Please Explain Rate Of Twist vs. Bullet Weight

[mwpslp]

In particular I have a Sako 75 Hunter in .270 Win. I am not sure what the rate of twist is on this gun so if anyone knows that would be helpful. I was wondering if a higher rate of twist leads to more accurracy with heavier bullets or is it the other way around? My gun seems to shoot Hornady 130 gr. bullets well but I was thinking about trying a 150 gr. Not sure how much difference the 20 gr. would make or even if it was feasable or necessary with this rifle.

 

[rbernie]

Twist rate dictates the LENGTH of a bullet that can be properly stabilized. Since heavier bullets tend to be longer than lighter ones, it's sort of a truism that a faster twist rate lets you stabilize heavier bullets and a slower twist rate may not. Dunno specifically about your Sako, but most 270s that I've encountered have a 1:10" twist rate, suitable for 110gr to 160gr bullets just fine.

Once a bullet is stabilized, spinning it faster won't make it inherently MORE accurate as best I know. Spinning it faster MAY make a bullet fly apart if it's a very lightly contructed varmint bullet and you're spinning it way too fast (e.g. shooting a 45gr .224 varmint bullet from a 1:7" twist barrel).

You're more likely to run into receiver-n-barrel harmonic preferences with a given weight/contruction bullet for a given powder than anything else.

 

[jpwilly]

Pretty sure the rate of twist is 1 in 10" in your Sako The 130gr pills tend to be a little more accurate in the 270 but the 150gr should shoot well enough...you'll have to try it out. Heavier bullets in any caliber are longer and therefore need to be spun faster to stabalize them.

 

[GunTech]

rbernie has it right. It's length, not weight. For example, you can go to a heavy, round nose bullet even if your twist won't stabilize the same weight bullet in a spitzer. Also, all copper bullets like the Barnes TSX require faster twists than lead bullets of the same weight, because copper is less dense.

The old way to determine required twist was to use the Greenhill formular, but that formula was derived from observing jacketed lead, round nose bullets. It is not accurate for modern spitzer or boattail bullets.

There are a number of more modern formula available for calculating required twist, most Based on Robert McCoys McDrag. I recommend this one:

http://www.eskimo.com/~jbm/calculations/drag/drag.html

Overspinning a bullet (more twist than required) can lead to increase Magus effect as well as bullets that precess or don't stay tangential to the the flight path, leading to a smaller effective BC. Note that these effects aren't notice at short range, but can be noted if you shoot long - like 1K yards.

 

[mwpslp]

The twist rate is in fact 1:10. I will give the Hornady 150 gr. a try and hope they are as accurate for me as their 130gr. Should a 1:10 twist rate be sufficient, if anyone has any personal knowledge with this caliber?

 

[Jim Watson]

The .270 came out in 1925 with a 10" twist for a 130 grain spitzer. They added a rather mild 150 grain deer load in 1933. Sometime later, probably right after resuming commercial production after WW II, they souped it up to compete with the .30-06. I have never heard anything against it.

I think rifling twist in hunting rifles is overstudied. The usual barrels and the usual bullets match up adequately well for their intended purposes. It takes a pretty good target shooter in the more difficult events to call for much different twist than has been used all along for standard calibers.

 

[frogomatic]

as previously stated, twist rate needs are based on the length of the bullet not the actual weight of the bullet. According to the formula's I got from school, a 1:10 twist will stabilize anything you can load in a .270

on a side note. While I haven't seen any evidence that a bullet can be over stabilized, I have personally seen thin jacketed rounds tear themselves apart when fired through very fast twist barrels.

 

[Stillwater97]

Ok,

I have a Stevens 200 in .243Win with a 9.5 in 12 twist. What bullets should I be reloalding?

MB

 

[frogomatic]

9.5 in 12?

do you mean 1:9.5 or 1:12?

if it's 1:9.5 you're good to go for anything but very thin jacketed bullets

if it's 1:12 you should be good to about 100 grains

 

[45crittergitter]

Length vs. mass or weight

Technically, it IS the weight, not the length, that is the primary concern. The twist formula (Greenhill) does ask for length, but the formula itself presumes a composite bullet density for a typical copper jacketed lead core bullet (I didn't check, but believe it assumes 90% lead and 10% copper). A bullet that is of significantly different density will not yield accurate results with this formula, even if it's length is properly entered.

 

[saltydog452]

Not to be contentious, but when folks mention 'length' of a bullet, I thjink that the amt of bearing surface is more accurately what they are refering to.

A BT HP has a different 'length' of the bearing surface.

That'd open a considerable sized can of worms.

Consider about 20-30 years ago the 1000 yard shooters were fooling around with 140 grain 6.5 mm BT HPs for the extreme long range events. Mostly they were using 300 Win or Wtbhy Mags necked down to 6.5 set to headspace on the neck rather than the belt.

In my .264, (1:10) twist, I could not get the 140s to group worth a flip @ practical distance. If I did my part, they'd do just dandy on paper at longer ranges though.

The .264 was a different breed of cat. Factory ammunition was, at that time, either 100 grain or 140. The 120s, using 4831, pushed at 270 speed worked well on paper at reasonable hunting distance.

I suspect though that, other than the 12 gauge, the factory loads and guns like the 30.30 has put more meat on the table and gotten more folks through the winter than just about any other caliber/gauge.

Tree rodents and 22rf exempted.

salty

 

[Jim Watson]

No, salty, he is talking about the length from base to tip. Your .264 would have stabilized a 140 gr roundnose better and it might well have been more accurate , even though the trajectory would not have been as flat.

 

[saltydog452]

I dunno Mr. Watson, never fooled around with the round nose 140s.

Even the BT HPs weren't designed for making meat, just punching holes in paper at long range.

It was interesting to fool around with, but served no practical purpose.

My .264 with Exhibition Grade wood and a Heavy Sporter 26" barrel built on a '17 Enfield action was a chore to tote around.

I still respectfully submit that the length of bearing surface is more relevant than total length.

I can't prove it, but thats still my notion.

salty

 

[GunTech]

Greenhill is out of date. The Greenhill formula is a rule of thumb based on observed behavior of round nose, jacketed lead bullets. It's over a hundred years old and is not appropriate for 'modern' spitzer and boat tailed bullets. I'm not sure why people keep referring to it.

Most modern calculations are derived from the work of Robert McCoy, who spent 30 years at the Ballistics Research Laboratory and was responsible for developing many of the standard mathematical models in use today by professional ballisticians.

For people interested in the nitty-gritty and who love doing DiffEQ in their spare time, I cannot recommend a more detailed and boring book than McCoy's Modern Exterior Ballistics

McCoys program McDrag and others are the basis for most modern ballistics software.

http://www.amazon.com/Modern-Exterior-Ballistics-Symmetric-Projectiles/dp/0764307207

 

[Swampy]

I still respectfully submit that the length of bearing surface is more relevant than total length.

Sorry saltydog..... not true. As long as average density remains relatively constant (As mentioned earlier in this thread...) between the various shapes being tested the bullet overall length is the determining factor as to how much twist is needed for stabilization.

Best,
Swampy

 

[saltydog452]

Reluctantly, I stand corrected.

I do have a kinda/sorta relevant guestion though...In the Sierra Match King BT HPs, if not to lessen bearing surface, what is the purpose of the BT?..or HP?

Thanks,

salty

 

[Jim Watson]

The boattail is what amounts to streamlining.
A bullet's trajectory and wind drift are determined by velocity and ballistic coefficient. The cartridge and the load control velocity.
Ballistic coefficient is sectional density divided by form factor.
Sectional density is bullet weight divided by diameter squared, so the caliber and weight are all that matter. Shape and aerodynamics are covered by the form factor. A boattail spitzer has a low form factor. There are whole books on the subject if you want details.

The SMK and many other match bullets are made as hollowpoints because that lets the bullet base be completely covered in smooth, even, jacket material for a uniform exit from the muzzle of the barrel. The hollow also sets the center of gravity back, which is favorable in a rifle bullet.

 

[saltydog452]

Thank you sir.

salty

 

[Swampy]

I do have a kinda/sorta relevant guestion though...In the Sierra Match King BT HPs, if not to lessen bearing surface, what is the purpose of the BT?..or HP?

A BT shape on the back of the bullet is more aerodynamic than a flat base. You will find that most bullets meant for ranges longer than 300 yards have a BT. It definitely ups the Ballistic Cooeficient.

The HP is just a by-product of the manufacturing process.... It is almost impossible to close up the jacket nose entirely with any uniformity. As a result, the small opening remains. Unless and until you decide to use a nylon-plastic tip like the "Blitz-King" bullet (Same as Hornady "Ballistic Tip") the small hollow end is just the point where the jacket comes together as far as it will go. The other alternative is to leave exposed lead ala "Game King" and other types.

Best regards,
Swampy

 

[saltydog452]

Thnx again.

salty