Making bullets with plastic ballistic tips...unanticipated results

[lpreddick]

I hunt varmints and have been partial to hollowpoints as they really come apart inside ghogs,esp. A friend made me some 65 gr 6 mm open meplates and i'v killed ghogs out to 300 yards that do not crawl away and often have no exit wound. The BC of these bullets is quite low and they drop quickly and don't like wind. He made some with a plastic tips in hopes of having a higher BC. Jackets were .820. The pointing die had to leave a good bit of jacket as bearing surface in order to leave a meplate large enough to accept the tip. My 6 mm BR is a 14 twist and would not stabilize them! My 12 tw 6x47 Lapua shot a nice tight group. I ran the numbers, after the fact, using Berger's stability chart and in a 14 tw they were unstable. I will be shooting them in an 8 tw 6 BR and expect good groups as well as seeing if their BC is higher than the open meplate ones. I recalled the 50 gr Barnes non lead core Varmint Grenade 22 call bullet that had a bearing surface of .50" and required a 10 two or faster.

 

[lpreddick]

I hunt varmints and have been partial to hollowpoints as they really come apart inside ghogs,esp. A friend made me some 65 gr 6 mm open meplates and i'v killed ghogs out to 300 yards that do not crawl away and often have no exit wound. The BC of these bullets is quite low and they drop quickly and don't like wind. He made some with a plastic tips in hopes of having a higher BC. Jackets were .820. The pointing die had to leave a good bit of jacket as bearing surface in order to leave a meplate large enough to accept the tip. My 6 mm BR is a 14 twist and would not stabilize them! My 12 tw 6x47 Lapua shot a nice tight group. I ran the numbers, after the fact, using Berger's stability chart and in a 14 tw they were unstable. I will be shooting them in an 8 tw 6 BR and expect good groups as well as seeing if their BC is higher than the open meplate ones. I recalled the 50 gr Barnes non lead core Varmint Grenade 22 call bullet that had a bearing surface of .50" and required a 10 two or faster.

 

[lpreddick]

I posted this observation on Saubier and a member suggested the instability was due to bullet's increased length, not it's increased bearing surface. I seriously doubt the .3 gr tip's weight can move bullet's center of gravity forward of it's center of pressure (balance point). My bullet maker is going to make a few with the same meplate used to hold the plastic tip, but with none, and shoot them...should be interesting.

 

[RGRobinett]

I posted this observation on Saubier and a member suggested the instability was due to bullet's increased length, not it's increased bearing surface. I seriously doubt the .3 gr tip's weight can move bullet's center of gravity forward of it's center of pressure (balance point). My bullet maker is going to make a few with the same meplate used to hold the plastic tip, but with none, and shoot them...should be interesting.

The [plastic tip] CG moves rearward, thus making the bullet more tail heavy, while reducing the specific gravity - both of which, require more spin! If the bullet is 0.3" longer, that's a real Sg (gyroscopic stability) killer - probably about 3.5" of twist rate worth! I'll try to post a pic of a side-by-side comparison of the two bullet types (well, close enough) for your consideration. Will take a while to get there. Using the Tioga Engineering (Bill Davis) Bullet design program, I'll show you a graphic comparison of two Sierra 55 Gr. bullets: the "old reliable 55 Gr. Spitzer BT(31365), and the newer 55 Gr. BlitzKing (31455). RG

 

[RGRobinett]

Here's a comparison, as stated above, between two 55 Gr., .224 caliber Sierra bullets. Of note, "the time honored 1:14" twist rate, though, "it works", is obviously marginal for the shorter of the two bullets! Clearly, a twist rate of 1:12.2" is necessary to produce a 1.5 Sg, in the calculated atmosphere. The red text, last line (4), indicates the twist rate required to produce a 1.5 gyroscopic stability factor: Sg 1.4 is the minimum factor to reliably damp all of the yaw & pitch at the nose.

Note the difference in specific gravity (second line down from the Ts - for tangent ogive), 10.7 vs. 9.7, due to the differing lengths, for bullets of equal weight. The full 2 inch faster twist rate requirement for the tipped bullet - in this example, only 0.10" longer - is the norm! . The last line, above the red text, is the atmospheric density - in this example, I used my default sea-level plus, for a safety-net B4 ordering barrels! RG

P.S. For those unfamiliar with Mr. Davis (William C. Davis Jr.), he was a ballistics engineer, and contemporary of Robert McCoy, at Aberdeen Proving Grounds, he also worked with Frankfort Arsenal, Rock Island Arsenal, and was a long-time contributor to THE AMERICAN RIFLEMAN.

 

[lpreddick]

The [plastic tip] CG moves rearward, thus making the bullet more tail heavy, while reducing the specific gravity - both of which, require more spin! If the bullet is 0.3" longer, that's a real Sg (gyroscopic stability) killer - probably about 3.5" of twist rate worth! I'll try to post a pic of a side-by-side comparison of the two bullet types (well, close enough) for your consideration. Will take a while to get there. Using the Tioga Engineering (Bill Davis) Bullet design program, I'll show you a graphic comparison of two Sierra 55 Gr. bullets: the "old reliable 55 Gr. Spitzer BT(31365), and the newer 55 Gr. BlitzKing (31455). RG

Thanks you Mr Robinette...i'm rereading Mr Litz section on bullet stability. The .3 mentioned is not length, but weight of the ballistic tips. Correct me if i'm wrong: If you balance a bullet at it's center of gravity and add. 3 gr to it's tip, the CG will move forward toward the tip, not rearward. This ballistic tip adds .12 inches to the bullets length and .3 gr to it's weight. If this moves the CG ahead of the CP, then bullet is probably unstable at the RPMs generated by the 14 tw barrel. The 12 tw barrel stabilized the bullet, so RPMS were sufficient. My friend will shoot the bullet with the open meplate but no tip to see if it is stable. If it is, then the tip,as small as it is, rendered the bullet unstable at lower RPMS. This is very interesting and challenging as there is a scientific explanation for what we are seeing.

 

[RGRobinett]

Thanks you Mr Robinette...i'm rereading Mr Litz section on bullet stability. The .3 mentioned is not length, but weight of the ballistic tips. Correct me if i'm wrong: If you balance a bullet at it's center of gravity and add. 3 gr to it's tip, the CG will move forward toward the tip, not rearward. This ballistic tip adds .12 inches to the bullets length and .3 gr to it's weight. If this moves the CG ahead of the CP, then bullet is probably unstable at the RPMs generated by the 14 tw barrel. The 12 tw barrel stabilized the bullet, so RPMS were sufficient. My friend will shoot the bullet with the open meplate but no tip to see if it is stable. If it is, then the tip,as small as it is, rendered the bullet unstable at lower RPMS. This is very interesting and challenging as there is a scientific explanation for what we are seeing.

I am pretty certain that you will find that the lack of stability revolves around the extra length - relative to the additional length, the .3 Gr. isn't going to amount to much: you have a 65.3 Gr. bullet, which is .120" longer. Again, per above, in order to maintain equal stability, lengthening the bullet by 0.10" necessitated a 2.0 inch faster twist rate. The specific gravity of the tip is likely way less than lead. Though the BC will suffer dramatically, I'm betting that the open tips shoot just fine.

Here, I'll attempt to provide a comparison of 65 to 66 Gr bullets, albeit, of .224 caliber (6mm won't prove much different), and @ Std. sea-level air. A lighter, longer projectile will require faster spin-rate - imagine an aluminum bullet vs. a lead bullet, of identical geography. Though I didn't make a pic, if I leave the specific gravity input equal, the longer bullet, in this illustration, would weigh in at 74 Gr., but still require a full 1.0" faster twist rate - you'll have to trust me on that. RG

 

[RGRobinett]

I am pretty certain that you will find that the lack of stability revolves around the extra length - relative to the additional length, the .3 Gr. isn't going to amount to much: you have a 65.3 Gr. bullet, which is .120" longer. Again, per above, in order to maintain equal stability, lengthening the bullet by 0.10" necessitated a 2.0 inch faster twist rate. The specific gravity of the tip is likely way less than lead. Though the BC will suffer dramatically, I'm betting that the open tips shoot just fine.

Here, I'll attempt to provide a comparison of 65 to 66 Gr bullets, albeit, of .224 caliber (6mm won't prove much different), and @ Std. sea-level air. A lighter, longer projectile will require faster spin-rate - imagine an aluminum bullet vs. a lead bullet, of identical geography. Though I didn't make a pic, if I leave the specific gravity input equal, the longer bullet, in this illustration, would weigh in at 74 Gr., but still require a full 1.0" faster twist rate - you'll have to trust me on that. RG
View attachment 1024606

Since I feel my ability to explain is lacking, here is a comparison of two 6mm bullets,, probably pretty close to what you are testing - one 65 Gr., the other 66 gr., and 0.120" longer. OOOps, I failed to equalize the atmosphere . . . but that won't be worth even 0.1" of twist rate change. Were the specific gravity left equal, the longer bullet would weigh 72 gr., and require only a 1:11.1" twist barrel for Sg 1.5:hope this proves useful. On 6mm, and smaller diameter bullets, 0.1" of length is a big deal . . .
RG

 

[lpreddick]

Since I feel my ability to explain is lacking, here is a comparison of two 6mm bullets,, probably pretty close to what you are testing - one 65 Gr., the other 66 gr., and 0.120" longer. OOOps, I failed to equalize the atmosphere . . . but that won't be worth even 0.1" of twist rate change. Were the specific gravity left equal, the longer bullet would weigh 72 gr., and require only a 1:11.1" twist barrel for Sg 1.5:hope this proves useful. On 6mm, and smaller diameter bullets, 0.1" of length is a big deal . . .
RG
View attachment 1024607

Absolutely! I have been playing with the JBM Ballistics program and was amazed what that little tip does to a bullets gyroscopic stability! The bullet in question is a BT based on an .820 jacket. Changing it to a FB with the tip doesn't produce a stable design in a 14 twist. A 12 tw barrel shoots the original bullet nicely. Very interesting

 

[Ballisticboy]

If you put a plastic tip onto the front of a bullet without changing anything else then as mentioned you will move the C of G forward very slightly relative to the base of the bullet. You will also change the relationship between the moments of inertia making the bullet very slightly less stable due to the bigger increase in the transverse inertia compared to the axial inertia.
However, the position of the aerodynamic centre will move forward relative to the base of the bullet much more than the C of G, due to the increased nose length and the change of shape, increasing the distance between them and making the bullet more aerodynamically unstable and thus reducing the gyroscopic stability for a given twist rate. Coupled with this there is likely to be a reduction in the dynamic stability of the bullet due to the change in the aerodynamic properties.
The net result will be a change in the necessary rifling rate needed to produce the same level of stability compared to the round with no plastic tip as you have found.

 

[ImBIllT]

I posted this observation on Saubier and a member suggested the instability was due to bullet's increased length, not it's increased bearing surface. I seriously doubt the .3 gr tip's weight can move bullet's center of gravity forward of it's center of pressure (balance point). My bullet maker is going to make a few with the same meplate used to hold the plastic tip, but with none, and shoot them...should be interesting.

It's the length. The air pushes on the tip regardless of its weight. The air is now pushing on the bullet with a longer torque arm...it's like adding a cheater bar to a wrench. The air now has an easier time pushing the bullet side-way. In fact, adding the plastic tip is the least stabile way to add length because it weighs less than copper and lead.

 

[lpreddick]

It's the length. The air pushes on the tip regardless of its weight. The air is now pushing on the bullet with a longer torque arm...it's like adding a cheater bar to a wrench. The air now has an easier time pushing the bullet side-way. In fact, adding the plastic tip is the least stabile way to add length because it weighs less than copper and lead.

Is there any reason to put plastic tips on short range bullets? Long range shooters really like sharp pointed tips. My interest is related to varmint hunting and prefer hollowpoints. These bullets usually have low BCs and I was hoping tipping them would improve their BC. Early results with shorter flat base ones, are encouraging.

 

[RGRobinett]

Is there any reason to put plastic tips on short range bullets? Long range shooters really like sharp pointed tips. My interest is related to varmint hunting and prefer hollowpoints. These bullets usually have low BCs and I was hoping tipping them would improve their BC. Early results with shorter flat base ones, are encouraging.

From my perspective, potential BC increase is the only reason: adding tips is just one more attribute to get misaligned.
R U old enough the recall the SPEER HP varmint bullets? They featured humongous me'plats and shot great - they just 'dropped' and drifted . . .but, when one properly crorrected, they were hell on diggers (ground squirrels) and rockchucks!
If you want great bullets, simply obtain a new point-up die, featuring a 0.045" knock-out pin: compared toa 0.0625" K.O. pin, you can expect a BC increase equal to adding a BT to the same weight bullet, made with the same jacket length. RG

 

[lpreddick]

From my perspective, potential BC increase is the only reason: adding tips is just one more attribute to get misaligned.
R U old enough the recall the SPEER HP varmint bullets? They featured humongous me'plats and shot great - they just 'dropped' and drifted . . .but, when one properly rorrected, they were hell on diggers (ground squirrels) and rockchucks!
If you want great bullets, simply obtain a new point-up die, featuring a 0.045" knock-out pin: compared toa 0.0625" K.O. pin, you can expect a BC increase equal to adding a BT to the same weight bullet, made with the same jacket length. RG

Thanks Mr Robinette. The improved BC is what i'm after. I don't think the tip will hurt terminal ballistics. I ordered some ballistic gel to check this property.

 

[lpreddick]

Thanks Mr Robinette. The improved BC is what i'm after. I don't think the tip will hurt terminal ballistics. I ordered some ballistic gel to check this property.

Thanks Mr Robinette. The improved BC is what i'm after. I don't think the tip will hurt terminal ballistics. I ordered some ballistic gel to check this property.

Field testing for an improved BC after tipping a large meplate varmint bullet was interesting. The tipped ones did not drop as much as open ones. Ballistic gel test was eye opening...open meplates came apart early and no projectile parts exited the gel. Tipped bullets opened later and pieces did exit the gel. This is being addressed, but might not make any difference in a varmint as the bullet did come apart.

 

[fyrewall]

The [plastic tip] CG moves rearward, thus making the bullet more tail heavy, while reducing the specific gravity - both of which, require more spin! If the bullet is 0.3" longer, that's a real Sg (gyroscopic stability) killer - probably about 3.5" of twist rate worth! I'll try to post a pic of a side-by-side comparison of the two bullet types (well, close enough) for your consideration. Will take a while to get there. Using the Tioga Engineering (Bill Davis) Bullet design program, I'll show you a graphic comparison of two Sierra 55 Gr. bullets: the "old reliable 55 Gr. Spitzer BT(31365), and the newer 55 Gr. BlitzKing (31455). RG

My observations shooting the Hornady .243 VMax from a 1-8 twist seem to agree with this. These 87's with their .15 inch plastic tips are incredibly accurate when shot out of 1-8 at 3200 fps. A normal recommended twist rate might be 1-10.

 

[lpreddick]

Can home tipped bullets be accurate? I inserted the ballistic tip from a Hornady 40 gr VMax into the open meplat of a 52 gr bullet made by my bullet making guru. They were unstable in a 14 twist 222. I shot them over LT 32 in a slightly modified 12 twist 222 Rem Mag. Each target is three shots, second one .2 gr more powder. These bullets are J4 jackets, so ghogs beware.