Does a plastic tip , mean I can get away with a slower twist ?

Get or give advice on equipment, reloading and other technical issues.

Moderator: Mod

Message
Author
GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Does a plastic tip , mean I can get away with a slower twist ?

#1 Postby GSells » Fri Aug 31, 2018 9:14 pm

Hey all ! So I have some of these Hornady 180’s measuring 1.550” with a twist of 1:9 at 2750 FPS , giving me a miller factor of 1.36 sg .

Is it safe to assume that because of the plastic tip which isn’t lead or copper . That the above miller formula over estimates yaw on the above bullet , and maybe with a 9 twist it’s adequately stable??

Tim L
Posts: 876
Joined: Mon May 19, 2014 7:11 pm
Location: Townsville

Re: Does a plastic tip , mean I can get away with a slower twist ?

#2 Postby Tim L » Fri Aug 31, 2018 10:16 pm

What calibre Gman. I run 185s in 308 through a 1:12. A bit faster than 2750 though.
I can't comment on Miller, I don't know the bloke.

GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Re: Does a plastic tip , mean I can get away with a slower twist ?

#3 Postby GSells » Fri Aug 31, 2018 11:04 pm

Tim L wrote:What calibre Gman. I run 185s in 308 through a 1:12. A bit faster than 2750 though.
I can't comment on Miller, I don't know the bloke.

Hi Timmy , 708 ai , how's things? Regards Golf Seira lol, aka The G Man ! :lol: 8)

Ps Hornady 180 elds 1.550" long .284 cal cheers !

Steve N
Posts: 462
Joined: Tue May 14, 2013 8:16 pm
Location: Gippsland Victoria.

Re: Does a plastic tip , mean I can get away with a slower twist ?

#4 Postby Steve N » Sat Sep 01, 2018 7:18 am

GMan the Hornady website shows the BC details for that bullet at various velocities for both 7.5 twist and 8.75 twist and the 7.5 figures are much better so maybe better stabilised. 9 twist would have a BC bit lower so less stable? Hope this helps
https://www.hornady.com/support/ballistic-coefficient

GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Re: Does a plastic tip , mean I can get away with a slower twist ?

#5 Postby GSells » Sat Sep 01, 2018 2:48 pm

Steve N wrote:GMan the Hornady website shows the BC details for that bullet at various velocities for both 7.5 twist and 8.75 twist and the 7.5 figures are much better so maybe better stabilised. 9 twist would have a BC bit lower so less stable? Hope this helps
https://www.hornady.com/support/ballistic-coefficient



Thanks Steve and that is my gut feeling. I hope it works , as I’ve mentioned before. If it does work , then I have 300 gr 338 cal lap scenner ballistics that is the same as my 338 edge in a 7/08 !!! I’ll trial it next weekend at about 885 ms as close as I can get 1000 yds , out here in the bush . In good conditions and see how she goes !!
Now Miller formula is as per Wiki :
https://en.wikipedia.org/wiki/Miller_twist_rule

Tim L
Posts: 876
Joined: Mon May 19, 2014 7:11 pm
Location: Townsville

Re: Does a plastic tip , mean I can get away with a slower twist ?

#6 Postby Tim L » Sat Sep 01, 2018 3:16 pm

If you take the data from Hornady and punch it into the Berger calculator you are compromised GMan!
It says you are only marginally stable (that's not news for most of us).
Things may get a bit wobbly as you push out further, with cold weather compounding the issue.
With all that said the maths sometimes doesn't hold up. Holes in paper are facts :D

GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Re: Does a plastic tip , mean I can get away with a slower twist ?

#7 Postby GSells » Sat Sep 01, 2018 5:56 pm

Tim it maybe a case of works up here at altitude but at Belmont it’s no good! But just have to try it and see as u said ! Thanks all !

GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Re: Does a plastic tip , mean I can get away with a slower twist ?

#8 Postby GSells » Sat Sep 01, 2018 6:54 pm

A little of topic , but today I had a wonderful day at my home rifle range shooting PRS !! It’s nothing really like f class and any achievements and pride , ego earned form f class can really be left at the gate lol! It is hard !! Especially with a rifle that doesn’t have a break , that u can’t spot your own shots !! That’s where I’m struggling and not to mention using 5 shot magazines and just being a plain duck out of water situations in shooting positions that is f classers just don’t do unless down the paddock taking care of feral pests !! Then the time constraints throws any wind reading skills out the window as there just isn’t any time to really sus out the conditions!! Very tuff but had heaps of fun !!
Ok Alan , sorry I was way of topic !! Back into it !!
I found 4 th degree of freedom (4DOF) on a Hornady website.
I plugged in 180 ELD’s with a 9 twist at 1250 ft alt ( my alt here ) and range from 100 out to 1200 and it shows it’s well above 1.4 and actually increases the further it goes out !
I’ll take a screenshot of the SG factor , each window is a hundred yards further down range .

1CCA8BD9-6CC5-433F-952A-09A837E21595.jpeg

Here is an explanation of from Hornady:

Gyroscopic Stability:

Calculated using the rifle’s twist rate, bullet physical characteristics, the center of gravity, aerodynamic properties, and velocity. The number increases as the bullet moves downrange. To attain acceptable aerodynamic performance, we recommend a muzzle Gyroscopic Stability (Sg) above 1.4.

NOTE: Gyroscopic Stability Factor (Sg) at the muzzle is highlighted in yellow. The barrel twist, velocity, and air density will determine the muzzle Sg along with other projectile properties included in the projectile file. Doppler radar data has shown that the maximum aerodynamic performance is attained, at higher velocities, with a projectile Sg of approximately 2.0 or higher. The calculator outputs the Sg for each range as the projectile travels downrange so the user can see how the projectile’s stability changes as it travels downrange.

https://www.hornady.com/team-hornady/ba ... rs/#!/4dof

So judging by the above info , we are quite fine ! This could be I believe because of the plastic tip !
Regards Graham ( GMan !!)
You do not have the required permissions to view the files attached to this post.

!Peter!
Posts: 150
Joined: Sun Oct 30, 2016 6:35 am

Re: Does a plastic tip , mean I can get away with a slower twist ?

#9 Postby !Peter! » Sat Sep 01, 2018 9:41 pm

Graham,
FWIW, I've attached some work on modifying the Miller stability formula for plastic tipped bullets by Michael Courtney & Don Miller.


So, if you're now known as the Gman does this mean that where you're standing is called the Gspot? :-k Sorry couldn't help myself :lol: I really tried!

!Peter!
You do not have the required permissions to view the files attached to this post.

GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Re: Does a plastic tip , mean I can get away with a slower twist ?

#10 Postby GSells » Sat Sep 01, 2018 9:56 pm

!Peter! wrote:Graham,
FWIW, I've attached some work on modifying the Miller stability formula for plastic tipped bullets by Michael Courtney & Don Miller.


So, if you're now known as the Gman does this mean that where you're standing is called the Gspot? :-k Sorry couldn't help myself :lol: I really tried!

!Peter!

Hahahahahahhaa! =D> Thanks Pete ! Hopefully I can work it out and that it’s simple to use lol!

GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Re: Does a plastic tip , mean I can get away with a slower twist ?

#11 Postby GSells » Sat Sep 01, 2018 9:59 pm

Like , just look at this puppy !! A .308 case pushing a 7 mm torpedo!!
image.jpg
You do not have the required permissions to view the files attached to this post.

GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Re: Does a plastic tip , mean I can get away with a slower twist ?

#12 Postby GSells » Sun Sep 02, 2018 2:35 pm

Thanks again Peter, I really appreciate your contributions !
Well according to the info given by Peter , looks like my gut feeling was right ! If anyone has the math skill to work it out , you are more than welcome to.
stability of plastic-tipped bullets, we settled on the
formula (Equation 2):
S g=
30 m
t
2
d
3
L(1+Lm
2
)
×(
V
2800 )
1

( FT+ 460)
(59+460 )
29.92
P
,
where Lm is the length of the metal portion of the
bullet. This adjustment was most reasonable,
because the length in the (1 + L2
) term is related to
the rotational moments of inertia. That term will be
more accurately estimated by the length of the metal
part of the bullet because the density of copper and
lead are close to 10 times the density of the plastic
tip. Consequently, the length added by the plastic tip
does little to change the moments of inertia. In
contrast, the other L in the formula originates in the
center of pressure, an aerodynamic property
determined by the shape of the bullet without regard
to material density, with a smaller effect from the
center of gravity. Thus it makes most sense for this
to remain as the total length of the bullet.

Ps look up the pdf for the formulor! \:D/

GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Re: Does a plastic tip , mean I can get away with a slower twist ?

#13 Postby GSells » Sun Sep 02, 2018 8:46 pm

I've struggled to do the math , but cal is .284 , length of a Hornady 180 eld is 1.55" overall and metal portion is 1.406" overall speed at muzzle is 2750 at a 1000 m is 1590 fps , ambiant temp is 75 deg ft and 30.09 hg . Please if someone can have a go at it ?
Regards Graham .

GSells
Posts: 798
Joined: Sun Aug 13, 2017 11:04 pm
Location: Qld

Re: Does a plastic tip , mean I can get away with a slower twist ?

#14 Postby GSells » Sun Sep 02, 2018 9:42 pm

Ok I’ve found a simple formula and it’s stable and I’ve backed it up with JBM ballistics http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
Sg=30 x 180
—————————
31.69 sq x .284 cube x 5.45(1+4.95 sq)
= 1.68 sg

Here it is with atomosphere and velocity of 2750 FPS
3D8E72E5-68B3-4616-8B2C-4BD13FDBD502.png

So it is adequately stable all the way out to 1.4 KM’s at 1200 FPS !! At a bc of .390 g 7 !!

Not too shabby for a plumber haha! Most likely easy for a sparkie lol!
You do not have the required permissions to view the files attached to this post.

bruce moulds
Posts: 2900
Joined: Sun Jun 19, 2005 4:07 pm

Re: Does a plastic tip , mean I can get away with a slower twist ?

#15 Postby bruce moulds » Mon Sep 03, 2018 7:16 am

gyroscopic stability looks good.
now let's start on dynamic stability.
keep safe,
bruce.
"SUCH IS LIFE" Edward Kelly 11 nov 1880
http://youtu.be/YRaRCCZjdTM


Return to “Equipment & Technical”

Who is online

Users browsing this forum: No registered users and 34 guests