Upon running the Miller2 program to determine twist requirements it informed me that the adequate stability values, over 1.4 for the 40 grain .204 diameter Vmax would occur with a twist rate of 1-10. My rifle has a 1-11 and shoots them just fine even up to 400 yards. Does the little plastic tip thingy count when measuring bullet length. If I subtract the length of the plastic point the 1-11 will produce a stability number of 1.4 or more. This is on a typical rodent shoot day with temperatures over 75 degrees F.
Those plastic tipped bullets
- Thread starter P72
- Start date
Bryan Litz Ballistics
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The plastic tips don't affect stability very much. True, they don't weigh much, but they do affect the aerodynamics by moving the center of pressure forward.
If you're seeing an SG of 1.4, I would expect the bullets grouping to be unaffected compared to an SG of 1.5 or higher. Groups aren't really affected much by marginal stability down to around 1.1.
At an SG of 1.4, there may be a slight decrease in BC, but I doubt the rodents would notice
Take care,
-Bryan
If you're seeing an SG of 1.4, I would expect the bullets grouping to be unaffected compared to an SG of 1.5 or higher. Groups aren't really affected much by marginal stability down to around 1.1.
At an SG of 1.4, there may be a slight decrease in BC, but I doubt the rodents would notice
Take care,
-Bryan
Bryan if you don't mind, can you explain how plastic tips are improving Sg for given length?Bryan Litz said:
It seems to me that moving Cp forward while leaving Cg where it is would lengthen the overturning moment arm.
And BC seems unimproved by plastic tips..
So where is the gain?
Thanks
Bryan Litz Ballistics
Site $$ Sponsor
mikecr and P72,
In laymans terms, there are stabilizing 'forces' and destabilizing 'forces' that trade off to determine the overall stability of a bullet.
The CP being in front of the CG is an unstable condition, so the further the CP is in front, the worse it is. You seem to have a good grasp on the CP being determined by the front of the bullet; plastic or not.
The 'stabilizing' factor that you're overlooking is the rotating mass. The Miller stability formula assumes a given mass distribution based on the bullet length and caliber, and being all metal. When a bullet has a plastic tip, the ratio of the spinning inertia is greater in comparison to the transverse inertia, which makes for a more rigid axis. Think of it like a top; short fat tops are easier to spin than tall skinny tops. The plastic is almost non-existent from a mass point of view compared to the metal, so the bullet is effectively 'shorter and fatter' from a mass perspective. This is the stabilizing effect that offsets the CP being pushed forward by the plastic tip. The two effects offset to a near zero effect.
P72's observation about the formula under-estimating stability for his bullet is due to that bullet being a flat base (or nearly a flat base). As stated above, the Miller formula assumes certain proportions, and one of those assumptions is the presence of a BT. The smaller the BT is, the more conservative the Miller stability estimates will be.
Possibly the Miller formula could use a modifier for bullets having shorter BT's or FB's to correct the stability predictions.
-Bryan
In laymans terms, there are stabilizing 'forces' and destabilizing 'forces' that trade off to determine the overall stability of a bullet.
The CP being in front of the CG is an unstable condition, so the further the CP is in front, the worse it is. You seem to have a good grasp on the CP being determined by the front of the bullet; plastic or not.
The 'stabilizing' factor that you're overlooking is the rotating mass. The Miller stability formula assumes a given mass distribution based on the bullet length and caliber, and being all metal. When a bullet has a plastic tip, the ratio of the spinning inertia is greater in comparison to the transverse inertia, which makes for a more rigid axis. Think of it like a top; short fat tops are easier to spin than tall skinny tops. The plastic is almost non-existent from a mass point of view compared to the metal, so the bullet is effectively 'shorter and fatter' from a mass perspective. This is the stabilizing effect that offsets the CP being pushed forward by the plastic tip. The two effects offset to a near zero effect.
P72's observation about the formula under-estimating stability for his bullet is due to that bullet being a flat base (or nearly a flat base). As stated above, the Miller formula assumes certain proportions, and one of those assumptions is the presence of a BT. The smaller the BT is, the more conservative the Miller stability estimates will be.
Possibly the Miller formula could use a modifier for bullets having shorter BT's or FB's to correct the stability predictions.
-Bryan
Thanks bryan.
I believe your description of the spinning metal mass being the dominant factor in Sg, and that it being relatively shorter here more than offsets the added length of Cp to Cg(with a light plastic extension).
I think it's a good point to reflect on the reality that the Miller formula is a rule of thumb(like Greenhill). Useful, but there will always be tests it won't pass.
I believe your description of the spinning metal mass being the dominant factor in Sg, and that it being relatively shorter here more than offsets the added length of Cp to Cg(with a light plastic extension).
I think it's a good point to reflect on the reality that the Miller formula is a rule of thumb(like Greenhill). Useful, but there will always be tests it won't pass.
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