08-13-2012, 12:43 AM
Bwild97 Wrote:I have a 24" tube with 1:9 twist and it is stableizing 130g NORMA's and 142g SMK's just fine!
Every tube will have its own "personality"...
Litz's data says the SMK 142 has a "nominal" stability factor of 1.36 and worst case of 1.18 (less than 1.4 indicating marginal stability; less than 1.0 being unstable) with a 1:9 twist. Stability factor improves to 1.72 and 1.49 respectively for 1:8 twist.
His data also indicates Berger 140 VLD has 1.22 nominal and 1.05 worst case from 1:9 twist; this improves to 1.54 nominal and 1.33 worst case for 1:8 twist.
Of course, this is all "theory" and ymmv...
As I responded to OP earlier, the 8, 8.5 and 9 twists would all work for his stated range of 120-130 grn bullets (unless he plans on Berger 130 VLD which Litz indicates has a stability factor of 1.35 "best case"; 1.02 worst case for 1:9 twist)...
FYI: From Litz's book (Applied Ballistics for Long Range Shooting):
Stabiility Factor (SG) using Miller Stability formula:
Best case: 1000' altitude, 100% humidity, 100 F, 3000 fps mv
Nominal case: S.L., 50% humidity, 59 F, 2800 fps
Worse case: S.L., 0% humidity, 0 F, 2600 fps
Stability factor, in theory, improves as altitude is increased...and becomes more important with the longer range at which one is shooting.
(btw, I'm an engineering geek turned fighter pilot...I enjoy the theory, but not as much as the application of firepower ;-)