Those little plastic tips & Sg stuff

[fyrewall]

I like plastic tip bullets for rodents - I think the splat effect is much greater. The reasoning is that the little tip thingy gets driven into the bullet upon impact causing immediate expansion, provided the bullet has a thin jacket & soft core. But the bullet has to not fly sideways and a good indicator for this is a stability program that provides a Sg (stability factor).

One of my most used bullets is the .204 diameter Hornady 40 grain Vmax. Looking at the on-line JBM on-line bullet stability program, this tiny bullet, having an overall length of .753 (measured using my calipers) should be an absolute stability failure when fired from a 1-11 twist rifle, producing a Sg value of .984. Things look better when using the JBM program and inputting .139 for the tip length and then the Sg jumps up to 1.430. This is at 59 degrees F, at the usual rodent shooting temps at 75 and warmer the bullet has even a greater Sg.

What does this mean - I have had less than 1 inch groups at 300 and have picked off hapless rodents at about 400. No photos of the rodents but I have a photo of a 5 shot group that measures less than 1 inch shot at 300. The bullet shoots real good.

All this should give some idea that Sg values are just not some esoteric stuff and the JBM stability program is a useful tool.

I also shoot the 75 grain .224 Amax out of a 1-7.7 twist .22-.250 at about 3175. Using the JBM calculation this bullet has a Sg of 1.351 at 59 degrees. It is plenty accurate on a nice warm day and the Sg increases (fair weather bullet?) . Would I use this bullet for shooting coyotes at 28 degrees - no - 59-28 = 31 degree temp drop and a resulting Sg of 1.270 (marginal) then the temp would keep getting colder - no - the 69 grain .224 Sierra tipped MK would be my choice. It might not be so good for fur collection.

I have confidence in this Sg stuff without trying it out for all calibers because it works so well for the .204's (.20 Practical & .204 R)

 

[Dusty Stevens]

real world results always trump keyboard shooting. if they shoot to your standards who cares what a computer program says?

 

[mikecr]

The Miller/Courtney stability rule of thumb works fairly well for the SOLID portion of a projectile.
With this, for better results, the plastic tip length should be subtracted from bullet OAL.
For hollow points, the depth of tip void should be subtracted.

 

[dcali]

Keep in mind that the miller approximation is sketchy when it comes to flat based bullets. As with any engineering calculation, use a safety margin if you want to be sure it’s going to work.

 

[fyrewall]

Shoot & see, then we have all copper or copper alloy bullets and pointy bullets having tiny volume but relatively long voids, being filled with squashed in bullet jacket. A Miller approximation run combined with actual twist rate specs might just prevent providing credit card info to buy a lot of 500 before shooting a dozen or so.

Also - is the twist in your rifle what you think it is? My old 6.5-06 barrel was ordered with a 1-9 twist but it would not shoot long 140 grain boat tail hollow point bullets. My other rifle, a 6.5-.308, AKA .260 Rem) shot the same bullets just fine. Both side by side on a nice warm day. Upon measuring the twist rate I discovered the 6.5-06 actually had a 9.75 (+- tiny bit) inch twist rate and the 6.5-.308 twist rate was right at 9 inches. I switched to 120 Hornady Amax bullets for the 6.5-06, it also did real good with 95 Vmax bullets. I still have the 6.5-.308 and will see if the long 140 HpBt bullets fly sideways when the temps are less than 20 F. Small changes of twist rate produce big changes in Sg's. Temperature changes produce moderate Sg changes.

I have the formula for the Miller approximation and can't remember seeing any allowance for flat base vs. boat tail bullets. I would guess that a flat base bullet would have a center of mass nearer to the base of the bullet. Then not all boat tails are equal - shoot & see.

I use the Miller approximation to provide a safety margin before buying.

I bet lots of math was done before we put a man on the moon. (not to compare my crude bullet analysis with that event)

 

[mikecr]

A rule of thumb is often useful, but rarely representing truths.
For actual stability calcs you need actual stability math. And believe me, you're on your own there..

 

[fyrewall]

A rule of thumb is often useful, but rarely representing truths.
For actual stability calcs you need actual stability math. And believe me, you're on your own there..

Any comments about the new (recent) Hornady ballistic calculator?? 4 Degree business.

 

[dcali]

I have the formula for the Miller approximation and can't remember seeing any allowance for flat base vs. boat tail bullets.

Miller approximates aerodynamic and mass properties based on empirical data. The data is a surprisingly good fit with length for boattail bullets. Less so for flat based bullets, and in some cases, not very good at all. Use it conservatively.

 

[fyrewall]

Thanks for your responses & interest - no shooting at the range or elsewhere because of armpit deep snow so I got to do something.

I ran some comparisons using the same bullet - .224 75 grain Hornady ELDM - using the Miller approximation and Hornady 4 degree of Freedom site.
The bullet Length = 1.116 OAL, plastic tip .156

Miller Sg JBM calculator with .156 plastic tip @ 3150 (.22-250, 1-7.7 twist) 100 degrees F Sg 1.986, 59 degrees 1.841, 15 degrees 1.685

Hornady 4 degree calculator with same bullet and results in Gyro column 100 degrees F Sg 1.57 , 59 degrees 1.45 , 15 degrees 1.33

Page 6 in the Hornady accompanying manual provides some explanation - like if Sg below 1.4 use a different bullet or faster twist (wait for a warmer day?). It was noted that the Sg increased as velocity decayed.

What all this tells me is to use the Miller approximation sensibly and don't buy a big bunch of a bullets without actually shooting some. Both methods provide insight of what to expect.The Hornady calculator relates gyro or Sg effects to bullet drag. My excel spreadsheet runs the Miller stuff good and displays the formula but apparently the Hornady calculations use actual measured data from Doppler radar (melting meplats?).

 

[dcali]

If I'm not mistaken, Hornady uses the correct mass properties (as opposed to the miller approximation), which should (if they do it right) account for hollow points and plastic tips. The more accurate mass properties will absolutely be more accurate than miller.

Last I heard, they calculate the required aero properties with PRODAS. In theory this should be more accurate than the miller approximation too. In practice there are some ways to screw it up, and the calculations required have some inherent uncertainty due to the empirically based methods used. But there isn't a practical way to improve on this currently. What I have done is recreate the Miller formula from scratch looking at the original data he used, and I have a sense for how much uncertainty there is in the aero data. It's not insignificant. But neither is the error in calculating it.

I do something similar with my own bullets. I don't use PRODAS - I wrote my own software to do this. But the results tend to be similar to Miller most of the time. I tend to trust my calculations more simply. I haven't been able to test that, but for the reasons I just gave above, it's probably better than Miller on average, and just as good as Hornady's. (It's basically the same method as Hornady's, which has been around for ages).

At the end of the day, you have to do exactly what you say - be careful and don't by a ton of bullets on the edge until you run a few through the barrrel under the worst conditions you'll face.

To put this all in context - this is really splitting hairs for people interested in the stuff. Being conservative and using miller (or just plain asking around) is a very good solution. There's no real need for all this math. I just do it for fun.

 

[fyrewall]

Fun with math? It feels good when you stop!

Some more info:

https://arxiv.org/ftp/arxiv/papers/1410/1410.5340.pdf

Looking at this stuff I was unable to make their (Courtney) suggested modification work with some modifications- I got identical results as the JBM with the 75 grain .224 Hornady ELDX and 40 grain .204 Hornady Vmax.

75 grain .224 Hornady ELD @ 3150 mv, 59 F, 29.92 in Hg for a Sg = 1.814; yes cold temps.

40 .204 VMax @ 3650 mv, 59 & 29.92 = 1.418; unknown cold temps

The attachment shows OAL calculations.

What I did was change the (1+L^2) to (1+ ((bullet length - tip length)/caliber)^2)). Upon doing this the modified Miller Sg for tipped matched the JBM for .204 40 Vmax's & .224 75 ELD's. This gives me a quick and easy program. The modified Miller approximation appear to be identical to that of JBM.

I do lots of hot weather rodent shooting, but what about the .20 40 Vmax @ under 20 F, will it hit under 2 inches @ 300 with nice round holes (paper)? I get lots of good under 2 inch groups at 300 with the .20 Practical and have hit small rodents @ 400. Nice to know before packing it up for some event. What about 5.56X45 ammo that might be used at over 100 F to below zero (not for me)? 1-7 twist rates?

I did play around with calculations based on density differences assuming the density of poly carbonate was about 15 percent of jacket/lead bullet material and also assuming the jacket & lead material were sort of density similar. I came real close to other calculations but then the density based calculations might be a better approximation.

Should I get another rifle or some LabRadar stuff?

 

[fyrewall]

The attachment shows the Miller/Courtney approximation made to work with (1+((bullet length-tip length)/caliber)^2)) To be compared with the previous thread attachment

 

[dcali]

Let me skip to the end, since you’re headed down this rabbit hole. Try calculating the moments of intertia in as much detail as you can stomach. It’s an enormous pain in the ass, but you only have to do it once.

Then use intlift to calculate the needed aero coefficients. Then you can use the equations straight out of McCoy without approximations. There are limits to intlift’s accuracy and suitability, but I think this method is as close as we can reasonably expect to get without going to great lengths deriving better aero coefficients.

This will get you stability numbers that are not all that far off Miller, which is prett crazy to me - that Miller works so well.

 

[moondog]

Went out last Friday with my Son who flew in to surprise me for my retirement. Went out to the neighbors ranch to shoot some steel.
105 hybrids in the Dasher seemed to work on Pdogs just fine

 

[fyrewall]

moondog's adventure sounds like more fun than "moments of inertia", but unless I get some relief from the tedium of forced inactivity I will be sucked into more of the intricate details of going down a rabbit (not a rodent) hole. I now have a Excel spread sheet that does the Miller stuff with plastic tipped bullets as good as JBM. I have decided on the .224 75 ELD vs the .204 40 Vmax for the next event assuming temperatures in the low 20's. I should have paid more attention in physics 201. The Dasher is a frequent winner at the 1000 yard BR shoot just miles from here.

 

[rwj]

Let me skip to the end, since you’re headed down this rabbit hole. Try calculating the moments of intertia in as much detail as you can stomach. It’s an enormous pain in the ass, but you only have to do it once.

Today’s 3D solid modeling programs make it easy to obtain the physical properties of any shape/material combination. For those willing, AutoDesk allows hobbiests to download a copy of Fusion 360...

 

[dcali]

Today’s 3D solid modeling programs make it easy to obtain the physical properties of any shape/material combination. For those willing, AutoDesk allows hobbiests to download a copy of Fusion 360...

That will work too, especially if you only want to do a few cases.

 

[fyrewall]

I will play around with the Hornady 4DOF Ballistic Calculator with its large library of bullets of different calibers, lengths, structures and weights, some of which are not Hornady brand. I have a modest stash of Hornady bullets in their list and some 70 grain Nosler .224 RDF bullets. I also need more computing resources other than Excel.

Thank-you for the suggestions. As soon as it warms up this hobbyist will put aside the numbers stuff or at least diminish "key board ballistics"

Moondog's dead rodent that could have been hit with a 6mm 105 Berger looks much like my rodents shot with a .204 40 Vmax.

In a former life I was a FORTRAN & BASIC drone.