Bullets breaking apart in flight at longer ranges?

[The boogalorian]

That wasn't very clear was it...........

Formula is: Bullet RPM = (velocity in fps x 720)/(barrel twist rate rate).
In the case of my 284, my RPM = (2825 x 720)/8.5 = 239,000 rpms.
I am betting your is close to: RPM = (2970 x 720)/8.0 = 267,000 rpms.

Hope that is clearer.

That is still below what is mentioned above as Hornaday's maximum recommendation for rpm but it is somewhat close. Heat weakens a jacket and the faster it spins, the more force the jacket has to absorb. In the end, the issue is the jacket is too weak for the condition. I was able to really minimize issues running barrel coolers between strings but that only limited the problem. I've since moved to other bullets with stronger jackets.

isn’t fps = feet per second, and barrel twist in inches? I think there should be a conversion factor somewhere. Maybe I’m wrong. That has happened before.

Muzzle velocity (feet/second) x 60 sec/min x 1.5 ((8 twist=1.5) rotation/foot) =rpm

for example my 6x47l shooting 109’s in a 1 in 8 twist
3,065fps x 60sec/min x 1.5 r/f= 275,850rpm

 

[rardoin]

I have shot less than 500 Hornady bullets in 6.5 and 7mm at moderate velocity in two different makes of barrels, neither high mileage, and no failures. The only bullets I have shredded were 150 SMK’s and a couple of 142 Precision Ballistics in a snug straight cut riflling barrel with over 1000rds and probably not cleaned well. Go figure.

 

[jelenko]

isn’t fps = feet per second, and barrel twist in inches? I think there should be a conversion factor somewhere. Maybe I’m wrong. That has happened before.

That's what the 720 does.

 

[The boogalorian]

I updated my post. Not sure how 720 converts inches to feet or seconds to minutes.

duh. I see what I was getting confused about.

 

[jelenko]

That will do the job. I was a junior in Sweden in the 70’s and nobody ever blew bullets up in their rifles. It’s a bullet problem not a rifle problem.
My CG63 is still with me here in Ohio, don’t shoot it much but it still has to blow a bullet up.

Berger did an investigation into what was causing some of their bullets to disintegrate. They found it's a combination of factors, one of which is the thickness of the jacket with spin rate and heat from friction being the other two major factors.

Spoiler alert: The thicker jacket does not keep the bullet from disintegrating because the jacket is stronger.​

See the post by Old Gunner http://benchrest.com/archive/index.php/t-65740.html

 

[jelenko]

I updated my post. Not sure how 720 converts inches to feet or seconds to minutes.

In the numerator, you've got length/time, in the denominator you've got lenth/revolution => when you divide the denominator into the numerator, the resulting dimension is revolution/time. If you want to know where the 720 comes from, you can do the math yourself.

 

[tomswede]

Berger did an investigation into what was causing some of their bullets to disintegrate. They found it's a combination of factors, one of which is the thickness of the jacket with spin rate and heat from friction being the other two major factors.

Spoiler alert: The thicker jacket does not keep the bullet from disintegrating because the jacket is stronger.​

See the post by Old Gunner http://benchrest.com/archive/index.php/t-65740.html

Some years ago I shot at Perry with some juniors from CA, they were shooting Berger’s and had the bullets blow up. It seems like Berger fixed the problem while Hornady has not.
Guess my point is why frustrate yourself with a known problem brand when the data says that others don’t have the problem.

 

[FeMan]

I updated my post. Not sure how 720 converts inches to feet or seconds to minutes.

the formula is: ???ft/sec x 12in/ft x 60sec/min x rev/??in = bullet rpm

 

[jelenko]

Some years ago I shot at Perry with some juniors from CA, they were shooting Berger’s and had the bullets blow up. It seems like Berger fixed the problem while Hornady has not.
Guess my point is why frustrate yourself with a known problem brand when the data says that others don’t have the problem.

Understand. Completely agree that it's better to simply avoid a potential issue than trying to deal with it. I love it when I can buy my way out of a problem.
But, even Sierra says they would like their target bullets spinning less than 250K - 260K. I've heard reports of SMK's disappearing on the way to the target - not as much as Hornady's, though.

My take away has been to consider what the RPM's will be on the bullets that are planned to be shot. My perception is that many [might have included myself] didn't consider RPM's when choosing the twist rate. For example, a 7 twist, 26", 223 barrel can easily produce MV of 2800 fps => 288K RPM. A 7.7 twist produces 261K which keeps it within both Hornady's and Sierra's recommended ranges. [In talking with a Hornady tech, he said they want the bullets to be less than 280K].

I haven't talked with a Berger tech, but it would be interesting to know what their recommendation is.

 

[logandiana]

My take away has been to consider what the RPM's will be on the bullets that are planned to be shot. My perception is that many [might have included myself] didn't consider RPM's when choosing the twist rate.

I am one of the many! I never even knew this was a thing until now.

 

[ebb]

So let me get this straight, Hornaday took Amaxes of the market cause the tips were melting and they were not living up to their BC claims. Then they brought out a new model doubled the price and they melt the core on the way to the target and don't even make the whole trip. Seems like you had a better chance of actually hitting the target with the Amaxes than the new bullets. But now Hornaday gets twice as much money for them. Something tells me the solution will not go back to Amax prices. I liked Amaxes a lot.

 

[Spook]

So let me get this straight, Hornaday took Amaxes of the market cause the tips were melting and they were not living up to their BC claims. Then they brought out a new model doubled the price and they melt the core on the way to the target and don't even make the whole trip. Seems like you had a better chance of actually hitting the target with the Amaxes than the new bullets. But now Hornaday gets twice as much money for them. Something tells me the solution will not go back to Amax prices. I liked Amaxes a lot.

Ebb, I shot 105 Amax to 600 regularly, they shot well, they ALL made it to the paper and they were relatively inexpensive. I miss the AMAX line.
I lived next to a guy who's Ford SUV got renamed " the ford exploder" (explorer), Gotta come up with something catchy for the ELD's... like popcorns or sugar pops or weasels ?

 

[ebb]

Many years ago when i first got a 6.5-284 I shot a ladder with 140 Amax and 142 Sierra the Amax made smaller groups than the 142s and IMR 4381 shot better that H 4381. Never would have guessed that. Shot Amax for years in the gun.

 

[STOMP442]

Ebb, I shot 105 Amax to 600 regularly, they shot well, they ALL made it to the paper and they were relatively inexpensive. I miss the AMAX line.
I lived next to a guy who's Ford SUV got renamed " the ford exploder" (explorer), Gotta come up with something catchy for the ELD's... like popcorns or sugar pops or weasels ?

ELD's = Exploding Lead Delivery System. or Extreme lack of dependability.

 

[saunaking]

I threw a post up in the gunsmithing area of the forum because of a similar problem I am having with 64 grain Berger FB varmint bullets. Once I get over 3400 fps in a 22BR they blow up. I did the calculation using the formula here and get about 310,500 with the 8 twist barrel on it right now.
So my question is whether that is too much for the light skinned bullet, or if I have a bigger problem with a barrel not lapped properly or maybe a carbon ring problem? I have not bore scoped it yet, but will be this week. The 75-80 grain bullets don't blow up (especially my 75 grain Amax...).