6.5mm Projectile Stability

[OXN939]

About to start working up a load for the 6.5 CM. Berger stability calculator which, in my experience, has always been pretty close to spot on, indicates that 120-130 grain projectiles, such as the TTSX and LRX from Barnes, will be only "marginally stable" in the standard twist rate of 1:8. So, what are y'all's experiences with this? Advice for how to tweak the load to get them closer if not across the line into "stable?" Planning to start with a middle-of-the-road charge and play with seating depth as my first variable, giving them plenty of jump to the lands. Thanks for any input.

 

[doverpack12]

LRX was fine in my 1:8 260. I didn’t stretch it out real far but never gave me reason to be concerned. Personally I wouldn’t rely on a Berger platform for anything other than Berger bullets. Would like to see a 127 LRX side by side with any 140 Berger that will really provide some clue as to their calculator having bias or not.

 

[OXN939]

Would like to see a 127 LRX side by side with any 140 Berger that will really provide some clue as to their calculator having bias or not.

Especially since the two have fairly similar physical dimensions. I know Berger's calculator is designed specifically for their own projectiles, but I have yet to see a load it wasn't at least pretty close for. That being said, Barnes' website lists "1:8 or faster" as the twist rate they recommend for the 127 LRX.

 

[wyosam]

The 127 LRX (in factory ammo) shot great from my tikka, no signs of instability to 600 anyway. I’m shooting the 124 Hammer Hunter now, and couldn’t be happier with the performance.


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[rayporter]

for a given bullet length you are limited to increasing the velocity to gain RPM to increase the stability if you are stuck with a certain twist.

i had a long discussion about the twist effects on a certain bullet with Walt once. he was sure i could not stabilize a 95 gr bullet with a 10 but i knew of several shooters that did get it to stabilize. so some barrels will and some wont is the moral.

 

[2five7]

Did you input your altitude into the calculations? After barrel twist, it's the largest factor.

 

[tdot]

I spoke to Steve at Hammer Bullets about the Berger stability calculation. Apparently it uses a common industry wide formula, it's not specific to Berger bullets. But it is specific to Lead core bullets. His feeling was that there were too many errors when using that formula for copper bullets, both his and others pure copper bullets. He felt that a formula, built from the data obtained from single metal projectiles would be more appropriate. But for now, the Berger formula was a rough guide.


Edit. I think Steve referred to the common formula as the Miller Twist Formula, or something similar.

 

[OXN939]

a formula, built from the data obtained from single metal projectiles would be more appropriate.

Anyone know if such a thing exists?

 

[Mike S]

What is stability number that it gives you?

 

[OXN939]

What is stability number that it gives you?

Weirdly, if you input ".264 inches" instead of "6.5mm" as the projectile diameter, it bumps the stability to a 1.51, over their line into "comfortable stability." Seems like a monometallic calculator is really necessary to get a better answer here. That being said, I'm sure Barnes wouldn't have designed a 6.5mm projectile that would lack stability in the vast majority of rifles made in that caliber that are 1:8.

 

[Mike S]

I shoot berger 155.5 fullbore bullets in my .308 Palma rifle. Depending on the temperature, the calculator shows around 1.44. They shoot great at 1000 yards with high x counts even with 4000+ rounds down the barrel. I would expect you will be fine, but the only way to know for sure is to shoot them...

 

[wind gypsy]

Keep in mind that the comparatively light plastic tip moves the center of gravity further back in the bullet which makes them perform like a shorter bullet in regards to stability. 127 LRX have stabilized fine in two different 8.5 twist saum barrels for me. You’ll be fine with an 8 twist.

140 target hybrid and 127 LRX

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[Varminterror]

Typically you will never be able to push enough extra velocity to significantly increase the stability factor. Play with the calculator a bit, you’ll see it typically takes 100fps or more to increase by even 0.1. So if a bullet is apparently not sufficiently stabilizing, rebarreling or changing bullets are really the only two options.

 

[OXN939]

Keep in mind that the comparatively light plastic tip moves the center of gravity further back in the bullet which makes them perform like a shorter bullet in regards to stability. 127 LRX have stabilized fine in two different 8.5 twist saum barrels for me. You’ll be fine with an 8 twist.

140 target hybrid and 127 LRX

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Check. Makes sense. Will update the thread with what I find as I'm working up this load.

 

[jfs82]

My Bergara 1:8 loves LRX