Holy Crap! New Nosler Long Range Accubonds!

[philw]

I see your concerns about the .270, Dave (your other calibers don't have a Nosler ALR listed), but 1:10 might be fine with the 150 in .277, given the long boattail and ogive will shorten bearing surface.

Doesn't the twist rate required to stabilize a bullet depend on the bullet length, not just the bearing surface?

 

[Matt Cashell]

Doesn't the twist rate required to stabilize a bullet depend on the bullet length, not just the bearing surface?

Bullets with shorter bearing surfaces and long boat tails tend to have a more balanced center of mass, and require less spin to stabilize, in my understanding. I am not an engineer though, and could be wrong.

 

[Shrek]

Bullet lenght is the dominant factor. The secant ogive long for caliber bullets are great but hard to tune. All I have used have been finicky about free jump and powder. The long bearing surface and fast twist really heats the barrel. That causes the throats to work harden and when fired again the surface cracks and flakes off. That is what throat errosion is. You need a microscope to see the cracking an flaking.

 

[Steve O]

Thanks for all the info guys.

 

[carnyman]

Call me new to this, but isn't heavy for caliber what gives you a high B.C.? If so than why make the 6.5 in 129gr when they already make a 140gr accubond?

 

[Shrek]

conjecture is that they are trying to make these work with standard twist rates and could not get the 6.5 140gr to stabilise. My thoughtis that they want to avoid confusion with the standard 6.6 140gr accubond. Nosler has a history of overestimating thier bc's and most shooters on another site devoted to shooting feel these will be too. Even at 10% less bc's they are still great hunting bullets just not as good for marketing. Only time will tell as they are not available until spri.g I hear.

 

[Justin Crossley]

I'll be trying them as soon as they come out in my custom 6.5-0.6 Ackley.

 

[philw]

Call me new to this, but isn't heavy for caliber what gives you a high B.C.? If so than why make the 6.5 in 129gr when they already make a 140gr accubond?

Length is one factor but shape is important in aerodynamics too, especially the long boat tail, the ogive, and tip. If you look up the BCs, you'll see that the number they've assigned to the new 129 is better than the number for the "old" 140.

 

[RosinBag]

Phil is spot on, weight, shape, length, ogive and tip. It is all about aerodynamics. The perfect bullet would have a 1.000 BC according to theorists. Perfectly aerodynamical. Since that is nearly impossible out of a hand held rifle, they experiment with all the variables and try and build an aerodynamic bullet. Obviously they found something if these numbers turn out to be true.

 

[Segan]

Is there a standard twist rate for a 7mm RUM? Sendero? And would the 168VLD's shoot better or worse than the 180's? I think they are for different twist's but I'm not sure.

 

[philw]

Remington lists the twist for the 7 RUM in the Sendero at 9 1/2. That's probably borderline for the 168 and no good for the 180. Long-for-caliber bullets like the 7 mm 168 and especially the 180 are not popular with most hunters so you won't find a lot of factory barrels twisted for them. For the 180 especially you'd be better off with an after market barrel in an 8-twist. If by "shoot better" you mean more accurately, the 168 will be better than the 180 in a 9 1/2 twist, because the longer the bullet the faster the twist needed to stabilize it and 9 1/2 is probably not fast enough for a 180. On Berger's website they have a chart showing the slowest twist that will stabilize a given bullet; for the 180 the slowest twist is a 9 and for the 168 they say it's a 10. There are lots of different opinions on this, a lot of guys would recommend a faster twist like an 8-twist for the 180s, reasoning that the fast twist is more versatile and doesn't hurt anything if you're shooting lighter bullets. Others like Berger maintain that you'd do best on accuracy and velocity with the slowest twist needed to stabilize the bullet you'll shoot most often. And there have been problems with thin-jacketed bullets coming apart at very high velocities and fast twist rates but most sooters don't need to worry about that.

 

[Shrek]

Over spinning the bullet causes them to ride nose up down range and shed energy fast. There is a just right window. Too fast or slow is bad. You want the slowest twist that stabilises the bullet.

 

[philw]

Over spinning the bullet causes them to ride nose up down range and shed energy fast. t.

Dave do you have a source for that? I'm finding that there are lots of different opinions but not a lot of definitive written information on this subject. If you know where I can find some, I'd love to check it out, thanks.

 

[Shrek]

I picked up that info years ago and couldn't sight a source today. I'm hunting now and working on my phone so I'm not going to try to search now. You might try Accurateshooter / 6mmbenchrest.com . It is the reason that gun makers don't build all their barrels with a 1 in 7 twist to stabilise the longest bullets made.

 

[Justin Crossley]

My gunsmith told me the same thing that Dave is saying.

 

[philw]

I know there are a lot of opinions on this, and I've talked to a few gunsmiths about it too. I haven't heard that particular reason (the bullet riding nose up and losing energy downrange) but I've heard that a slower spin gets you more velocity and accuracy. And on Berger's website, they say that they recommend the slowest twist that will stabilize the bullet because "Spinning a bullet faster than necessary can amplify any inconsistency in the bullet" resulting in a slight loss of accuracy.

But Dave's post got me thinking so I dug out my copy of Bryan Litz's book, second edition. If you're not familiar with him, he's the chief ballistician for Berger and shoots competitively at long range. He has a pretty good discussion on this topic starting on page 141, and pretty much debunks the theory of the nose up causing an increase in drag or a loss in velocity. He modeled the differences in pitch (nose orientation in the vertical plane) and yaw (horizontal plane) and their effects on trajectory and velocity for the 7 mm. 180 Berger vld. out of a 9-twist and a 6-twist barrel. That's a pretty radical difference in twist and I've never run across any 6-twist 284 barrels.

According to Litz. the difference in pitch (nose orientation), drag, and velocity between two 180 vld bullets launched at 2800 fps out of 6-twist and 9-twist barrels is insignificant. The difference in velocity is less than 1 fps at 1200 yards. Litz goes on to say that there would be a slight difference in trajectory, with the bullet out of the 6-twist impacting a half-inch higher at 1000 yards.

There's a much more pronounced difference in yaw between the two twist rates, resulting in an increase in spin drift (in the horizontal plane) of about 4 inches at 1200 yards for the 6-twist over the 9-twist.

 

[Shrek]

I'll take his word on that. Brian knows his business. Still , rhe best practice is slowest twist that stabilises the bullet.

 

[philw]

I'd tend to agree but if you compared a 9-twist to an 8-twist with the parameters Litz used the horizontal difference in POI would be somewhere in the neighborhood of 1/3 of the difference Litz modeled for the 9-twist vs. 6-twist, or about 1.3 inches of spin drift at 1200 yards, and no difference in velocity or trajectory. Not much practical difference, and the downside of picking a twist that's an inch too slow seems a little greater than the downside of picking one that's an inch too fast.

 

[Segan]

Philw, thanks for the info I did mean shooting more accurate. I appreciate what you have put on here. I have shot the 180VLD's for a few years now. I went with this bullet for 2 reasons. First being high B.C. And second was the heavy bullet bucking the wind better at extended ranges. Looks like I'm going to have a research project for winter. If I was getting a custom barrel what would you recommend? And what twist rate would be optimal for my setup? Or would I be better to shoot a smaller bullet? Thanks for all the help.

 

[Justin Crossley]

Good info there Phil! Thanks