Bob Kingsbury
New member
What happens to a bullet, say solid copper that is given an SG
of say 3. Assuming it is a perfect bullet, what will it do
of say 3. Assuming it is a perfect bullet, what will it do
solid bullet and SG of 3
- Thread starter Bob Kingsbury
- Start date
I'm GUESSING that;
-it'll have a BC of about .180
-it'll drift in the wind like a round ball
-BUT, it'll be spinning so awful fast that it'll actually precess so far as to produce an upwind drag factor which will somewhat offset it's downwind drag....... until it rolls over so far that it gains a vertical component. In other words, in crosswind it'll print a diagonal group slanted from 10:00 to 4:00
them's my guesses
al
-it'll have a BC of about .180
-it'll drift in the wind like a round ball
-BUT, it'll be spinning so awful fast that it'll actually precess so far as to produce an upwind drag factor which will somewhat offset it's downwind drag....... until it rolls over so far that it gains a vertical component. In other words, in crosswind it'll print a diagonal group slanted from 10:00 to 4:00
them's my guesses
al
Bob Kingsbury
New member
30 br typically prints more 10--4 than a 6. This bullet need not be a solid,
just that it can't strip the core. Could be a typical .825 jacket in a 7" twist.
I guess a better question is , By theory, what does over stabilization do ?
just that it can't strip the core. Could be a typical .825 jacket in a 7" twist.
I guess a better question is , By theory, what does over stabilization do ?
As I read Vaughn, the primary "bad" effect of overstabilization is that it increases shot dispersion due to any Center-Of-Gravity offset in a bullet. CG offset is the amount the center of mass in a bullet differs from its physical center. Practically, that would mean voids in the core, or wall variation in the jackets.
I can give Vaughn's formula for for calculating CG offset, replacing the Greek symbols with words in curly brackets:
{sigma} = 24{pi} (V/t) (TOF) {delta}
where
{sigma} = bullet deflection in inches, radius of dispersion, or miss distance.
{pi} = 3.14159
V = velocity at muzzle in fps.
t = rate of twist inches.
TOF = time of flight in seconds (use about 0.1 second at 100 yards)
{delta} = CG offset in inches.
OK -- the important thing is that the larger the twist number, the smaller the dispersion -- it's in the denominator. A 1:10 will give greater dispersion than a 1:14, with no other changes.
This has nothing to do with Sg per se. A .30 caliber with a 18-twist will have less CG offset dispersion than a 6mm with a 14-twist, assuming all other variables are the same. In this, with typical benchrest bullets, the Sg of the .30 will be higher, but we have just compared apples to oranges if we're talking about "overstabilization".
When you're talking about "overstabilization", you're usually talking about one caliber, one bullet, etc. So again, as the rate of twist increases both Sg and shot dispersion increase for that one bullet.
From Vaughn, page 193:
OK, note that Vaughn is assuming a certain jacket runout, and it isn't clear what he's assuming in this paragraph. He's also assuming an Sg of 1.4 for a 14-twist barrel, and that's questionable, if he's also assuming a bullet on an .825 jacket.
Anyway, that's the CG-offset story. After a point (pretty early), I'm not a ballistician and should shut up & recommend "further reading."
* * *
Another effect which may be "bad" is that in a crosswind, the vertical component of wind drift will be higher. The higher the Sg, the more vertical component. There will always be some vertical component, the question is "how much." But it is constant -- as you learn to hold for that wind drift, you have to learn how much horizontal, how much vertical. As long as you don't change barrels, what you learn will always hold true. I suppose if you get excited and forget, you might think some vertical has crept into your load where it has not, but that's just a shooting mistake.
With right-twist barrels, up with a right wind, down with a left wind -- alinwa's 10- to 4 line.
I can give Vaughn's formula for for calculating CG offset, replacing the Greek symbols with words in curly brackets:
{sigma} = 24{pi} (V/t) (TOF) {delta}
where
{sigma} = bullet deflection in inches, radius of dispersion, or miss distance.
{pi} = 3.14159
V = velocity at muzzle in fps.
t = rate of twist inches.
TOF = time of flight in seconds (use about 0.1 second at 100 yards)
{delta} = CG offset in inches.
OK -- the important thing is that the larger the twist number, the smaller the dispersion -- it's in the denominator. A 1:10 will give greater dispersion than a 1:14, with no other changes.
This has nothing to do with Sg per se. A .30 caliber with a 18-twist will have less CG offset dispersion than a 6mm with a 14-twist, assuming all other variables are the same. In this, with typical benchrest bullets, the Sg of the .30 will be higher, but we have just compared apples to oranges if we're talking about "overstabilization".
When you're talking about "overstabilization", you're usually talking about one caliber, one bullet, etc. So again, as the rate of twist increases both Sg and shot dispersion increase for that one bullet.
From Vaughn, page 193:
OK, note that Vaughn is assuming a certain jacket runout, and it isn't clear what he's assuming in this paragraph. He's also assuming an Sg of 1.4 for a 14-twist barrel, and that's questionable, if he's also assuming a bullet on an .825 jacket.
Anyway, that's the CG-offset story. After a point (pretty early), I'm not a ballistician and should shut up & recommend "further reading."
* * *
Another effect which may be "bad" is that in a crosswind, the vertical component of wind drift will be higher. The higher the Sg, the more vertical component. There will always be some vertical component, the question is "how much." But it is constant -- as you learn to hold for that wind drift, you have to learn how much horizontal, how much vertical. As long as you don't change barrels, what you learn will always hold true. I suppose if you get excited and forget, you might think some vertical has crept into your load where it has not, but that's just a shooting mistake.
With right-twist barrels, up with a right wind, down with a left wind -- alinwa's 10- to 4 line.
Bob Kingsbury
New member
The 30BR with its typical twist is know to produce a bit more tilt in its wind line than
a 6. As Harold Vaughn shows it about 17 something degrees. Thats about 10-4
the 30 shows more at maybe 10:45- 4:45.
Now, given a 6mm and jackets with all the same runout and same length. Wouldn't
bullets made with a shorter core ( less lead) offer less CG offset ? There is certainly
less mass in the offset.
a 6. As Harold Vaughn shows it about 17 something degrees. Thats about 10-4
the 30 shows more at maybe 10:45- 4:45.
Now, given a 6mm and jackets with all the same runout and same length. Wouldn't
bullets made with a shorter core ( less lead) offer less CG offset ? There is certainly
less mass in the offset.
This isn't calculating CG offset, which cannot be calculated or predicted(unless put there in a test), but must be measured.
Your posted formula is for dispersion caused by CG offset, once known.
Right. Thanks for the correction. Should I fix the post, or assume people will see your correction?
Edit:
@ Bob:
It isn't the total amount of mass, it is the difference in the center of mass from the physical center. As there would be less lead, assuming the source of the offset is the jacket, the CG offset would be greater with less lead -- that for one caliber. When you compare a .30 to a 6mm, I'm less sure, but I think the center of mass offset would be even better with the .30
Ask Al Nyhus or Randy Robinett?
The 10-4 line is a factor of the higher Sg, unrelated to CG offset. -- It's really more like 9:30 - 3:30 . . . pretty small change from Sg 1.2 to 1.5
Last edited: