BC is King for 6.5x47?

[bosulli]

This week I posted a question about powder density, and “BC is King” debate came up about my bullet weight choice. I agree that BC is king, but after reading Applied Ballistics For Long Range Shooting I decided to do an analysis of my bullet weight options using the Litz ballistic software included with the book.

Assertion – The higher BC of a 6.5 142smk is a better choice over the 6.5 123Scenar for shooting 1000yds.

Data
142smk 123Scenar
Velocity(muzzle) 2750fps 2950fps
G(7)coeffeicient .301 .270
Length 1.375 1.304
Atmosphere for both – standard, 10mph cross wind, no spin drift calculated.

Results
142smk 123Scenar
Drop(1000yds) 323in 293in
Time of flight 1.52s 1.46s
Drift 74.7in 77.4in
Stability 1.71 1.52

Conclusion – the assertion is false. The higher BC bullet is not significantly ballistically superior to the lower BC bullet. The stability of the 123scenar is above the 1.4 threshold for stable flight.

Is this process correct for deciding bullet choice?

 

[bohem]

Re: BC is King for 6.5x47?

The stability factor can be as low as 1.0

Notice that you have another thing to worry about here. Commonly the "BC is King" talks about dissimilar bullets where the difference in wind drift is very large. You have a difference of appx 4% in favor of the 142 SMK. This is nothing to sneeze at, especially if you're shooting known distances.

However, if you shoot an unknown distance (UKD) like in hunting or a UKD specific match then I'd re-evaluate the choice of the 142 over the 123. The difference in wind is negligible over the differences in drop. The extra "flatness" of that 123's trajectory will make small differences in ranging errors less likely to cause a high/low miss, while the difference in wind is less critical (assuming you have reasonable wind skills).

When it really comes down to it though, pick a bullet and shoot it lots and lots and lots. Calculators and computer generated dope sheets don't trump the gut-check experience of shooting a few thousand rounds with the rifle you are competing/fighting with.

ETA: You may find out that neither of these bullets shoot as well as you'd like in the rifle but the 123 Amax or 140 Amax or 139 Scenar or 130 Berger VLD happen to shoot much better. I'll take a small hit in BC to go to a bullet that shoots significantly tighter groups than a couple % edge in BC.

Hence why I'm shooting 123 and 140 Amax from my 260 upper, I couldn't get the 130 JLK's to run well in it. Even though they have a blistering BC they don't shoot sub MOA... shoot, they don't even go 1 MOA from my rifle, so they're useless to me for the 260.

 

[427Cobra]

Re: BC is King for 6.5x47?

I think the heavies buck the wind better, I learned that lesson while shooting 155's out of my 308, 155's shoot a lot flatter but 190's have less drift.

 

[Advokaten]

Re: BC is King for 6.5x47?

in my x47L,

the 123 scenar shoots the best, 123 A-max as a close second and the 130 AB, 130 Norma GL, 140 SST and so forth,

I choose 123 A-max for hunting based on those results, ar far as I shoot for game, actual and possibly, there is nothing to be gained by using a higher BC bullet.

Placement is everything.

/Chris

 

[raptor99]

Re: BC is King for 6.5x47?

I have shot the barrels out of several 260s and a couple of 6.5x284s and still have two 260s and now a 6.5x55.

I shoot the 123s in the 260s and the 139 in the 6.5x55. I honestly feel the 260 case is to small to really get the best out of a 139 or 140gr bullet. It seems to do its best with the 130 to 120gr bullets. Mine both love the 123s and they do shoot well even at 1000yds but the heavier bullets are better.

I have a 6XC that is a nearly a ballistic twin to the 260 with the 123s and 6XC with a 105. The 260 will do better in the wind at 1000yds than the 6XC and the 6.5x55 will do better in the wind with the 139 than either of the other two.

I know computers may say light and fast is good but in real world shooting the heavier bullets just seem to do better in the wind. Even to a higher BC lighter and faster bullet. At least this has been my findings.

 

[Greg Langelius *]

Re: BC is King for 6.5x47?

Flatness is a reason I am evaluating the .280 Rem as a performance cartridge. In the aspects discussed here, it has the potential to outperform the .30-'06, and maybe even the .260. It may permit magnum-like trajectories without a need to employ a magnum bolt face.

Putting it a different way, the velocity and BC come from a lighter projectile. I am more succeptible to recoil than many, and I want to get as much performance as I can for what recoil I am forced to tolerate. Ten years agop, I wouldn't have considered a 7mm cartridge, but nowadays, as with the 7-08, the .280 has far more interesting bullets to choose among.

In evaluating 'nearly ballistic twins', I consider that to be rather like comparing apples and oranges. There's no disputing they're fruit, but there is <span style="font-style: italic">still</span> a meaningful difference.

Greg

 

[sandwarrior]

Re: BC is King for 6.5x47?

You want a high of a BC as you can get...that shoots accurately in your rifle at the ranges you wish to shoot.

Meaning: If you narrowed it down to those two bullets, which one is more accurate at the range you wish to shoot? Which one bucks wind better? On any given day trajectory is going to be a given. You need to figure out what it is, as it will change. But, day by day it's a given, unless you witness some big front that changes it drastically. Wind, is almost not ever a given. It will change enough to take points away from you on every shot. So, having the bullet that bucks the wind the best, and is accurate, is what you want. Once you decide on that you need to know the drifts for that bullet/velocity combo inside and out.

This li'l anecdote always rings true when I hear this discussion:

A windbucking bullet with high BC is great if you miss a wind call as it may halve your mistake. The problem is the guys on your left and right aren't making those mistakes....

 

[ChadTRG42]

Re: BC is King for 6.5x47?

When guys talk 1K yards and beyond, BC will be king. At distance shorter than 1K, the lighter bullets will generally shoot flatter. But the slower, heavier, higher BC bullet will always drift less in the wind with the same caliber. Once you know your distance, then elevation doesn't matter. It's the wind that will get you. So, if you want fast and flat, go lighter. If you want better long range performance, go heavier with higher BC. Also, the heavier bullets generally hit a little harder, and aid in spotting your hits.

Also, most 260 shooters push a 140-142 grain bullet 2800-2860. So, 2750 is a little slow.

 

[clay_breaker]

Re: BC is King for 6.5x47?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 427Cobra</div><div class="ubbcode-body">I think the heavies buck the wind better, I learned that lesson while shooting 155's out of my 308, 155's shoot a lot flatter but 190's have less drift. </div></div>

X2. I was shooting from 500 a few days ago and I couldnt believe the difference in 178 A-Max and 155 Scenars at that distance. 155's blowing way off in 15 mph crosswind.

 

[bosulli]

Re: BC is King for 6.5x47?

Good point regarding wind. Verticle drop is consistent and predicable, which you get with the light bullet. Wind dispersion is difficult to predict, so compensate with the more wind resistant bullet. A few inches off of wind compensation is difference in hearing a "ding" and not.

The 1750fps I estimated is for a 6.5L and 41g of H4350, and may be on the hot side. I just loaded the first 142's, so we shall see.

 

[ChadTRG42]

Re: BC is King for 6.5x47?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bosulli</div><div class="ubbcode-body">Good point regarding wind. Verticle drop is consistent and predicable, which you get with the light bullet. Wind dispersion is difficult to predict, so compensate with the more wind resistant bullet. A few inches off of wind compensation is difference in hearing a "ding" and not.

The 1750fps I estimated is for a 6.5L and 41g of H4350, and may be on the hot side. I just loaded the first 142's, so we shall see. </div></div>
Doh! Yes, you did say 6.5x47. My fault. 2680 to 2750 ish is a decent area for them.

 

[vman]

Re: BC is King for 6.5x47?

I would prefer a higher BC over a bullet that shoots flatter.

Reason for this being that range is easier to estimate imo and we have tools available that pretty much whipe out distance as a variable... rangefinders and such.

Wind on the other hand is difficult to master, if it even can be. And for that reason I chose the bullet that offers me better balistics to reduce the effect of the wind on my shooting.

 

[Greg Langelius *]

Re: BC is King for 6.5x47?

Wind deflection is what it is, whether it's more for one, or less for another.

Ramping up the load energy in the hope that a screamin' load will 'save' you with a bad wind call is optimistic at best. Regardless of how suscepible a load is to wind, it's the shooters' wind skills that save scores, and not ballistics.

The 'ballistics is the whole ball game' approach has very small benefit in return for an excessively high cost in bore life, IMHO.

Greg

 

[sandwarrior]

Re: BC is King for 6.5x47?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Greg Langelius *</div><div class="ubbcode-body">Wind deflection is what it is, whether it's more for one, or less for another.

Ramping up the load energy in the hope that a screamin' load will 'save' you with a bad wind call is optimistic at best. Regardless of how suscepible a load is to wind, it's the shooters' wind skills that save scores, and not ballistics.

The 'ballistics is the whole ball game' approach has very small benefit in return for an excessively high cost in bore life, IMHO.

Greg</div></div>

That's the point though. You're not ramping up the load to <span style="text-decoration: underline">spend</span> more energy. You're using a more ballistically efficient bullet to <span style="text-decoration: underline">keep</span> more energy. In that envelope is included heavy for caliber bullets. Which buck the wind better.