Shredding bullets in 6mm CM

[Nicktheshooter]

Hello,
I have been reloading heavy target loads for my 6mm CM and wanted to try my hand in some light Varmint loads for hunting. I chose to go with 58gr Vmax and IMR 8208 XBR. I used published load data from Hornady for this load. Even at the lowest charge weight I was shredding the jackets and bullets were unstable. I moved my target up to about 30 yards and was getting fragmented bullet strikes. Some completely missing paper. Unfortunately my battery was dead in chronograph so didn’t get speed but should be in 3600 to 3800 FPS range. Can anyone shed some light on what might be the cause and steps to try and correct? Specs on setup are:
Savage 10T 6mm Creedmoor
1:8 twist, 26” barrel
Hornady 58gr vmax
COL: 2.56
IMR 8208 XBR powder
Photo of target attached

 

[Texas10]

You're spinning them apart.

Velocity X 720 / twist= 324,000 RPM.

Shoot a heavier bullet, slower. I used the 86 V-Max out of my .243 for deer but that was a 9 1/2 twist.

 

[Nicktheshooter]

Thanks Texas10, is there a rule of thumb for max bullet RPM to stay below?

 

[ricco1949]

Prior to my Vartarg days I used 22-250's for PDs. One year I installed a new Shilen 9 twist on one Rifle but didn't have time to do any load development so I bought a case of Winchester 45 grain on the way to the PD wars.

Those Winny bullets couldn't survive the 9 twist after about 10 rounds heated the barrel. When I got home I pulled the Winny bullets and inserted 40 Vmax's on top of the factory powder. The V-max's did fine on top of that factory powder......survived the 9 twist RPM''s at over 4000 FPS with very nice groups too.

Moral of the story......try another bullet.

 

[wedgy]

Take a look down the barrel to see what it looks like as it could have some nasty tooling marks, heavy fowling, etc.
A good cleaning may help regardless of what bullet you shoot.

 

[Sniper338]

Bullet blow ups.. poof!

 

[Nicktheshooter]

I was thinking about tooling marks, etc because I have had other bullets (heavier) come apart as well, but some shoot great. Here is another photo from same day shooting 108 ELD-M, 10 shot group @ .08 MOA. Believe it or not though, they will come apart on me at higher velocity as well

 

[Webster]

View attachment 1149807 Hello,
I have been reloading heavy target loads for my 6mm CM and wanted to try my hand in some light Varmint loads for hunting. I chose to go with 58gr Vmax and IMR 8208 XBR. I used published load data from Hornady for this load. Even at the lowest charge weight I was shredding the jackets and bullets were unstable. I moved my target up to about 30 yards and was getting fragmented bullet strikes. Some completely missing paper. Unfortunately my battery was dead in chronograph so didn’t get speed but should be in 3600 to 3800 FPS range. Can anyone shed some light on what might be the cause and steps to try and correct? Specs on setup are:
Savage 10T 6mm Creedmoor
1:8 twist, 26” barrel
Hornady 58gr vmax
COL: 2.56
IMR 8208 XBR powder
Photo of target attached

13.5 - 14 twist for light bullets. Like Texas10 said to much RPM. Try 75-85 gr varmint type bullets.

 

[Zero333]

I have same problem in the summer with 80 gr Bereger FB Varmint.

In the freezing winter cold when the barrel doesn't heat up they shoot like a dream.

Too much twist.

 

[SBS]

rpm, smoothness of throat & rifling, heat, bullet construction/hardness, even type of rifling -- all play a part. Lots of info on this subject in other threads.

 

[TimS]

If you can control the velocity and distance when they come apart, you might be onto something good.

 

[Ranger188]

My smith suggested below 310,000
Berger wants you to stay below 300,000

 

[jepp2]

Thanks Texas10, is there a rule of thumb for max bullet RPM to stay below?

I was blowing up the Speer 52 gr HP's at 290,000 RPM coming out of my 8T upper. But when I tried them in other barrels at even higher RPM (from a 7T), they worked just fine. So in my case it was specific to the one barrel. So I'm not sure you can define a maximum since it is:
- bullet dependent
- twist dependent
- specific barrel dependent

 

[chop house]

sbs got it... and i have scoped many a savage factory bbl and they aren't smooth and copper quickly, typically. some frangible bullets (like speer tnt) can be particularly susceptible.

 

[mikecr]

I would have predicted failure beyond 3400fps in 8tw for 24cal.
My route:
KrpmLim = ((0.308-Your bullet dia.)*238.095+290) = 305Krpm
Velocity limit = 1.38889*KrpmLim*Twist = 3394fps

This predicted original Berger VLD failures(now 'hunting' VLDs) - exactly.
But other bullets tend to be close ahead/behind.
Don't even need to know the cartridge.

 

[carlsbad]

I would have predicted failure beyond 3400fps in 8tw for 24cal.
My route:
KrpmLim = ((0.308-Your bullet dia.)*238.095+290) = 305Krpm
Velocity limit = 1.38889*KrpmLim*Twist = 3394fps

This predicted original Berger VLD failures(now 'hunting' VLDs) - exactly.
But other bullets tend to be close ahead/behind.
Don't even need to know the cartridge.

I follow your calcs and have done them myself but what is the parameter where bullets fail?

If you want to have some fun calculate the centrifugal accelaration of a mass element at the surface of the bullet. It will be almost unbelievable.

--Jerry

 

[mikecr]

I can only see it as overall energy applied to reach a failure spin rate.
That is, not just high spin rate itself, nor great heat energy itself, but the combination.
If you review it, there is always both in extremes with these failures.

 

[chop house]

centrifugal accelaration of a mass element at the surface of the bullet. It will be almost unbelievable.

well, there ya go. how do they EVER stay together? especially once we engrave those discontinuities in them

 

[SBS]

[QUOTE="carlsbad,
If you want to have some fun calculate the centrifugal accelaration of a mass element at the surface of the bullet. It will be almost unbelievable.

--Jerry[/QUOTE]

Yep, I KNEW there had to be a simple answer!

 

[Texas10]

Thanks Texas10, is there a rule of thumb for max bullet RPM to stay below?

If i'm not mistaken, Hornaday sets the redline at 280,000. Sierra and Berger are at or slightly over 300,000. Barrel internal finish makes a lot of difference, but at the speed you're moving them down an 8 twist, not many will survive that kind of force.