Barrel length vs Bullet weight

Mike / Tx said:
Bart, I did have groups that were bigger. I worked for a month or three with several powders, bullets, cases and primers to dial in the very best possible load i could develop.

The end result however will shoot better than I can, day in and out, and has turned plenty of consistent clover leaf groups at 100 and 200. It is an awesome combination and I'm proud to own it. I also have a .223 AI barrel that has made just as impressive a track record.
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What's the biggest group sizes?

I've been reminded by national and international champions and record setters that accuracy is best defined by the largest groups fired. And the smallest ones are mostly luck and near impossible to repeat. Statistical phenomena on both sides of the mean. Don't claim an accuracy level you cannot equal or better when challenged or every time you test.
 
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Savagehawg said:
Loading for a 7mm-08 which is new Caliber for me, new rifle with a 1:9 1/2 twist 20inch barrel. Loaded 140gn Accubonds with 46gn of IMR 4350 groups are consistent but tightest group is 1 inch, trying to figure out do i want to play with the powder load, seating depth , or bullet weight . I could try different powder but would probably have to purchase some new powders, I have other powders but none for what loading books are recommending , since most of our reloading is for magnum style rifles.

So my main question is for shorter barrels do i go heavier or lighter weight or do you think that it makes no difference. Would like to stick with the Accubonds since i have 2 boxes of them and nothing do do with them, but also know some barrels just don't like some bullets.

Thanks in advance for any help
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I have 3 7-08. All of them love 110 ttsx over cfe223.
The 140 accubond is definitely not my first choice unless I were going to shoot brown bear or moose. Honestly, I wouldnt shoot either of them with a 7-08. If you want to stay with a 140, try Staball 6.5.
 
Bart B. said:
What's the biggest group sizes?

I've been reminded by national and international champions and record setters that accuracy is best defined by the largest groups fired. And the smallest ones are mostly luck and near impossible to repeat. Statistical phenomena on both sides of the mean. Don't claim an accuracy level you cannot equal or better when challenged or every time you test.
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Dont have the targets right here with me but I believe the largest group was 1-5/8" , got some more test loads done up with some Varget and some more with the IMR 4350 haven't had a chance to go the range yet. Also a buddy gave me some 150 grn ELD-X may play with those some but if they dont work up i will use those in the 7mm Mag.
 
I think the magic correlation you're looking for is the relationship between bullet sectional density (SD), rate of rifling twist, and velocity. Twist seems to be the element of more radical change. The SD of any given caliber will change only slightly by variations in weight. Barrel length mostly relates to achieving peak pressure behind the bullet. Good luck with your project.
 
caligula,

Bullet sectional density is the ratio of bullet weight to its diameter squared. All 7mm bullets of the same weight have the same SD regardless of length. Their ballistic coefficient varies.

https://www.vcalc.com/wiki/MichaelBartmess/Sectional+Density

Here's a place to look them up.

http://www.shootforum.com/forum/bulletdb.html

A given load will produce the same pressure curve across all barrels with the same chamber, bore and groove dimensions. Pressure on the bullet for each inch of barrel length will be the same. Highest a few inches down then getting less for each inch it goes.

Use this program to calculate the stability factor for bullets. Ideally about 1.4 is best. Bullet data required is in the above link.

https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
 
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caligilgula said:
The SD of any given caliber will change only slightly by variations in weight.
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It is directly proportional to weight. In other words, the percent change in SD is exactly the same as the percent change in weight when the bullet diameter remains the same.

As Bart said, in ballistics, SD is the weight of a projectile in pounds divided by the square of its diameter. This is not true in aerodynamics or physics. In those disciplines, it is accurately described as the mass of an object divided by its cross-sectional area in the direction for which sectional density is calculated. But in ballistics, it was realized just after the Civil War (and probably before then by some people) that all projectiles commonly used have a round cross-section in their direction of flight. That means their cross-sectional area (2×ϖ×r) is a constant 1.2732954… times smaller than just the square of their diameter.

Thus it came to pass, to save calculating time (this was devised in the 19th century when no pocket calculators or computers were available for artillery officers to make their targeting calculations with) and since sectional density is just used as a ratio to the sectional density of a standard projectile to work out ballistic coefficients, it was decided to skip making people deal with pi and halving the diameter to get a radius, and just let them square the diameter. The result is proportional, if not a numeric match to the physical sectional density used by the previously mentioned disciplines who don't have the ballisticians luxury of having all their objects and devices perfectly round in directional cross-section.
 
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