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Which factory 308 Win

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  #15  
Unread 05-03-2009, 04:29 PM
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Re: Which factory 308 Win

Quote:
Originally Posted by trueblue View Post
It seems like the 1:10 twist would be the way to go if shooting heavier than 175gr, but have not thought about that. . Kevin Cram posted that he couldn't get 175gr to shoot out of his 1:12 twist, so 1:12 might be borderline. I want to shoot at least 168-175gr, and now maybe try the heavier ones as per Michael's recomendation, so 1:10 is probably the way to go. Only offered in a few models., so back to the drawing board to narrow it down.

There are no gurantees that a given barrel will like a give bullet weight or style. Trust me, if Kevin couldnt get a 175 to shoot good in his barrel, the 12x had nothing to do with it. 12x with 175's isnt even close to marginal at 308 velocities.
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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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  •   #16  
    Unread 05-03-2009, 04:46 PM
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    Re: Which factory 308 Win

    Michael,
    I might have misstated Kevin's position on the 175"s. I went back and reread his ad for his 308 he has for sale. He stated the 1:12 was more suitible for the 168gr, and that he tried the 175's. I interpruted that to mean the 175's didn't work. That might not be the case. Kevin can elaborate further if he feels the need to respond. Thanks for commenting on the twist issue. If 1:12 is suitable for 175's that opens up more options on rifle choice.
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      #17  
    Unread 05-03-2009, 05:45 PM
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    Re: Which factory 308 Win

    That makes sense. I am sorry if I miss-interpreted the post.

    According to the old greenhill formulas, the 168 became standardized for the 12 twist 308 barrels. Also this is where the 11.25 was born for the 175. Greenhill has always been on the fast side of things and only takes into concideration bullet diameter, length and twist and only two velocity ranges. The problem is that going from 2799 FPS to 2800 FPS isnt a magical jump. More modern and IMO more accurate formulas take into concideration actual velocity, weight, diameter, length and many other factors. These formulas along with real world proof show that the twists used by many of todays shooters work very well such as 12x for 175-180's and even 13 for 168's and 14x for 155's. On the surface, they seem too slow but in reality are very optimum twists for these bullets. Actually Berger bullets recomends 12x for their 190 VLD's and 13 for the 175's. This should help put into perspective how overkill many shooters are in their twist selection.

    This is only my opinion, so please take with a grain of salt, but in my 308 experience, the 12x is a perfect ballance for the 175-180 where the 11.25 is perfect for the 190 SMK and the 11 is perfect for the 190 VLD's. The 10x is perfect for the 208-210 variety. Dont get me wrong though, these just seem like the optimum balances. You can still shoot 168's very well out of an 11x barrel even though 13 is more optimum. I have shot the 155's well from the 11.25 even though I prefer a 14x. It seems like when you use the optimum twist, you put the bullets to sleep closer to the muzzle than when you spin them too fast or too slow. It was taking 600 yards to put the 155 AMAX to sleep with the 11.25x where the 190's were put to sleep at the end of the barrel. This is my opinion simply due to the 190's shooting the same small MOA at 100 and 1000 and every range in-between where the groups with the 155's are horrible until after 600 yards, then they get better. This is why I am of the mind set "dont spin the bullet any faster than you have to". I say that 1/2 hearted because a bullet will be stable with a SF of 1.1 and 1.5 is the widely accepted optimum balance. If you reach 1.5 in normal conditions, then when you go to death valley (below sea level) during the next ice age (35 degrees below zero F) then your bullets will still be well above 1.1 and perfectly stable. This why we shoot for 1.5 in standard air. There is absolutly no need to try and reach higher than 1.5 unless you want more spin drift, more yaw and to see a bullets jacket imperfections manifest themselves on paper.

    I dont meen to rant, I guess the point was to show where 12x and the 168/11.25 and the 175 standard came from. Unless you want to shoot bullets heavier than the 190's, I would highly recomend the 12 or 11.25X. 10x will work, but you will get better velocity with less pressure and stability factors closer to 1.5 with the 11.25 or 12x. If you need 208's - 220's then go with the 10x as that is what you will need.
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    Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

    Last edited by Michael Eichele; 05-03-2009 at 05:51 PM.
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      #18  
    Unread 05-03-2009, 06:02 PM
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    Re: Which factory 308 Win

    I probably won't be shooting anything heavier than 175gr in the 308. Although it would make a good close range thumper. If I want to go heavier I would probably be better off shooting 338 cal or a faster 30 cal with heavier bullets, correct ?
    So, if I understand your response, 12x for 168gr, but would need to go 11.25 x for 175gr, which would require a custom barrel as I don't see a factory barrel with that twist ?
    So, if I buy a factory rifle 10x will work for 175gr , but the bullets might not go to sleep as quickly and be as acurate at say 500yards or less.
    Maybe, I am overthinking this ?
    Also, I am not familiar with SF and the 1.1/ 1.5 goals you referred to . Where can I read about what this is and how to determine optimal twist required for bullet design?
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      #19  
    Unread 05-03-2009, 06:27 PM
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    Re: Which factory 308 Win

    I think you missunderstood my post.

    The standard for a while was 12x for 168's and 11.25 for the 175 based on the out dated greenhill methods.

    The newer and better methods of choosing twists will show you 12x for 175-180 and 13 for 168's. This corosponds well with real world reults.

    I was just trying to illustrate to you why many shooters tell you 12x for 168. These are old standards and overkill.

    As far as reading up on stability factors, I am not sure where to point you for reading. There arent alot of references that go into detail about it, just little snippits here and there.

    In short a stability factor is the gyroscopic stability of a spinning object such as a bullet. When the factor is less than 1.0-1.1 it is unstable. The goal is about 1.5. this allows room for changing elements such as very dense air that lowers stability factors a bit. You wouldnt want to develop a load in 70 degree air at 5000 feet that has a SF of 1.1and then go to alaska and hunt moose in 10 degree air at sea level. This would cause a SF of under 1.0 and subsequently you would be unstable and the bullet will never reach the traget straight on or with any degree of accuracy.

    In a 12x barrel at 2650 FPS a 175 SMK has a 1.65 SF in standard air (59 degrees F and 29.92 BP) In -30 degrees F at sea level, it is still 1.36.

    Again, if you say 175 is your max bullet, get the 12x. a 10x will give you more than 2.3. This will cause sleep issues as well as possible jacket concentricity imperfection issues.

    If you want a program for calculating SF, send me your email and I can send you a couple of different calculators to help you see the difference.

    Also, there is one factory rifle that uses a 11.25 twist. This is a 700 5R (milspec). Agian though, this twist works better with the 190-200's.
    __________________
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    Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

    Last edited by Michael Eichele; 05-03-2009 at 06:32 PM.
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      #20  
    Unread 05-03-2009, 07:10 PM
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    Re: Which factory 308 Win

    Thanks for your reply. You are a wealth of imformation. You obviously have been at this awhile. My e-mail is on it's way.
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      #21  
    Unread 05-03-2009, 09:42 PM
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    Re: Which factory 308 Win

    so Michael could the twist rate be the reason im struggling to find a good load for my .308 with the 165 sst. i have a remi 700 sps with a 1 in 10 twist. cant seem to find a good load. do u think i should go up to something like a 200g amax. i just feel like i could throw the bullet flatter to the target than it will end up lobbing them since the velocity will be like 2300. whats your thoughts on that
    cheers
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