Was Mr Greenhill Wrong ?

I have no interest in benchrest, but like F-T/R. I haven't seen any F-Class posted for Byers for 2012 yet. Most of the F-Class shooters use 12-42X56 NightForce scopes and I don't $ee one in my future. - nhk
 
nhkuehl said:
I have no interest in benchrest, but like F-T/R. I haven't seen any F-Class posted for Byers for 2012 yet. Most of the F-Class shooters use 12-42X56 NightForce scopes and I don't $ee one in my future. - nhk
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As you have no interest or experience, maybe you'll be a touch careful in your posts. I think your posts to the new & not-yet benchrest shooters have a fair bit of poor information, for benchrest-level decisions. You are of course entitled to both your opinions and to report your experience, but sometimes the lack of knowledge about BR level accuracy doesn't come through.

Foe example: I know of NO (nada, zip) benchrest shooter who would purposefully set up a competition rifle to have an Sg over 2. Most don't set up to have an Sg of 1.7 to 1.8. There are reasons for that; it is not a matter of fashion.

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Here's another one. It is your tested experience, so I'll not say wrong, but can leave the wrong impression.

I've experimented by purposely loading to reduce Sg below 1.0. I've seen groups open up immediately and at about Sg 0.8 holes are oval and it doesn't get any better past 100 yards even though I've been told stability improves down range.
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Usually, what happens is not oval holes, but pie-plate size groups. The BR gunsmith & competitor Dave Tooley had a 14-twist barrel that would shoot match-winning groups with the 187 BIB flatbase bullets, but pie-plate size groups with the Sierra 200 MKs.The 200 SMKs were 8 inches at 200 yards. The 187s were less than that at 1,000.

As Tim mentioned, right on the edge is very accurate, going over it a disaster. Secondly, while the fast and slow arms usually damp, and you're right that the Sg *number* improves downrange (usually, there are exceptions with some bullet designs in the transonic region), this only applies if the bullet was "stable" to begin with. In other words, as you said, if you were getting pie-plate size groups, they won't improve down range. Nor, for that matter, will any groups improve in terms of MOA.

If ballistics interests you, try reading Bob McCoy, or Harold Vaughn, or Brian Litz.
 
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I didn't intentionally set up my rifle for an Sg of 2.0+, it's a factory rifle I bought for the action with a 1:10 twist with no intentions of shooting it, but I needed a rifle for F-T/R training on short notice and pressed it into service and it delivered. I said I have no interest in benchrest as a competitor, but I've followed benchrest for years and picked their brains and applied what I could to my shooting. Some of them even coached me on their rifles. You are looking at things from a benchrest perspective and I'm looking at it from a hunting perspective (this is a factory/hybrid thread). I do load developement for conditions that range from winter coyotes to summer prairie dogs with Sierra, Nosler and Hornady (not Berger or custom bullets) in button rifled and hammer forged barrels. I would be perfectly content if you would like to delete my profile from the forum. I've tried to delete it before and wasn't able too. I'll go back to shooting by myself on the prairie and be perfectly happy. I have Bryan Litz's book. - nhk
 
No-- If you'll look, there are no more moderators, Wilbur had to delete that program feature to "close a hole." I'm finally able to speak as nobody in particular, and say things on my mind.

If you think your experience useful to people -- and I surely won't say it isn't -- post away, just please make sure people know what perspective you're coming from.

Best,

Charles
 
Thank you Charles. I thought I had made a disclaimer in post #16 about what I shoot. In the initial post Murphy stated, "Popular thinking in the 1960s was that bullets could be easily overstabilised but my own tests do not bear this out. Conversly I have had plenty of understabilised bullets go through the paper sideways and give on again, off again accuracy. " and my experience is similar to his. Take for example if you want to purchase a factory bolt-action .223 Remington across the counter it's likely to be either a 1:12 or 1:9 twist. If I want to shoot a 55 gr Nosler Ballistic Tip (BC .267) at 3100 fps at 59 degrees you either have an Sg of 1.03 with the 1:12 twist or 1.84 with the 1:9 twist, which rifle am I going to choose? I might get away with shooting the 1:12 until I go coyote hunting at 0 degrees and the Sg drops to .92. If I go with the 1:9 and when I'm shooting prairie dogs at 90 degrees I have an Sg of 1.94, but if I use it for the coyote at 0 degrees I still have an Sg of 1.63. I've shot both and found the 1:9 to be the better choice for a one gun (twist) approach, since a 1:10.5 twist wasn't available. My PERSONAL preference would be an Sg between 1.4 - 2.0. Incidentally the Greenhill formula would suggest a 1:11 twist and the Velocity formula would suggest a 1:11.5 twist. - nhk
 
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Specific gravity

Harold M said:
nhkuehl: Does your 1:11.5 include a correction for the specific gravity of the bullet?
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I tried to find my notes on the velocity formula (I believe it's also called the Corbin formula)... I've had it embedded in a spreadsheet for some time and rarely use it, but its: .06 (a constant) * Velocity * (Caliber squared) / Length

I'll have to venture a guess that the constant has something to do with specific gravity and I'm assuming specific gravity and relative density are the same thing.

SG, specific gravity is not the same thing as Sg which is gyroscopic stability. http://kwk.us/twist.html. I only use the Don Miller Twist Rule which has been published in Precision Shooting and calculates Sg for bullet/twist combination. I believe this site http://www.jbmballistics.com/ballistics/calculators/calculators.shtml does that also. - nhk
 
The reason I bring up the correction is that plastic tip bullets have lower bulk density, lower bulk specific gravity, which reduces their polar moment of inertia hence resistance to upsetting torque about their lateral axes. I had guessed that this is one reason the 55 grain Nosler, for example, requires a faster twist than one normally would expect. I don't have the calculations I did on this bullet at hand, unfortunately, but your 1:11.5 sounds familiar - hence my question.
 
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