really surprised to learn a 22 caliber 1:8 twist has the same rifling angle to its bullets as a 30 caliber 1:11 twist has to theirs.

Then compare the range of bore and groove dimensions to all the bullet diameters and you my well think there's a greater spread in peak pressure and velocity caused by this than small changes in the rifling's angle to the bullet. I think this is the most common cause.

 

[Tempest 455]

On Sierra's website, they have a PDF charts that shows the BC changes with velocity. They say the 175 SMK have a .505 at 2,800 but less for example if you are at 2650.

http://www.sierrabullets.com/bullets/BallisticCoefficient-rifle.pdf

They say the 168's closer to sea level will start to tumble at 1,000 yards and is why most people use the 175's for 1,000 yards.

 

[MTSCMike]

168's go sub-sonic at extreme ranges (900-1000 yards) but the 175's don't. The disruption caused when going sub-sonic can affect accuracy.

 

[Jimro]

Why don't you go back to JBM ballistics and use the (Litz) data?

That will explain why so many shooters still choose 175 SMKs and heavier VLDs for long range loads.

One of the ways bullet manufacturers sell bullets is by publishing G1 data. G1 data is really useful in comparing flat based spitzer bullets. G7 data is much more useful comparing hollowpoint boat tail bullets. Use the (Litz) data for the Sierra 155 Palma 0.214 G7 and 175 Match King 0.243 G7.

Things make more sense that way.

Jimro

 

[trg42wraglefragle]

Jimro;
That is a very good point, and exactly the answer I was hoping for.
I forgot about G7 BCs as mostly G1's are thrown around.

BUT, Just done a bit of research and with JBM comparing Berger 185gr with G7 of 0.291 doing 2550fps, compared to 155gr Berger G7 0.247 doing 2950fps.
The 155gr will buck the wind better and have less drop out to 1100 yards, and will also have more energy out to 800yards.
After 1100yards the 185gr will buck the wind better, but the 155gr will still have much less drop.

Tis all very interesting this ballistics stuff.

 

[trg42wraglefragle]

Quite interesting that the G7 of that Berger 155gr Match Hybrid Target is a lot better than the G7's of many 165/168/175gr bullets.

And with G1 numbers how certain light bullets will be a lot higher in a few 155gr bullets than other companies higher grain bullets.

Edit to add:
Also the G7 BC of the .224 90 gr Match VLD Target is 0.281, which according to the data given buy other forums from other shooters is you could get one to around 3000+ fps from a 22-250.

I can't be bothered looking up ballistics at the moment but I imagine it'd be pretty good in the wind.

 

[Metal god]

Here is the normal 155gr MK , the BC never gets over .450
http://www.sierrabullets.com/index.cfm?section=bullets&page=bc&submit=1&stock_num=2155&bullettype=0

Here is the 155gr palma MK the BC tops out at .504
http://www.sierrabullets.com/index.cfm?section=bullets&page=bc&submit=1&stock_num=2156&bullettype=0

Clearly there is a design difference in these bullets . I have to assume the palma bullet is longer and that is what gives it the better BC

In the picture of the two bullets next to each other the palma looks a tad longer . You may have to scroll down to the 155gr bullets
http://www.sierrabullets.com/index.cfm?section=bullets&page=bullets&brandID=1&displayAll=1

EDIT : Does anybody know where to find the specs for the OAL of each of these bullets

 

[natman]

Now I know that Palma rifles have very long barrels to push the lighter bullets much faster, but with such close BC's of the bullets and the ability to push the 155gr bullets faster would it not be better to use the lighter bullets in any barrel length?

Velocity comes and goes, but BC is forever.

In other words, a bit more initial velocity is helpful at first, but after a few hundred yards it's BC that matters in the long run.

 

[taylorce1]

EDIT : Does anybody know where to find the specs for the OAL of each of these bullets

JBM Bullet Lengths

0.308 155.0 HPBT MatchKing™ 1.133
0.308 155.0 HPBT Palma MatchKing™ 1.194"
0.308 155.0 Palma [2155] (Litz) 1.131"
0.308 155.0 Palma [2156] (Litz) 1.210"

 

[Jimro]

trg42wraglefragle,

Ok, here is the "rest of the story" as Mr. Harvey might say...

Those high BC 155gr bullets have a secant ogive to obtain "VLD" numbers, (Very Low Drag). The traditional tangent ogive isn't as "slippery" in the air, but because of how it interfaces with the rifling in the throat is usually much less picky about seating depth.

Everyone I know who is pushing VLDs is pushing them from a custom chamber. Everyone I know shooting a SAAMI spec chamber is pushing a tangent ogive bullet (175 or 190 Match kings).

For example, a 210 VLD with secant ogive has a better BC than the 220 SMK. So why did Crane NSWC choose the 220 with less velocity as the newest 300 Win Mag sniper bullet? Because of how the bullet performs across the entire range of ballistics. The tangent ogive insures that the bullet will perform well in a variaty of chambers across the DOD, and the heavier bullet goes through the transonic range much better than the VLD.

I realize that you are asking about the 308 Win, but Crane NSWC did the same comparitive testing with the new Mk316 Mod 0 version of M118LR, testing multiple bullets looking for accuracy across multiple platforms (m24, M110, M14/21, etc). They stayed with the 175 because it was the most consistent performer as far as accuracy went.

Anyways, Berger recognized that they had a problem with SAAMI spec chambers and came out with their new "hybrid" ogive, which is secant for the bulk of it and then transitions to tangent before getting to the full diameter body of the bullet. This, in theory, allows the benefits of a VLD for high BC with the benefits of a normal BTHP with regard to not being seating depth sensitive. http://www.accurateshooter.com/ballistics/tangent-vs-secant-vs-hybrid-ogive-bullets/

I hope this explains why a lot of people are sticking with heavier bullets.

Jimro