Future H.A.T B.C. Test shoot

[noel carlson]

Jon,

ZA30/5.8-W designation breakdown;

- ZA = Zethilius Associates
- 30 = caliber
- 5.8 = length in calibers (5.8x.30=1.74")
- W = bullet type (in this case tungsten core)

I am omitting the weight in the nomenclature, because when these are released there will be only two types in a given projectile length;

1 - The "M" will be a target solid, optimized for form factor.

2 - The "W" is a frangible optimized for both form factor, and mass. This is ideal for ELR, or for hunting. The shortest "W" projectile in each caliber series will be dimensioned for magazines, and twist rate compatibility with readily available, off-the-shelf, barrels. In the ZA30/5.8-W example, the 8.3" twist requirement is available from Hart (8.0"), Shilen (8.0"), Lilja (8.0"), and Pac-Nor (8.0" & 7.0"). If you want a cut-rifled barrel, Bartlein, and a few others will produce it on a special order. The 27 caliber twist will work for all 6 caliber, or shorter, "W" projectiles.

For ZA projectiles in excess of 6.0 calibers, a gain twist in a linear, exponential, or acceleration compensated, geometry is ideal... but they will spin the jackets off of conventional bullets, so you are committed to a dedicated system once this line is crossed.

If I have totally confused you, feel free to ask me about a specific bullet.

Mark,

Both the 175, and 240, should stabilize in the 8" twist, but obviously the 175 SG will be lower. A higher MV will add stability to this projectile.

P.S.,
I did not anticipate going into great detail in this thread on the ZA projectile specifications, and am not holding back information without cause. I will post photographs of these at sometime prior to the meet.

 

[jmden]

I'm just curious as to the length in inches of these projectiles and the min. twist rate in inches needed to stabilized them at sea level.

Thanks,

Jon

 

[MontanaRifleman]

I'm just curious as to the length in inches of these projectiles and the min. twist rate in inches needed to stabilized them at sea level.

Thanks,

Jon

Jon,

If you take the caliber length of the bullet, i.e. 5.8, and multiply it by the bore caliber, .308 you will get the bullet lenght in inches, in this case 1.78". For comparison the 210 Berger is about 1.48" or 4.8 calibers. The 265 HAT is 1.785" or 5.28 calibers.

Noel can correct me if I'm wrong, but I'm guessing the twist rate is bsed on the diameter to length ratio of the bullets (measured in calibers). The Twist for the 5.8 caliber bullets being 27 clibers (or 8.3" in a .308) and for the 6.5 caliber bullets being 18 calibers (or 5.5" in a .308) Not sure if that remains the same when going to a different bore size?

-Mark



4,08

 

[jmden]

Jon,

If you take the caliber length of the bullet, i.e. 5.8, and multiply it by the bore caliber, .308 you will get the bullet lenght in inches, in this case 1.78". For comparison the 210 Berger is about 1.48" or 4.8 calibers. The 265 HAT is 1.785" or 5.28 calibers.

Noel can correct me if I'm wrong, but I'm guessing the twist rate is bsed on the diameter to length ratio of the bullets (measured in calibers). The Twist for the 5.8 caliber bullets being 27 clibers (or 8.3" in a .308) and for the 6.5 caliber bullets being 18 calibers (or 5.5" in a .308) Not sure if that remains the same when going to a different bore size?

-Mark


4,08

Hi Mark,

I know the caliber and length of bullet thing, but how did you arrive at the min twist numbers? I must've have missed that otherwise, but have not combed through every post on this thread with a fine tooth comb. My bad, if I missed that.

More than that, I just think it'd be easier for most of us if we saw these numbers in units we're more familiar with, namely inches. That's the language most of us have used for years hear and so it gives a quick and easy reference. Obviously differnent 'languages' and that's fine, I just think it'd be easier for potential customers to understand what they are dealing with potentially if someone converted that list of bullets from Noel to inches.

Thanks,

Jon

 

[noel carlson]

Jon,

Gyroscopic stability can be determined mathematically. Bryan Litz has publicized the formula, based on projectile mass, and length, in Precision Shooting Magazine I believe. This sets a minimum parameter for dynamic stability. To really nail down the actual twist requirement, nothing substitutes for empirical testing.

- Noel

 

[MontanaRifleman]

Jon, I did a little searching and found this...

Barrel Twist Calculator

Iit says VLD and ULD tyoe bullets may need a little tighter twist than this calc provides. JBM also has one that takes a lot more input.

 

[noel carlson]

Mark,

Just to provide some idea of the non-equivalence of "gyroscopic", and "dynamic" stability; The ZA375/6.5-M is supposed to be stable in a 1: 10" twist based on the calculator which you linked.

In reality, it requires a 1: 6.5" exit twist at 3,000 fps. Some very serious errors have been made by projectile manufacturers who relied too heavily on mathematical models. Bullet flight stability is very complex.

Best,
Noel

 

[noel carlson]

Mark,

"... Not sure if that remains the same going to a different bore size?"... I missed your appendix last night, and it does bring up some interesting issues.

Bore size, and twist-rate are essentially scalable. In 5.5+ caliber projectiles there is another factor that comes into play which has an effect that must be taken into account.

The ZA relies heavily on tail geometry to maintain high velocity stability. In order for the tail design to be effective hovever, laminar flow needs to be preserved across it's surface. The Reynold's effect states, in essence, that an object of "A" diameter, at "B" velocity, will produce the same laminar/turbulence flow properties as an identical object of (1/2) "A" diameter, and (2) "B" velocity.

To simulate a 50 caliber projectile at 2,500 fps, I would need to run a 25 caliber model at 5,000 fps. Bryan might want to weigh in here, but the practical result is that a projectile needs to change tail geometry, across caliber transistions, in order for the scaled twist-rate to work.

Best,
Noel

 

[MontanaRifleman]

Noel,

I guess I'm a little confused why you reffenced Bryan's formula for calculating BC to Jon? In any case, I would get bullet manufacturer's twist recommendation bfore having a barrel made. The provider of that calculator did make this statement....

The basic twist rate calculator above uses Bowman's equation modified with the SG correction quoted by Howell. For flat base bullets, the calculator should give a good estimate for the twist needed. The estimates should work for some boattail designs, but VLD and ULD style bullets may well need more twist.

I assume you'll provide the required twist for your bullets when you are ready to sell them.

The twist calc seems to be fairly accurate for some other common bullets, but your's would definitely be in a different category.

Am interested in seeing the results of the shooting event and getting more info on your bullets when the time comes.

Cheers,

Mark

 

[noel carlson]

Mark,

Jon was asking how I arrived at twist requirements. While there are some mathematical means of generating minimum parameters, such as Bryan's referenced formula, the bottom line is that actual testing is necessary.

- Noel

 

[MontanaRifleman]

Thanks Noel, understood.

 

[MontanaRifleman]

Mark,

"... Not sure if that remains the same going to a different bore size?"... I missed your appendix last night, and it does bring up some interesting issues.

Bore size, and twist-rate are essentially scalable. In 5.5+ caliber projectiles there is another factor that comes into play which has an effect that must be taken into account.

The ZA relies heavily on tail geometry to maintain high velocity stability. In order for the tail design to be effective hovever, laminar flow needs to be preserved across it's surface. The Reynold's effect states, in essence, that an object of "A" diameter, at "B" velocity, will produce the same laminar/turbulence flow properties as an identical object of (1/2) "A" diameter, and (2) "B" velocity.

To simulate a 50 caliber projectile at 2,500 fps, I would need to run a 25 caliber model at 5,000 fps. Bryan might want to weigh in here, but the practical result is that a projectile needs to change tail geometry, across caliber transistions, in order for the scaled twist-rate to work.

Best,
Noel

Noel,

I missed this post until just now. Thanks for the reply. Something else I wonder is how much bearing surface comes into play. We lengthen tails and noses to reduce drag but I would think that at some point we get dimishing returns in terms of the bullets stabilitiy if the bearing surface gets short enough the bullet may begin to "rock" going through the tube. Interesting stuff and beyond me.

Mark

 

[noel carlson]

Mark,

Your point is well taken. In-bore cant is the single greatest cause of dispersion, and you do not need a short bearing surface to have the problem.

The longer the column of lead, present in jacketed VLD projectiles, has a tendency to distort upon acceleration. As a result, a "cant" is induced in the projectile itself.

 

[GREYGHOSTt]

Noel
Very interesting read impressive numbers.
Shall we stick the torch back in to the gunpowder.
Show us your projected bc's.