The Greenhill formula says a .277 115grSMK will stabilize in a 14 twist barrel at 2800fps. Think so? I hear there is another formula but I can't seem to put my hands on it. What do you guys think?
Greenhill formula dead on?
- Thread starter constructor
- Start date
I think it will.
Using QuickTarget parameters for this bullet including a nose length producing an ICAO BC of .335,StdMetro of .341) I get:
Sg with 14tw under ICAO conditions:
1.11 [McGyro]
1.22 [RSI]
1.31 [Lija]
1.29 [PS rule of thumb]
1.37 [Mine]
Stable is Sg= >1.00
The rule of thumb out of Precision Shooter is
30*Weight/,,Tw/Dia)^2*Dia^3*,Length/Dia)*,1+,Length/Dia)^2))
It disregards velocity,which is minor), and air density beyond standard,which is major). But it's relatively safe, as with Greenhill.
Using QuickTarget parameters for this bullet including a nose length producing an ICAO BC of .335,StdMetro of .341) I get:
Sg with 14tw under ICAO conditions:
1.11 [McGyro]
1.22 [RSI]
1.31 [Lija]
1.29 [PS rule of thumb]
1.37 [Mine]
Stable is Sg= >1.00
The rule of thumb out of Precision Shooter is
30*Weight/,,Tw/Dia)^2*Dia^3*,Length/Dia)*,1+,Length/Dia)^2))
It disregards velocity,which is minor), and air density beyond standard,which is major). But it's relatively safe, as with Greenhill.
Bryan Litz Ballistics
Site $$ Sponsor
Greenhill was for blunt bullets, and isn't a very good prediction for the style of bullets we shoot today.
Don Miller published a great equation in the March, 2005 issue of precision shooting:
Sg = 30*m/,t^2*d^3*L*,1 + L^2))
Where:
Sg is the static stability. Must be greater than 1.0, desired to be at least 1.4 for practical purposes.
m is mass in grains
t is twis rate in calibers per turn
d is diameter in inches
L is bullet length in calibers
I don't know how long the .277 bullet is you're talking about, but using 115 grains, .277 diameter, and 1:14 twist, here are some Sg's for some likely lengths:
If bullet length is: 1.1" Sg is: 0.95
If bullet length is: 1.0" Sg is: 1.25
If bullet length is: 0.9" Sg is: 1.69
If bullet length is: 0.8" Sg is: 2.36
So according to Don's equation, the longest .277 cal 115 grain bullet that you can 'comfortably' stabilize with a 1:14" twist is 0.962". This length gives an Sg of 1.40. Of course, you might get away with up to 1.08", which gives Sg=1.0, but that's not an advisable combination.
Don's equation compares very well with more sophisticated methods of Sg prediction.
Hope this helps,
-Bryan
Edit: You beat me to it Mike! We were thinking the same thing. I take it from your math that the length is 0.99"
Don Miller published a great equation in the March, 2005 issue of precision shooting:
Sg = 30*m/,t^2*d^3*L*,1 + L^2))
Where:
Sg is the static stability. Must be greater than 1.0, desired to be at least 1.4 for practical purposes.
m is mass in grains
t is twis rate in calibers per turn
d is diameter in inches
L is bullet length in calibers
I don't know how long the .277 bullet is you're talking about, but using 115 grains, .277 diameter, and 1:14 twist, here are some Sg's for some likely lengths:
If bullet length is: 1.1" Sg is: 0.95
If bullet length is: 1.0" Sg is: 1.25
If bullet length is: 0.9" Sg is: 1.69
If bullet length is: 0.8" Sg is: 2.36
So according to Don's equation, the longest .277 cal 115 grain bullet that you can 'comfortably' stabilize with a 1:14" twist is 0.962". This length gives an Sg of 1.40. Of course, you might get away with up to 1.08", which gives Sg=1.0, but that's not an advisable combination.
Don's equation compares very well with more sophisticated methods of Sg prediction.
Hope this helps,
-Bryan
Edit: You beat me to it Mike! We were thinking the same thing. I take it from your math that the length is 0.99"
Thanks guys, The 115SMKs I have are .986. Have you seen the German KLG bullets?
Similar threads
Upgrades & Donations
This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.