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Old April 27, 2013, 07:53 PM   #26
JimPage
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Bart: as an amateur at this subject I'll venture a guess. Couldn't the additional weight make the heavier bullet maintain velocity better?
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Old April 28, 2013, 08:07 AM   #27
Bart B.
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Jim, sometimes heavy bullets hold velocity better. Other times, they won't. Depends on their shape in addition to their weight.

Given a choice of a bullet that has 20% better accuracy or one that bucks the wind 20% better, I would use the most accurate one. I can correct for wind drift. But I cannot correct for accuracy.
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Old April 28, 2013, 04:20 PM   #28
Jim Watson
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Ballistic Coefficient covers both shape (form factor) and weight (sectional density) in one term. No need to fall back on old saws like "heavy bullets carry up farther." A high BC bullet carries farther, flatter, and closer to the wind at the same velocity as a lower BC projectile.
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Old April 28, 2013, 06:15 PM   #29
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Jim Watson,

What you write is true, but BC isn't a truly static value, as it is an approximation of flight against some other projectile. As velocity decreases, so does BC, which is why Sierra advertises a BC range for their bullets with a velocity spread. Things always get interesting when crossing the transonic range too.

Some of the VLD designs fly really great until they hit the transonic range, which is why Crane chose the 220 SMK over the 210 VLD when working on the new 300 Win Mag SOCOM load. On paper the 210 has a higher BC and can be driven faster, in reality the accuracy isn't there once the bullet hits the transonic range. I'm really interested to see how Berger's hybrid bullets handle the transonic range as the it would be nice if all the advertising hype is true.

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Old April 28, 2013, 08:07 PM   #30
Bart B.
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Jimro, not all of Sierra's bullets have lower BC's as velocity drops. Check out the .308" 180-gr. HPMK's BC shown below:

.475 @ 2800 fps and above
.496 between 2800 and 2200 fps
.494 between 2200 and 1600 fps
.494 @ 1600 fps and below
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Old April 28, 2013, 08:30 PM   #31
Jim Watson
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Jimro,

If you would go by G7 BC you would not have to apply the Sierra kluge of variable G1 BC for boat tails.
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Old April 29, 2013, 03:59 AM   #32
trg42wraglefragle
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Quote:
The average shooter doesn't develop a load for a particular gun. The average handloader will do that, and not every handloader has the skill to really make a rifle sing with a bullet that is really picky about seating depth. Even if a handloader does have the skill, the rifle itself can be an issue if you want other functions like being able to load from the magazine.

I knew a Presidents 100 tab holder who had to change his seating depth every 500 rounds in his service rifle to keep the jump to the lands consistent with the bullets he used for the 600 yard line. That is not a "single load for a single rifle" that is continual load development to ensure maximum accuracy from a rifle as it ages.

Most High Power shooters aren't that picky, but when you are competing for the Presidents 100 tab, then you are competing against other people who are that picky. So then it pays off to use the best bullets perfectly loaded for your rifle. That being said, the AMU is using the 185gr Berger's for their M110 loads for service rifle 1000 yard High Power shooting.

Heck, some top competitors use a meplat uniforming tool which is advertised to take 2% off of the bullet BC, but give much more uniform BC as a result. For those shooters looking to drop from .6 MOA to .5 MOA this is a good thing. For the "average shooter" 0.1 MOA is meaningless.

Anyways, look at the loads that win matches. The only people I know who routinely use the 155gr bullets are Palma shooters for whom the bullet was designed based on international shooting competition rules.

Jimro
This was a very interesting read, I'd hate to have to keep changing the seating depth as my barrel got worn, that'd be a huge PITA.

But you are saying that bullets with a Tangent Ogive will not give this headache?
And also the Berger Hybrid bullets will not present this problem also?
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Old April 29, 2013, 07:15 AM   #33
Bart B.
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Jim Watson, Sierra does not have any "kluge" regarding their bullet's ballistic coefficients at different speeds. They know what they're doing. Even airplanes and cars have different drag coefficients for different speeds.

trg42, all it takes to change seating depth .010" to adjust for leade erosion is backing out your seating die stem the same amount. Surely that's not a PITA. Competitive shooters have been doing that for decades. It's worth it for the accuracy attained. Life's full of compromises. This is just another one to consider.

Maybe you have other priorities and objectives. If so, then size your cases so neck tension is a bit less, then seat all your bullets 1/10th inch longer than leade contact. They'll push back when the round's chambered and that'll do for 3000 to 4000 rounds of barrel life. Just don't unload a live round once it's chambered else the bullet might stay in the barrel and you'll get to clean all the powder out of your rifle's action.
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Old April 29, 2013, 04:49 PM   #34
trg42wraglefragle
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At the moment I only reload for 223 and am not hard out into it, but no doubt in the future I will get more into it and worry about extreme accuracy.

But was I correct in thinking that with the bergers the problem jimro is out lining is solved?
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Old July 14, 2013, 09:47 PM   #35
Phil McGrath
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Jimro, not all of Sierra's bullets have lower BC's as velocity drops. Check out the .308" 180-gr. HPMK's BC shown below:

.475 @ 2800 fps and above
.496 between 2800 and 2200 fps
.494 between 2200 and 1600 fps
.494 @ 1600 fps and below

Bart B, I'm shocked no one caught you on this one. This BC is for the current 180SMK that shares the same BT angle as the 168SMK.

Too keep this apples to apples use the old 180SMK with a BT angle of 9* its BC was .505 and it didn't drop like a rock.
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Old July 15, 2013, 02:10 AM   #36
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What you see with the 180 MK G1 numbers is a typical "bell curve." You have 4 different values.

If we put velocity at 0, then BC is 1.0 for any projectile (after all, the referrence projectile, a 1 lb lead flat based 2 radius spitzer iirc, also has the same drop at that forward velocity). As velocity changes projectiles will fly better or worse than the referrence projectile.

BC gets better as velocity increases until it doesn't anymore. This makes complete sense as laminar flow differences are not linear with velocity differences. This is why it is called "aerodynamics" and not "aero linear functions."

Anyways, the reason that the BC of the 180 SMK goes down as it goes over 2800 fps is the actual vacuum area behind the bullet. As velocity increases the area of vacuum increases, the boat tail design is to guide gases behind the blullet to minimize that vacuum.

Whirling and whirling, increasing in velocity, onward and onward into viscosity. An old rhyme that explains how increasing in velocity can cause an increase in "whirling" fluids and increase drag.

So yes, if you push a bullet too fast BC will decrease, just as BC decreases as velocity decreases. It isn't anything but a relative measure to how the referrence projectile flies at those velocities.

Jimro
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Old July 15, 2013, 09:36 AM   #37
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Phil, I wasn't making a comparison, just showing an example of how BC's can very with the bullet's speed through the air. The old 180 HPMK from Sierra held velocity better at long range and that's why the US Army Team had Sierra make them using the older dies after Sierra changed the boattail to that of the 168's. Those new 180 HPMK's would not hold supersonic velocity past 800 to 900 yards; just like the 168's. Extra tight barrels would shoot them fast enough to do so as their muzzle velocity was higher.

Having worn out a few 7.62 NATO Garand barrels, I've not noticed any significant degradation of accuracy as the throat eroded away after 3000 rounds and using M118 Match ammo with their bullets all seated to the same OAL. The several hundredths inch increase in bullet jump over that many rounds didn't effect any noticable accuracy change. The barrels still held at least 3/4 MOA accuracy at 600 yards from brand new to 2/3rds worn out. But the last 1000 rounds of the 5000 round barrel life did show accuracy had gone to about 1 MOA at 600. Which is why the Match Conditioning Unit rebarreled them at 5000 rounds. The bore erosion gauge read "5" at that point. Normal service rifle barrels were replaced when the gauge read "10" at about 10,000 rounds.
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