Inherently accurate?

[rasterman77]

Mod 70 and I where having a discussion about what makes certain cartridges inherently accurate, such as sharp shoulders, stout brass, etc. What science is there to show what, exactly, it is that seems to make certain cartridges more accurate than others?
If you are curious you can read the discussion Mod 70 and I had in "best general caliber" on page 4.

[Bravo]

As far as I can tell, 2 things determine how accurate a projectile will be.

1. The casing and powder charge. Really 2 separate things, but both are very closely related. The optimal casing will have a certain angle in the neck, and will be a certain length and width. This will allow for a perfect powder charge to be loaded for a specific bullet. How the case is shaped will determine how quickly the bullet will accelerate within the barrel, and what velocity it will eventually reach. For each bullet, there is an optimal acceleration and velocity.

2. The bullet itself. Bigger bullets, like .33-.50 cal bullets have very poor ballistic coefficients. The ballistic coefficient is a measure of air resistance (both head on and cross wind) during flight. The bigger the caliber, the more drag there will be head on, and it will be more effected by cross wind.

Too small of a caliber, and the bullet won't have sufficient mass to make it down range accurately.

Like with brass, you have to compromise when dealing with bullets. it is pretty much been determined over the course of the past 100+ years that the optimal bullet is somewhere between 6mm and 7mm in caliber.

The 6mm-7mm bullets also have an advantage in that a lot of them are still supersonic at and past 1000 yards. When a bullet goes from supersonic to subsonic, it is slightly disturbed, and develops a small wobble. Anyone that shoots .22LR knows this, where after 100 yards, no matter how good your rifle is, the accuracy of the bullet falls off incredibly.

Which is why the new top contenders in long range benchrest are the 6.5mm Grendel and .260rem (which happens to be 6.6mm). Both of which have somewhat sharp necked - short cases for optimal powder use, and both maintain supersonic speeds beyond 1000 yards.

I don't know a whole lot about the subject, but I hope this helps a little

[MOD 70]

Just some personal thoughts regarding Bravo`s first comment for now.I believe the short,fat cases require a faster burning powder than a longer more cylindrical case.This is due to the different shapes of the powder charges.In a shorter,fatter case the ignition of the powder charge is more centralized than in a longer powder column.The primer flash reach's a higher percentage of the surface area of the faster burning powder and builds it`s pressure very rapidly and theoretically more consistantly.The longer case requires a slower burning powder due to the fact that it`s primer flash cannot ignite as much of the surface area of it`s powder and it needs a little more time to build pressure within the case body.Any pressures produced must peak before the bullet leaves the barrel.Acceleration is finished when the expanding gases no longer act on the bullet.Any unburnt powder results in excess barrel erosion,lower velocity and muzzle flash. Who`s next?

[lefty o]

the gun is a much more important factor than the cartridge itself. though a really well designed cartridge will get the last couple thousandths smaller group than a good cartridge will. to set the accuracy world on fire, you need both a great gun, and great cartridge, and someone behind the trigger that can make use of it.

[Lizard]

Originally Posted by Bravo

2. The bullet itself. Bigger bullets, like .33-.50 cal bullets have very poor ballistic coefficients. The ballistic coefficient is a measure of air resistance (both head on and cross wind) during flight. The bigger the caliber, the more drag there will be head on, and it will be more effected by cross wind.

The data that I've seen for .224", .257", .308" and .510" bullets show increasing BC as the bullet gets larger. The .510" bullets - meant for extreme range shooting - have BC's in excess of 1.000, while the best .308"s only run close to .700.

To increase the BC on a .224" bullet, I think you'd have to shape it like a dart, and there are too many other problems that would create.

If I could make a cartridge that wasn't bothered by manufacturing problems, I'd make a case that was hemispherical with a cone shape on the bullet end, and a primer tube that fired back towards the powder. The ignition would spread outwards in a cone and burn towards the bolt face, burning all of the hemispherical shape in an even time period, developing a consistent pressure increase, with the powder completely burned at the same moment the bullet left the barrel.

("That's just what I'd do, if I ran the zoo," said young Sam McGrew.)

[MOD 70]

Originally Posted by rasterman77

Mod 70 and I where having a discussion about what makes certain cartridges inherently accurate, such as sharp shoulders, stout brass, etc. What science is there to show what, exactly, it is that seems to make certain cartridges more accurate than others?
If you are curious you can read the discussion Mod 70 and I had in "best general caliber" on page 4.

Leftyo Of course you`re right about the rifle being a major factor in accuracy down range but in order to make use of the rifle`s accuracy it needs a load that works with it.Variables such as type of powder,weight of the powder charge, bullet weight and bullet design are the difference between minute of angle and lucky to hit the paper at 200yrds.

[nathangdad]

Having a cartridge neither under nor over bore helps. For instance, in the Weatherby cartridges the .340 is often regarded as the best balanced with the others mostly seen as over bore. Also, the proper rifling twist mated to the bullet being fired is a major factor.

[Bravo]

Not related to the cartridge, but a place where people rarely look:

The crowning on the end of the barrel plays a huge role, especially at long range shots. This is because the crown at the muzzle is the last spot that the bullet and gas make contact. A poorly shaped crown results in uneven gas distribution, and can result in a wobble in the bullet. A simple fix to a rough crown is to just throw on a muzzle break. This gets rid of the gas so that it doesn't effect how the bullet moves down range.

Getting the crowning fixed is usually really easy for a gunsmith, and should be done to any high precision rifle.

[MOD 70]

Proper neck tension can also be factor in getting the most accuracy from a cartridge.(I not referring to the cramp in your neck from typing long posts).The length of the bearing surface of any given bullet affects chamber pressure and this is why you can`t just substitute a different brand of bullet in the same weight with the same powder charge without re-working the load.Boat-tail bullets reduce drag and really make a big difference down range. Does anyone have any theories on the affects these re-bated bullet bases have on the bullet before it exits the barrel?

[rasterman77]

Great info guys, so what I can determin, my assessment (page 4 "best general caliber") that the sharp shoulders and stout case of, seemingly, all the "inherently accurate" cartridges contributes to the even and complete powder burn, giving it an advantage, was right.

What about headspacing, does anyone think that the sharp shouldered, stout cases give an advantage to more accurate headspacing? from a manufacturing point of view,.. generally.. because we, of course, are working with over-all averages here.

"Having a cartridge neither under nor over bore helps"
Nathangdad, under or over bore for the optimal size for that brass, yes, but till we play with the over/under bore we don't really know what that optimal caliber is. The worlds top benchrest calibers are almost ALL over/under bores.. rather than try to make "the perfect round" from scratch someone usually takes an existing "inherently accurate" case and tries to find the "perfect" bullet for it. Rate of twist is definitely very important and the reason "match" ammo is sometimes an odd weight.. to match the rate of twist for that caliber.

Bravo, for not knowing alot you posted alot of insightful info, if nothing else, you have good research skills, Thanks!

Originally Posted by MOD 70

Proper neck tension can also be factor in getting the most accuracy from a cartridge.(I not referring to the cramp in your neck from typing long posts).The length of the bearing surface of any given bullet affects chamber pressure and this is why you can`t just substitute a different brand of bullet in the same weight with the same powder charge without re-working the load.Boat-tail bullets reduce drag and really make a big difference down range. Does anyone have any theories on the affects these re-bated bullet bases have on the bullet before it exits the barrel?

Wow, more research ahead..

[MOD 70]

I am sure that participation in this thread will be beneficial to all who hand-load or would just like to know what is really involved when we pull that trigger.How about the TSX bullets.Those ribs reduce the bearing surface,but do they also reduce neck tension? Hmmmm....

[LarryO1970]

Between the primer, casing, powder and bullet itself, that will tell the tale. A close friend of mine is one of the top military high power shooters in the country... and was one of the first to devise a 1,000 yard 5.56. All of the above components contribute to long distance accuracy... in their own way. After these components are obtained/used, mic every round to make them identical in length and also... most importantly, trickle the powder to make them exact.

What caliber and grin bullet are you using ?

[MOD 70]

.375 H/H 270grn 2700+ fps

[SwedeSteve]

The data that I've seen for .224", .257", .308" and .510" bullets show increasing BC as the bullet gets larger. The .510" bullets - meant for extreme range shooting - have BC's in excess of 1.000, while the best .308"s only run close to .700.

Lizard- There is no such thing as a BC >1. A BC of 1 only exists mathematically as the "standard bullet" by which all others are measured. This model is equivalent to the one use in the Ingalls Tables. When discussing projectile drag coefficients, which we are, it is also known as the G1 model (computer wont let me drop the 1). It is impossible mathematically to develop a projectile that is >1, as then IT would become the G1 model, or =1 !

[rasterman77]

What about headspacing, does anyone think that the sharp shouldered, stout cases give an advantage to more accurate headspacing? from a manufacturing point of view,.. generally.. because we, of course, are working with over-all averages here.

I'm repeating this question from my above post.. would like to see some ideas,

[MOD 70]

Originally Posted by rasterman77

What about headspacing, does anyone think that the sharp shouldered, stout cases give an advantage to more accurate headspacing? from a manufacturing point of view,.. generally.. because we, of course, are working with over-all averages here.

I'm repeating this question from my above post.. would like to see some ideas,

Absolutely! Long cylindrical cases such as the 45-70 were designed with a rim to avoid headspacing on the case mouth like some handgun cases or the 30 M1 carbine. The original H/H family of cartridges had long sloping shoulders and no reliable way to accurately control headspace, so a belt was added.Due to variation in base to top of the belt dimensions, most handloaders will only neck-size these cases resulting in removing the belt from the equation.It would stand to reason that a generous shoulder would control headspacing much easier and possibly more accurately on a case that has neither belt or rimmed design.

[rasterman77]

Originally Posted by MOD 70

Absolutely! Long cylindrical cases such as the 45-70 were designed with a rim to avoid headspacing on the case mouth like some handgun cases or the 30 M1 carbine. The original H/H family of cartridges had long sloping shoulders and no reliable way to accurately control headspace, so a belt was added.Due to variation in base to top of the belt dimensions, most handloaders will only neck-size these cases resulting in removing the belt from the equation.It would stand to reason that a generous shoulder would control headspacing much easier and possibly more accurately on a case that has neither belt or rimmed design.

I just recently did alot of reading on the belted magnum (because of the debate on mags vs short mags in "big game hunting") and the shape of the neck was almost shoulderless because of the powder charge "cordite" therefore a rim/belt was needed to headspace.
It definetly makes sense but only form a logic point of view, I have yet to see any stats that prove steep shoulders are easier headspaced; Your affirmation makes me feel better about my assumption.
Thanks!