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Twist and bullet weight

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Old March 6th, 2017, 04:13 PM   #1
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Twist and bullet weight

If a faster twist is necessary to stabilize a heavier bullet...

Is there an optimum twist rate for 150 grain .308 bullets? 165/168 grain? Optimum range for bullet speed?

What happens to a lighter bullet in a "fast twist" barrel? does it become de-stable? Would one reduce the charge for a lighter bullet (reducing from 2900 to 2500 or less fps)?

I am working up loads for a bolt action .308 with a 1:11 twist barrel and want to experiment with 125, 150, and 168 grain bullets. I intend to use whatever load I come up with mainly for 200 yd prone shooting.

Any experience that could help me get close and save some time is appreciated!

I've been reloading for awhile but this is my first time working up a load for a bolt rifle.

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Old March 6th, 2017, 04:44 PM   #2
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There are literally volumes written regarding your question and those with experience loading usually find the right combination of bullet weight/twist combinations that work for them. I shot Long Range matches and Palma Matches for several years and contrary to some I used the SMK's with Krieger 30" barrel, 12 twist and bullet weight was 190gr.
Was told that was the wrong combination, but did well at the game even though in theory the bullet would be sub sonic at the 1000yd. target face. Not so, but if it was got plenty of 10's and X's. Primary reason for going this route was to defend against wind drift with the heavier bullet. If you give Sierra a call, believe you will find they are more than happy to discuss the issue with you in detail and answer all of your questions. Not trying to avoid an answer, but Sierra folks have those very expert in such matters and they have never steered me wrong to date. Would imagine other bullet mfg.'s be equally willing to give you expert advise as well. A Sierra load manual has quite a bit of info on internal and external ballistics using scientific research data. Just a suggestion.

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Old March 6th, 2017, 04:57 PM   #3
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Quote:
Originally Posted by topclass2017 View Post
If a faster twist is necessary to stabilize a heavier bullet...

Is there an optimum twist rate for 150 grain .308 bullets? 165/168 grain? Optimum range for bullet speed?

What happens to a lighter bullet in a "fast twist" barrel? does it become de-stable? Would one reduce the charge for a lighter bullet (reducing from 2900 to 2500 or less fps)?

I am working up loads for a bolt action .308 with a 1:11 twist barrel and want to experiment with 125, 150, and 168 grain bullets. I intend to use whatever load I come up with mainly for 200 yd prone shooting.

Any experience that could help me get close and save some time is appreciated!

I've been reloading for awhile but this is my first time working up a load for a bolt rifle.
A fast twist is necessary to stabilize a long bullet, not necessarily heavier....

All you ever wanted to know about bullet stability, but were afraid to ask....

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Old March 6th, 2017, 05:39 PM   #4
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The conventional wisdom over the last 115 years is that you need a 1/12 twist for a 150 and below, a 1/11 for a 168 and a 1/10 for 175 and above.

However, there is no guarantee to any of it. Much relies on the quality of the barrel and the consistency of the ammunition.

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Old March 6th, 2017, 05:52 PM   #5
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Unless you shoot in the Alaskan winter, or at velocities below 2500 fps, 1 in 12 will stabilize a 190 grain SMK adequately....

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Old March 6th, 2017, 06:08 PM   #6
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Unless you shoot in the Alaskan winter, or at velocities below 2500 fps, 1 in 12 will stabilize a 190 grain SMK adequately....
That also depend on the barrels length don't you think, I know the Navy M1's had great success with there 7.62mm 1/12 tubes with the 190SMK but I don't think a 16in barrel with 190's would fair as well.......

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Old March 6th, 2017, 08:01 PM   #7
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Originally Posted by Phil McGrath View Post
That also depend on the barrels length don't you think, I know the Navy M1's had great success with there 7.62mm 1/12 tubes with the 190SMK but I don't think a 16in barrel with 190's would fair as well.......
Phil I tend to think that the 'velocity' achieved TRUMPS the barrel length here?

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Old March 6th, 2017, 08:07 PM   #8
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A well known top NW shooter of my acquaintance said on the Palma team they were using a 1-14 twist for the 155 gr bullets.

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Old March 7th, 2017, 05:30 AM   #9
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For 200 yard shooting, I don't think the twist rate itself will make a noticeable difference. Some barrels will shoot better than others, but that's much more complicated than just the twist rate.

Having more twist than is necessary can cause several problems in certain situations -
1) bullet jacket failures if MV is beyond the design limit for the bullet.
2) makes 'out of balance' (i.e. bad) bullets behave worse.
3) at long range (say beyond 600 yards) inhibits the bullet's ability to 'nose over' and stay aligned with its flight path - the bullet is 'too stable'.

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Old March 7th, 2017, 07:16 AM   #10
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Most of the Palma shooters I shot with "back in the day," used the SMK 155 Palma bullet and twist was 13 in those days. Notice that several of the factory rifles these days in 308 are using a 10 twist and supposedly for use of heavier bullets, but I stick with the 12 twist. Have read that the ideal twist(whatever ideal twist is) is 11.7 and see some Krieger barrels offered accordingly. Am told that other than cut rifled barrels the actual twist can vary as much as 1 versus what you are told that it is but also told that is nothing to worry about for the difference between 1 twist one way or the other does not alter the accuracy that much, extremely small amount in few thousands. Just recently built a Rem. 700 using Douglas barrel and specified 12 twist/4 groove/26" Varmint profile and upon checking the twist it is 12 or very close to it. Use cleaning rod with inch marks and count the rotations as pulling out of bore, not super accurate but gives a close reading.

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Old March 7th, 2017, 10:23 AM   #11
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It's not only the weight of the bullet that determines accuracy in any given twist. It's also the shape of the bullet.

The more metal that contacts the bore/twist the more accurate.

Thats why 6.5x55 bullets are generally more accurate. They have a long shape and more contact with the bore.

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Old March 7th, 2017, 05:49 PM   #12
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Question

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Originally Posted by JEFFJP_N_JJ View Post
Phil I tend to think that the 'velocity' achieved TRUMPS the barrel length here?
Ahhh my thought was a 16in tube wouldn't get a 190 up too 2500fps. Using a fast powder would generate higher chamber pressures and using a slow powder would make for excessive muzzle blast or inefficiency. In short, I think a 16in barrel with the 190SMK together is a bad combo that is unless your rifle is set up for subsonic use with a can?

The Navy put the stomp too a lot of Service Rifle shooters with the 7.62 M1's and the 190SMK combo. There then LR load data was 44. IMR-4320 under the 190SMK @ 2.90 M/V just checks in at a tad over 2600fps and while that is the min velocity threshold too cover 1000yds the 190SMK has a outstanding reputation for remaining stable when going transonic. I use the same data with the 185 Berger Hybred @ 2.95 M/V checks in just under 2650ish. The Berger is 5grs lighter and has a lot less bearing surface compared too the 190SMK that's why you see a boost in there velocity, not just there BC.

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Old March 7th, 2017, 07:40 PM   #13
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Built a Ruger NO 1 with 26" SS Douglas barrel and twist is 7(actually a little faster) and did so to shoot the heavier 22 bullets such as 75, 77, 80 and the 90's. The 69's give good accuracy but the best is the SMK 80gr bullets. I thought the 90's would do OK, but can not get the accuracy or even close as the 80's. Have not spent enough time developing loads for the 90's as I need to do and would imagine with more work involved could solve the problem?? If you have a problem with recoil and still want to compete in matches from 600 to 1000yds the 22-250 w/ the 80gr SMK's will get the job done providing you have a fast twist barrel. The round reminds me of shooting the 308 with 155 bullets in terms of wind adjustment needed, but my Ruger w/ Unertl 16x scope weighs in at some 10lbs or more and you barely know when it goes off, very pleasant to shoot for extended periods of time. Was told upon starting this venture that the preferred twist for the 90's would be no more than 6.5 twist and can't imagine what that would look like peering down the bore, awful fast it seems and the rpm's on that bullet would definitely be up there. I could get near 3400fps with the 80's but accuracy dropped off and find that just over 3000fps gives best performance.

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Old March 7th, 2017, 08:41 PM   #14
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Originally Posted by Phil McGrath View Post
That also depend on the barrels length don't you think, I know the Navy M1's had great success with there 7.62mm 1/12 tubes with the 190SMK but I don't think a 16in barrel with 190's would fair as well.......
It's the RPM that stabilizes the bullet, hence the 2500 fps qualifier.....

Twist X velocity = RPM

(with twist in revolutions per foot and velocity in feet per minute)

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Old March 8th, 2017, 08:33 AM   #15
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Originally Posted by topclass2017 View Post
Is there an optimum twist rate for 150 grain .308 bullets?
Depends on the 150 gr bullet!

A classical jacketed 150 gr 308 bullet is rather short. The entire volume inside the jacket is filled with lead.

A modern high BC 155 gr bullet is longer than a 175 SMK. All of the lead is away from the nose making the bullet back heavy and the front 20gr-volume is empty. So the aerodynamic profile is more like a 175 SMK (or a 190 SMK) than a 150. These bullets like faster twist rates.

The old 2155 155 SMK would shoot well in 1:14 PALMA rifle barrels (~30")
The new 2156 155 SMK needs 1:13 to shoot well and is considerably longer.
The 155.5 Berger FB actually needs 1:12 to shoot well.

A 150 gr pure copper body bullet will need a faster twist rate still as it has to be long in order to have that weight.

All of this points to the notion that it is no the weight of the bullet that makes the difference, it is the length.

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