What 'newbies' don't understand for lack of experience is that one can utilize all the data, formula, BC input, MV, stability factors, etc. and in the final analysis....each rifle is going to deliver it's best accuracy with one load, one bullet, and one speed which the owner thereof has to discover by shooting!!
Berger's new "Optimal Twist" ??
- Thread starter JDMock
- Start date
Let me add a little more information to Coreys post. What the new OPTIMAL TWIST RATE is based on is a "worst case scenario of 59 degrees Fahrenheit at sea level. This means that if you have the optimal twist rate listed you are good from sea level on up as far as stability and full BC of the bullet. So, as Corey stated " use our twist rate calculator to figure what twist rate you need for optimal stability at the ALTITUDE (S) you will be shooting at. Always use the Lowest altitude to figure your twist rate. Now , having said all that. If with your twist rate you end up with a stability factor of 1.4 or so it dosent mean that the bullet will not fly point forward at range. It just means that your BC is compromised by the percentage listed and external affects such as wind , rotational stability ETC will have more affect that they would at a stability factor of 1.5 or greater. In fact out to 600 yards you probably will see not affect on the bullet at all. Please keep in mind that if something(s) changes due to a change in bullet design we will tell you that its because of the design change. If you see change(s) without a design change statement it is because of advancements in testing instruments ETC that we have been able to get even better information on our bullets and are passing this on to you so you all can make adjustments if necessary and have better information to make decisions with rifle builds ETC.The bullets have not changed, just the information surrounding them. Some of the recommended twist rates were sped up in order to achieve an SG of 1.5 or greater. This does not mean you can't shoot them through a slightly slower twist like you have in the past. It just means you are potentially hurting the BC of the bullet a little bit. As for guys who have had good results shooting certain bullets through a now slower than recommended twist, "If it ain't broke, don't fix it"
If in doubt, run your particular setup with your own variables and see where it may fall: http://www.bergerbullets.com/twist-rate-calculator/
Here is a little more detail on the updated twist rates: http://www.bergerbullets.com/berger-bullets-twist-rate-recommendations-by-eric-stecker-bryan-litz/
But as Corey so aptly stated above " IF IT AINT BROKE! DONT FIX IT!" If you have a load that performs at .5 to 1 MOA at the range(s) you shoot out too. More accuracy can be found through learning how to read conditions , Trigger control and position than can be found fooling with the load. Get out there and practice! Even if its blowing a gale !
Thanks Phil. Our range at which we shoot a 600 yard match each month has an elevation of ~ 100 feet. We shoot 12 matches a year and it can be pretty cool during the winter months. So far the gain twist (8.25-7.75) has performed a little better than the 8 twist barrel in my 6 Dasher. I use mainly the 105 Hybrid and Varget powder for a velocity of 2950-2975. Should my next barrel be a 7.5 twist or perhaps a 7 twist? James
I shoot at 50' sea level and I find at 600 yards I can't see the difference between the 8" and the 7.5 . At 1000 the 7-5 is better. Larry
I'd agree with the general sentiments of this post, but this is not a worst case, rather a standard case. Atmospheric pressure is always quoted at sea level in weather forecasts and barometric charts - and it varies as does air temperature. So, it may be below 29.92 inches mercury (raising the Sg value), or higher (reducing Sg).
Whatever it happens to be at sea level, the pressure drops as you climb, by roughly an inch per 1,000 ft altitude. So, the actual Sg (and if the twist rate is marginal, BC too) will always be higher at Raton than at Camp Perry as the 7,000 or so ft altitude difference will invariably exceed any weather related (low and high pressure areas) variations occurring over continental USA.
When Bryan Litz tests bullets with his sound microphone method, no doubt others such as Lapua using Doppler radar too, the temperature, humidity, altitude and atmospheric pressure are measured to get those applying during the bullet flight, and the results are 'corrected' to what they would have been at sea level on a day when atmospheric pressure is 29.92 inches, air temperature 59-deg F etc.
I'm safe by 50' and it's 62 . Larry
JD,
Run THE NUMBERS ON THE twist rate calculator . Savagedasher does make a good point about going out to 1000 yds. Spin rate will help stability as the bullet slows down. Especially on a long bullet. Don't go crazy though ! MORES LAW can get you into issues in everything in reloading! lol !
Don't be so sure about that. Have a look at:
http://forum.accurateshooter.com/threads/ideal-twist-for-215-hybrid.3862132/
Team Michigan, coached by Bryan Litz, has obtained some 1,000 yard 'possibles' with this bullet with an apparently sub-optimal twist rate, but so far has failed to do it with rifles with (faster) optimal twist-rate barrels. My reading of this thread is that there were smaller long-range verticals with a slightly too slowly spun bullet. That may just apply to this one bullet of course ..... but it makes you think!
There are also a lot of people here getting superb results at out to 1,200 yards with the 180gn 7mm Hybrid in 1-9" twist barrels at Sg values which are a little short of the optimum 1.5 value, especially in winter. The 1,000 yard elevations which some people get from their .284s and 7mm WSMs with this combination are staggeringly small.
Laurie,
You are correct but you are running on the edge here and may have compromised your BC by a certain percentage. Now , if you are a top notch shooter like Bryan and his team you can call the wind a lot closer than the average bear and make up for this. Most shooters dont practice enough to get there so the lost BC due to the spin rate stability loss affects them much more. A slower spin rate MAY help with vertical but a hard hold helps more.
That BC/windage v verticals trade-off is a key one Phil. Before the heavy .308 Hybrids appeared, many GB FTR top competitors loaded 155.5s and the 210gn Berger LR BT (missing out the 185gn Juggernaut). Several continue to do so having tried the Hybrids and decided to carry on as before. Ammo choice was / is made on the day depending on weather conditions and an estimate of which is more important - verticals or windage related factors. The 210gn is noticeably superior in the wind, but pays for that partly in the ocassional point lost to a larger vertical spread. So, as always one makes one's choice and pays one's money - and at the end of the day when the results are posted hope that you don't rue the decision. What Bryan was saying here suggests another variation on this theme in which if (I stress if) there is a real and enduring correlation here and the competitor feels competent to make the decision, a small amount of BC (of these exceptionally high-rated models) might be deliberately sacrificed for reduced vertical spread. Your average F-Class and FTR competitor really hates 'verticals'. The F-Class people have it easier as their sevens often seem to combine both attributes - small verticals and reduced windage / super-high Speed + BC combination; FTR and the 308 still seem to req1uire some compromises.
We shoot at 50' sea level humidity of 98 percent . At 600 nod difference between 7-5 and 8 twist 1000 the7.5 shoots better And the hole in the paper is smaller. Will have a 7 to compair The hi humidity slows the bullets . Larry
If you are WAY over twisting the bullet in a long barrel at 3400 fps and up you could most likely compromise the bullets jacket due to the excessive heat and friction. It can be one, two, or all three of the these factors that may be the cause. Shooting a bullet that has an optimal twist rate of 1-10" in a 1-8" would in most cases probably not cause any structural issues with the bullet. The only real way to find out is to shoot it as you would in a string of fire like you have in competition where you are limited by time and the rifle does not have a chance to cool down any ( if at all) between shots. Get Bryan Litz's book MODERN ADVANCEMENTS IN LONG RANGE SHOOTING (vol 1) and the first chapter will explain everything pertaining to your question.
Laurie,
Bryans book APPLIED BALLISTIC for LONG RANGE SHOOTERS . Chapter 10 on page 135 " Bullet Stability" really goes into more detail than his article on this website on our discussion. Do you have the book?
That this was so was widely believed at one time. Kevin Thomas (on this forum under his own name) wrote about this indirectly in Precision Shooter magazine way back in the 80s in his Sierra Bullets days. It was an interesting short feature on the background to the design and introduction of the 175gn Sierra MatchKing and its adoption by the US military in what became the M118LR round. Long-Range (1,000 yard) Service Rifle shooting had started to become popular using the 7.62mm M14 / M1A rifle with its 20-inch 1 in 10" twist barrel. The venerable 168gn Sierra MK (originally designed for 300 metre ISSF competition) and used in the M14 in Service Rifle match ammo for the normal 200/300/600 yard XTC programme wouldn't perform reliably at this distance, so something else was needed and it had to comply with the M14's gas operating system parameters which basically means under 180gn. The original 155gn SMK was already there in use by Fullbore / Palma shooters and seemed an obvious answer for the L-R SR shooters, but it apparently didn't work out as well as anticipated. One hypothesis for this was that it was 'over-stabilised' and at its peak MRT point when it should start to point downwards towards the target, the degree of spin caused the nose to point a little high trying to continue on its original angle of departure from the muzzle and to descent a la space shuttle (but to nothing like the same degree) thereby incurring unwanted extra drag, also risking instability.
Bryan adresses this issue in book (Applied ballistics for the Long-Range Shooter) and discounts it in trajecory / BC / stability terms provided the bullet is well enough made in terms of its weight balance around a line through the centre. Those of us who've shot long-range for a while remember the days of ultra-slow 30-calibre twists and many still thought in these terms in the early days of FTR. 1 in 14" twists were common in British and Commonwealth TR throughout the 70s and 80s, and the still popular 1 in 13" pitch is really a spin-off (pun!) of this attitude possibly being rather slow for many of today's long 155gn bullets. Because we shot military ammo (in the UK and British Commonwealth, this was near mandatory using 145-146gn so-called 7.62mm sniper ammunition such as the UK Radway Green Ordnance Factory 'Green Spot' round) the bullets were so totally cr*p in terms of their concentricity, jacket thickness consistency around their circumference etc, the aim was to spin them as slowly as would just stabilise them. Basically, the higher the rate of its rotation, the more the bullet deviates from its intended aimed path if these adverse conditions apply. Bryan's point is that modern match bullets are in a different league and that there is therefore no benefit in pushing the envelope in this direction any more . In fact, there are much more likely to be disbenefits - and this was before the research on a higher spin rate (Sg at 1.50 instead of the long recommended 1.40) being needed to achieve the full BC value.
As I tend to take Bryan's findings pretty seriously, I for one have tended to use some rather fast twists in 308 for the last few years / rifles / barrels and have used 1 in 10" for quite a while now even though most of my shooting was with the 155.5gn Berger until recently when I took a liking to the 168gn Hybrid. Berger's Twist Rate Calculator says my Sg value for the 155.5 BT is 2.28 in the twist. It shoots very well indeed with that (or despite that depending on your point of view). At Raton a couple of years back given the altitude and air temperatures, Sg values ran in excess of 3.0 for me and most GB competitors and I didn't hear anybody complain.
Figured that but had to ask sir. Good string!
Upgrades & Donations
This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.