88gr Bergers in 1:12 Barrel

[ChrisNam]

I have a 1:12 twist 28" Krieger barrel chambered in 6 Dasher. I normally shoot the 80 Sierras.

I've repetedly been told that the maximum bullet that will stabilise in the 1:12 twist will probably be the 85gr Sierra MK, but a friend used 88gr Bergers with his 1:12 twist barrel in F Class,club meets) at 300, 500 and sometimes out to 600m in calm conditions. His rifle is extremely accurate at 300m,long delays as he shoots out the disc every second shot or so).

I know I'll probably not be competitive with the 1:8 twist barrels shooting the 105/107 gr VLD bullets, but would like,just for the sake of doing it, possible to safe my 1:8 Dasher) to use the 1:12 Dasher at 300 and 500m.

So what's the problem? Try the 88gr Bergers and confirm that you have been warned!!!

I live in Namibia and it is extremely difficult,and time consuming) to get good quality BR bullets.

I laid my hands on 40 88gr Bergers and wasted a couple on ineffective loads. I then used 32.5 gr of Somchem 335,SA powder and the only brand available) and shot a 0.35 group at 3290fps, thereafter 33.0 gr for a slightly larger group at 3350fps. No signs whatsoever of pressure. I however only have 15 bullets left.

My question is: will a higher velocity,33.5gr or 34gr for say 3400fps?) cause the bullet to stabilise and shoot tighter groups?

If not, what other bullet can I use out to 500m,600y)?

 

[EricStecker]

Shooting a bullet faster can allow for the bullet to become stabilized if it were not stabilized at a slower velocity. It is not necessary to increase your velocity for the goal of making the bullet more accurate as making the bullet more stable after it has already achieved stability does not effect accuracy. In fact,anyone can correct me if I am wrong) I believe that once a bullet has become stabilized it cannot be made "more" stable. Stability is when the bullet is made to fly through the air while spinning around its axis rather than tumbling like a buzz saw. Once this is achieved a bullet cannot spin more efficiently around its axis. A bullet is either stable or its not.

From an accuracy,not stability) point of view you may find that higher velocity in your rifle can produce better accuracy. Tweaking the load to obtain better accuracy is an effective way to find the best accuracy that your combination is capable of shooting. If you have to tweak your load,increase velocity) to make the bullet become stable your are too close to the edge on stability and will likely observe that a bullet will keyhole under certain conditions.

To my knowledge the only connection to bullet RMP and accuracy is that the slower you spin the bullet,while maintaining stability) the less you magnify any concentricity problems within the bullet. This is the basis on which we recommend twist rates for our bullets.

For example, let's say that the jackets used to make a particular bullet have a wall thickness variation of .001 TIR,this is very bad but will make it easier to illustrate the point). The portion of the jacket that is thinner will have more lead between the inside of the jacket wall and the true center line of the bullet. This results in this portion of the bullet being heavier than the rest of the bullet,like an out of balance tire). When the bullet is spinning at 180,000 rpm,3,000 fps in a 12" twist) the amount of pull this heavier section creates by being out of balance is less in a 12" twist than if you were to shoot this same bullet at the same velocity in a 8" twist which produces 270,000 rpm.

Out of balance bullet + faster RPM = reduced accuracy
Out of balance bullet + slower RPM = improved accuracy

Having said that it is important to point out that bullets made on good jackets with a TIR of .0003 or less,J4 or a good batch of another brand) are not significantly effected by the above situation because the bullet is as balanced as is possible. There are many 223 shooters who will use a 52 gr or a 60 gr in a 8" twist at 200 yards then switch to an 80 gr VLD at 600 yards. Since the 52 gr and the 60 gr are made with good jackets,.0003 TIR or less) the accuracy is still very good even though these bullets are spinning considerably faster than is needed for stability.

I hope this helps. I am not sure I answered your question directly so if you need further assistance send me an email at eric.stecker@bergerbullets.com.

Regards,
Eric Stecker

 

[ChrisNam]

Eric, thank you very much for your assistance. I've used your 7mm 168 gr VLD bullets for some time and I'm extremely satisfied.

I received the 88gr LD bullets in a bag, so I'm not sure what recommedations appear on your box.

I presume the recommended twist for the 88 gr is 1:10?
If I can get the velocity up to stabilise the bullet,as suggested by you), what approximate velocity will, in your opinion, be required for the 88 gr bullet? You don't perhaps have loading data and velocities available?

I further presume that you will rather suggest the 88 gr FB - you don't perhaps have loaing data and velocities available?

Eric, thank you very much for your help!!!

 

[tightneck]

"To my knowledge the only connection to bullet RMP and accuracy is that the slower you spin the bullet,while maintaining stability) the less you magnify any concentricity problems within the bullet. This is the basis on which we recommend twist rates for our bullets"

Eric, For a given caliber, say .243", how do you determine the velocity of any particular bullet to come up with the listed twist recommendation for said bullet? Obviously a 6br cant push a 115 gr VLD nearly as fast as a 6mmAI, so are you playing it safe and using a low velocity? Two bullets I'm interested in are the 88gr LD bullet and the 115 VLD mainly because they seem to have a similar bearing surface length but the 88 is listed to stabilize in a 10 twist and the 115 in a 7. Is this a function of weight or bullet design or just the fact that you can push the 88's a lot faster? I'm getting ready to do a 6mmAI build and the 115 will probably be the primary bullet, so I'm interested to know if I could get away with a slower twist than a 7. I,d really like to use the slowest twist I can get away with. I've got some 105's and 115's and a 243AI barrel now with a 10 twist that I may do some of my own testing on, but I'm thinkin' the 10 twist may not do it for the 115's. Any insight would be appreciated.TIA

 

[EricStecker]

Chris,

The twist rate we recommend for the 88 gr is a 10" twist. Keep in mind that our recommended twist rate is based on achieving a stability factor of 1.5,most stable) at the greatest range of velocities. This means that a 10" twist will work for just about every cartridge and in any environmental condition.

Having said that we do know that a slower twist MAY work if the velocity is high enough and/or if you are at higher elevation. Now the question is "at what velocity and/or elevation will it work?" Unfortunately, I don't know right now however Walt Berger can answer that question. He does not visit the forums so I suggest you contact him directly at bergerltd@aol.com. I know for sure that he can give you loading data.

Tightneck,

To determine the velocity Walt chose a broad range of velocities that cover most common cartridges. We then use an average. So that our twist recommendation works for everyone it is on the conservative side,1.5 as mentioned above). What I mean by that is a bullet requires only a 1.05 or higher stability factor to stay nose forward. However as the bullet slows it can drop below the 1.0 mark which will result in a bullet that keyholes through the target.

For example, we recommend a 13" twist for our 6mm 68 gr. This same bullet and many other brands of the same weight and length have been shot by short range bench rest shooters for years. I would bet a fair sum of money that you could go to any bench rest match in the country and you would not find two people using a 13" twist barrel. They all know that a 14" twist works just fine and produces a lower RPM which can improve accuracy.

Regarding the 88 gr vs the 115 gr in terms of twist rate the reason for the faster twist recommended for the 115 gr is because the OAL of the 115 gr is longer. The twist rate needed to stabilize a bullet is based on the length of the bullet not the weight. The length of a bullet will determine how many RPMs it needs to become stable. Before you ask "how many RPMs does a 115 gr really need?" I will tell you that we are putting that information together. I believe that twist charts are within other brands loading manuals however we are going to provide this information for our bullets as well.

Providing this data is a big project that Walt has been working on for some time. We are getting closer to having answers but for now the best thing you can do is contact Walt directly with your questions. Until we can publish our information he is the only one who can answer your questions on our bullets.

Regards,
Eric Stecker

 

[chino1969]

Eric Stecker,
First of all I would like to say that it is a pleasure dealing with you and your company. Several years ago, I contacted you with a question regarding the 80 grn. M.E.F.'s that I had been shooting in my 1-10" .243 A.I. with amazing results. This was about the time that you changed the name from M.E.F. to Varmint Match and the literature on the Sinclair website stated that the bullets were the same and that only the name had changed. I ordered three boxes of the newly named bullet from Sinclair and when they arrived, they were indeed a different bullet. Somewhat perplexed, I called and talked to you about this issue. You explained that there had been an error with the type of dies used and that it had been corrected. You offered to replace the bullets or I could try them as is. The ones that I had just received were even more accurate than the old ones and there was no need to return them. Your company produces an excellent product, backed by equal customer service and I tell everyone this story.
Chino69

 

[ChrisNam]

Eric, a typing error crept in!!!

I meant to ask whether you have any loading data and velocities for the 80 gr FB Varmint Match?

Will this be the best choice for the 1:12 twist?

Thank you again for all your advise.