Twist rate and bullet selection

[WestKentucky]

Is there a calculation for determine what an optimal bullet weight is for a given bore size/speed/twist? I’m going to be working up new loads for several guns in the near future and with that I want to start off with something that is most likely to give me good results. I will be loading 223ai, pointy 30-30 (single shot) and .270 win. And out of curiosity I was trying to see what would logically be a good starting point for those. I know that throat erosion in the .270 is forcing me away from the 130gr I want to use so it looks like 150s loaded to magazine length are in my future.

 

[Kp321]

Research Greenhill Formula. It will tell you the desired twist rate for a given bullet but with a little algebra, it could be used to solve for bullet variables. Some assumptions would have to be made on bullet shape and material.

 

[Jim Watson]

The Miller formula is more accurate with modern spitzers but I don't know where it might be shown as an equation so you could back out a bullet for a given stability factor.
I would look at the Berger or JBL calculators and just keep running examples until I had some matchups.

That is just the math. A lot of rifles are more accurate with lighter/shorter bullets than the maximum their twist will stabilize. My 6.5 twist 90 gr .223 shoots 75 gr Amax very well if I don't load it so hot as to blow the bullets up. My 9 twist rifle and my 12 twist rifle are just backwards to what the numbers say with 50 and 60 gr bullets.

So what twist are your rifles? The .223 AI could be anything from a pure varmint rifle to a midrange (600 yard) gun.
The .30-30 is likely either a 10 or 12 twist and will handle pretty heavy bullets; my 788 shoots 173 gr military match pulls. But my Contender Super 14 did best with 150 Speer boattails.
It sounds like your well worn .270 is not a theoretical ballistics exercise, you are chasing the lands and a 150 is about what you can use.

 

[WestKentucky]

So what twist are your rifles?

Sadly, I don’t know. They are both factory super 14 contender barrels, but neither of them are marked. The 223AI was a limited run and I found a post online saying that they are 12 twist but I don’t know. I haven’t looked up the numbers on the 30-30 barrel yet but it’s not listed on the barrel either. Come to think of it I’m not sure that it’s listed on my .357 mag or 7-30 waters barrels either. The .270 is a Remington 700 and it’s in the 5 digit range for round count. I’m not convinced that the barrel is salvageable, and if it’s not then the gun will simply be retired. It’s as budget as a 700gets and is worth less than any barrel I would consider putting on it. It’s killed a whole bunch of critters though in its 20 years of service. It’s killed a whole bunch of paper too over the years... and car hoods, milk jugs... couldn’t ask for a better rifle. I may just go ahead and load extra long and single load it if I have to just to keep it going.

 

[Jim Watson]

It is not usual to put the twist on a barrel other than ARs that are made with all different rates.

A tight patch on a swivel handle cleaning rod, a mark or piece of tape on the rod so you can see it turn, and you will soon know the twist.

What will the .270 actually do on target now?

 

[Blue68f100]

1:12 is what was introduced when the AR's first came out. Great for light bullets, 55gr and less. Would not stabilized heavy bullets, tumbled. I have 1:10 that loves the 52gr SMK. I also have a 1:9 (preferred) and 1:8. The 1:8 is only good for heavy bullets if you want the ultimate accuracy.

 

[WestKentucky]

What will the .270 actually do on target now?

It’s hard to say. It has trouble finding the target. Whatever issue it has happened pretty suddenly. It was shooting nice and tight, then it stuck a loaded round hard enough that I ripped a chunk of the casehead off trying to get it open, then I totally disassembled the gun and used a 4 lb maul and an aluminum rod to finish extracting the case. Bore looks fine and I did not foul up the crown, but it refuses to shoot better than shotgun pattern now. I have done everything I know to do with it and a gunsmith friend looked at it and couldn’t diagnose any actual issues other than erosion. Bore looks beautiful, throat looks like a cutting torch has been after it.

 

[Jim Watson]

Beyond my remote diagnostic ability.
There is a factory barrel on eBay for $145. Claimed new.

 

[Bfh_auto]

1:12 is what was introduced when the AR's first came out. Great for light bullets, 55gr and less. Would not stabilized heavy bullets, tumbled. I have 1:10 that loves the 52gr SMK. I also have a 1:9 (preferred) and 1:8. The 1:8 is only good for heavy bullets if you want the ultimate accuracy.

I have a 1-8 556 that stacks 40 grain vmax better than it does a 77 SMK if the wind isn't blowing.
I've learned to use the twist rate for calculating maximum bullet length.
The lightest bullet it will shoot depends on the particular barrel.

 

[WestKentucky]

Factory barrel $145
Gunsmith fees to install ??? At least as much as the barrel costs. I don’t have a 700 action wrench either. let’s call it 300 flat as a WAG.

Value of 20 yr old polymer and matte blue rem 700 in a common caliber- $300.... I don’t gain anything by not simply replacing the gun.

Value of Savage Axis or TC Compass to replace worn out Remington 700 listed above- less than other options.

 

[Jim Watson]

1:12 is what was introduced when the AR's first came out. .

Not exactly. The very first ARs had the 14 twist common to .222, .222 Magnum, .22-250 and most other .22 centerfires later than .22 Hornet. Fine for 55 gr flat base spitzers and 52 gr boattails.
But it would not stabilize cheap 55 gr FMJ boattails in cold weather. Hence the 12 twist.

After that, attempts to improve on range, penetration, and accuracy led to faster and faster twists. With a lot of confusion and Authoritative Internet Posts that the long time standard 14 twist varmint barrel was only good for sub-50 gr bullets.

 

[AJC1]

Factory barrel $145
Gunsmith fees to install ??? At least as much as the barrel costs. I don’t have a 700 action wrench either. let’s call it 300 flat as a WAG.

Value of 20 yr old polymer and matte blue rem 700 in a common caliber- $300.... I don’t gain anything by not simply replacing the gun.

Value of Savage Axis or TC Compass to replace worn out Remington 700 listed above- less than other options.

Ramage barrel from shilen 250ish nut and wrench 300... the ability to do it again or switch calibers when bored priceless