6.5 284 vs 260 ai

[codyadams]

I wouldn't consider myself an expert and probably will be chastised for my position, but simply declaring that everyone should simply buy a fast twist(7)barrel may not be the no-brainer that many are implying. Yes, the heavies(155) do require a faster twist if that makes sense for your use. Push a faster 130/140 gr bullet into that faster twist barrel at higher velocity and depending on the bullet construction there be a negative effect on accuracy and throat wear. There is a reason that Benchrest shooters pick the slowest twist they can get away with while still stabilizing the bullet. It takes a longer time in the barrel with more energy for a bullet to take a bite into the rifling of a fast twist barrel. The fouling produced in this process can be an issue effecting accuracy and throat erosion, thus the principle behind Bartliens gain twist barrels. With LR hunting as my primary objective, until I understand the terminal performance on game of the heavies, I'll stick with. 1:8 twist. "Pinpoint" accuracy with the resultant precise bullet placement with well proven bullets is what makes the 6.5x284/260AK so effective as a LRH cartridge. I wouldn't want to trade that off until the terminal performance(and BC) of the heavies are understood. Unlike the long/heavy high BC 30 caliber bullets that have more mass and frontal area, the 6.5's rely heavily on the bullets ability to shed energy in the animal. I'll be quick to jump on the fast twist "bandwagon" IF the 6.5 heavies perform well on game.
The opinion of an old fart!

I will say, I talked to the guys at Extreme outer limits, and they loaded the first gen pre-production heavy Berger 6.5 (the 155) on mountain goats, in a 6.5x284 going 29XX fps, and they said they killed great. However, that is only 2 animals. Also, Broz documented a kill on a deer with the 155 as well, and reported typical great Berger performance. The bullet that will be coming out is a 156, as berger is redesigning the 155 because it did not achieve the bc they wanted, so apparently (according to shot show) it will be a 156.

As far as twist, if you live that high, I would go with a 7.5 twist is you plan on shooting the 150+ class bullets, or high bc mono's like Hammer bullets. If berger makes the 156 and recommends a 7 twist, at 9000 ft. A 7.5 will be fine. If you plan on shooting 120-130 class or lighter bullets (limiting the long range performance...) than go 8 or even 8.5 twist. If you just wanna use 140's, 8 twist.

 

[B23]

I wonder if Sierra's new 150gr SMK with a BC of .713 had anything to do with Berger feeling the need to step up their BC.

 

[RockyMtnMT]

I too classify as an old fart.

From my perspective as a mono bullet maker the 7" twist will get you into the 140g class of vld style mono which would be physically about the same size as the 150g class lead core.

I will throw in that terminal stability and ballistic stability are 2 different things. The added altitude doesn't add rotational vel thus the speed that a bullet slows down its rotational vel on impact remains the same. With less spin the bullet will spend less time nose orriented forward after impact, effecting terminal performance.

So if you choose to use the new heavies in lead core I would choose full stability at sea level if hunting is the name of the game. Shooting strictly targets or competition go with whatever works. It should be noted that elr competion is moving to ever increased twist rates that would be considered over stabilized by the old standards.

Steve

 

[Greyfox]

Like many, I'm excited about the introduction of the high BC heavies from Berger and Sierra. I've been running models based on the flow of information that is coming out on a regular basis. My current load in my 6.5x284 is a 142gr JLK VLD with a BC of .323(G7) driven at 3015 FPS. My 1100 yard dope at 4500'/45F is 24.5MOA, for 1800FPS/1000FPE. The LR performance on medium game out past 1000 yards with this setup has proven to be very effective.
Modeling the 155gr Berger using current predictions of a .355BC and velocity of 2920FPS(current reports) with the same atmospherics listed above, the 1100 yard dope is 25.25MOA for 1800FPS/1100FP. If these are accurate, a additional 100FP isn't significant enough making me question if the view is worth the climb. I'm hoping for a more meaningful outcome,but suspect the material payoff in performance with the heavies will come from the larger capacity 6.5 cases(ie.260 Nosler, 264WM)) where velocities can be driven considerably faster then is achievable with the 6.5x284/260Ack case capacities.

 

[bbraden]

Also considering the 260AI vs 6.5x284 debate as it pertains to barrel life.
If a person buys 100 pieces of brass and gets 10 firings out of that brass before chunking, that means no less than 100 additional shots fired through it during fire forming. If 1000+ rounds is expected, it would actually add 200 to the round count in simply fireforming, right? Does the 260AI add 200+ rounds to the barrel life or is the margin slimmer than that?

 

[DocDoc]

You can fire form with pistol powder and COW, with basically no effect on barrel life. You can also do fire forming as part of seat depth testing with .260 cases and get information on seating depth and a fire formed case at the same time.

 

[bbraden]

DocDoc, please expand on seat depth testing during fireforming!

 

[DocDoc]

Basically run a Berger seating depth test as you fire form.
http://www.bergerbullets.com/getting-the-best-precision-and-accuracy-from-vld-bullets-in-your-rifle/
Set at magazine length, or touch at a start load (works well for fire forming). Then increase jump in increments to find best seating depth. Follow up with some check around that depth in formed cases, increase powder for find best accurate "node" and your max powder charge.