Range Report 6.5 CM: optimally stable 140 ELD-M vs. marginally stable 147 ELD-M

[DJL2]

I debated putting this is the "stupid question" section because it seems like a no brainer...however, there's the very real chance I could be missing something...perhaps even something glaringly obvious.

The Berger stability calculator (using data from the JBM library) considers the Hornady ELD-M marginally stable in my KRG SOTIC (1:8 twist) and indicates I'm losing roughly 1-3% of my effective BC. The 140 ELD-M is "optimally" stable...but, the BC between the two is roughly 10% in favor of the 147 gn...which means losing 1-3% is still strongly in favor of the 147, particularly given the roughly equivalent MV of each.

Anyhow, to the question: like it says in the title, is the 147 ELD-M still the clear choice here in terms of aerodynamics, even marginally stable, or is there something I'm missing?

Yes, that's an "all else equal" type of question.

 

[patriot07]

BC isn't everything - you're going to be able to push the 140 faster.

Honestly, there isn't a ton of difference between those 2 unless you're really trying to push the limits of the 6.5CM and be precise, and if you are then you should just go get a 7-08. If your gun happens to shoot one better than the other, that will be more important than any performance difference.

 

[ShtrRdy]

The reason the 147 would loose some BC is because the bullet starts to pitch and yaw. What this means is your accuracy will likely suffer. You could give the 147 a try but be on the lookout of any unexplained behavior downrange.

 

[bfglowkey]

Your talking about 1-3% loss of effective BC. If it was 10-13% you might argue but 1-3%.....your bullet is not going to be pitching and yawing. If so...on such microscopic levels most shooters if any could not tell how it is effecting them down range. Decide what the distance is your aiming to hit and then run the numbers again. If your staying in 1000 yards or less then either bullet will perform admirably. Its when your pushing out further that you might want to keep to the heavier pills.
Side note: If you are reloading your own ammo I would see at what speed you need to get to with your 1:8 twist and 147s to hit the magic optimal mark...you very well could be just a few FPS off and can get there with a different powder/brass/primer combo....we are seeing great results with RL26/H1000 and those 147s....just saying.

 

[bfoosh006]

Be sure to check your actual ( measured ) twist rate... some barrels can be a slightly faster ( or slower rate )

Just to be sure the numbers aren't in your favor.

Use the snug bore brush ( new ) and flagged cleaning rod method.

 

[DJL2]

Your talking about 1-3% loss of effective BC. If it was 10-13% you might argue but 1-3%.....your bullet is not going to be pitching and yawing. If so...on such microscopic levels most shooters if any could not tell how it is effecting them down range. Decide what the distance is your aiming to hit and then run the numbers again. If your staying in 1000 yards or less then either bullet will perform admirably. Its when your pushing out further that you might want to keep to the heavier pills.

Side note: If you are reloading your own ammo I would see at what speed you need to get to with your 1:8 twist and 147s to hit the magic optimal mark...you very well could be just a few FPS off and can get there with a different powder/brass/primer combo....we are seeing great results with RL26/H1000 and those 147s....just saying.

Temperature/Density Altitude is the big problem here. At 0 ft ASL (and with the humidity here, that’s not a stretch for DA and might even be conservative), the Berger calculator tells me I’d need 2950 fps to get full stability at 32F. If I saw above 2600 it would be good improvement from my first results out of a new barrel...obviously, 2950 isn’t in the cards. However, at 72F, the 147 ELD-M stabilizes at modest/obtainable MV.

As to pushing the 140 ELD-M faster...maybe so, but the advantage it enjoys isn’t meaningful when loaded by the factory. When I get a chance, I’ll edit in my chrono data and the theory crafting I did in the Hornady 4DOF calculator.

The 2550 is a 40 deg day, the 2587 is a 65 deg day and a warmer barrel, if that's of interest.

 

[just browsing]

Also consider that the projectile will not be traveling at that speed as it approaches its target. A 147 at 2950, for example, may be “optimally stable” but as it slows and transitions to transonic speed, how is the stability? At what range does it lose reasonable stability?

The point being that you can’t use MV solely as the deciding factor for stability. You need to look at the distance you’re shooting and the projected velocity at each of those corresponding distance points.

I can push a 147 over 3200 FPS in my 28” GAP4S. In an 8t barrel, at 0 feet density altitude, and assuming 1120 fps as supersonic range, my SF is under 1.20 or “marginally stable.” I am otherwise sacrificing 10% of my BC at that point.

 

[DAVETOOLEY]

The reason the 147 would loose some BC is because the bullet starts to pitch and yaw. What this means is your accuracy will likely suffer. You could give the 147 a try but be on the lookout of any unexplained behavior downrange.

Not really. The faster twist rates help some bullets through transaction.
Shot what's the most accurate. You'll never learn wind calls at distance with a mediocre shooting rifle.
We shot a lot of 140's through 1-9's. 150's through 8's.

 

[DAVETOOLEY]

How did you calculate a 10% decrease in BC? Or did you measure it?
I've shot 187 gr. flat base bullets at 1K with a stability of 1.1 and no accuracy problems. In fact I wore people out with that bullet barrel combo for two years. The target tells all.

Also consider that the projectile will not be traveling at that speed as it approaches its target. A 147 at 2950, for example, may be “optimally stable” but as it slows and transitions to transonic speed, how is the stability? At what range does it lose reasonable stability?

The point being that you can’t use MV solely as the deciding factor for stability. You need to look at the distance you’re shooting and the projected velocity at each of those corresponding distance points.

I can push a 147 over 3200 FPS in my 28” GAP4S. In an 8t barrel, at 0 feet density altitude, and assuming 1120 fps as supersonic range, my SF is under 1.20 or “marginally stable.” I am otherwise sacrificing 10% of my BC at that point.

 

[just browsing]

How did you calculate a 10% decrease in BC? Or did you measure it?
I've shot 187 gr. flat base bullets at 1K with a stability of 1.1 and no accuracy problems. In fact I wore people out with that bullet barrel combo for two years. The target tells all.

Measured vs. stated. Not sure how accurate of a comparison that is to make and I’m sure you could argue the actual percentage... But the idea of lower RPMs translating to lower stability and therefore lower BCs stands.

That being said, I definitely do not disagree with the target tells all idea. I shoot 150s in that same 8t GAP4S with great results. My main point was that MV is not/should not be the tell-all indicator for stability.

 

[ingraham]

IMO go with the 147s. Laser accurate.

I'm running 42gn of RL-17 with my 260 and 147s

Friend is running 42gn of RL-17 in her 6.5CM and 147s.

 

[CavReconScout]

147 ELD-M at 2650fps. I have shot this out to 1158 yds at Lodi and any missed targets were my fault. Spend less time worrying and more time shooting.

 

[DAVETOOLEY]

Measured vs. stated. Not sure how accurate of a comparison that is to make and I’m sure you could argue the actual percentage... But the idea of lower RPMs translating to lower stability and therefore lower BCs stands.

That being said, I definitely do not disagree with the target tells all idea. I shoot 150s in that same 8t GAP4S with great results. My main point was that MV is not/should not be the tell-all indicator for stability.

I agree that velocity has a lesser affect on stability than twist rate. I beg to differ about a lower stability number means lower BC. Unless you have access to extensive dopplar data for a multitude of bullets you're generalizing. The main thing is do no harm. Excessive twist rate can and will have a harmful effect on accuracy. To many other factors. Bullet design for one. Barrel dimensions and rifling profile. The list goes on. The only absolute is the extremes don't work out in the long run.