30 cal stability question

[Hammer47]

in a 308 with a 28 inch bbl what would be the correct twist rate for the Berger 185 BTLR? Would appreciate some load data too. Many thanx.

 

[711rollerdog]

http://www.bergerbullets.com/twist-rate-calculator/#

 

[M14AMU]

1-10"!!

 

[Laurie]

At 2,700 fps and in standard ballistic conditions (59-deg F; 29.92-inches mercury atmospheric pressure) the following twist rates produce these Sg (stability coefficient) values:

12 ....... 1.43
11.5 .... 1.55
11 ....... 1.70
10 ....... 2.05

As Berger now recommends an Sg value of 1.50 or higher, 11.5 is the slowest optimal twist rate for these conditions. Shoot at Raton in 90-deg ambient temperatures though and that figure changes to 13.5 (thin air due to temperature and altitude), or if at sea level on a high pressure day of 31-inches mercury and 32-deg F it increases to 10.9 inches.

Sub-optimal twists for the bullet and condition will often shoot well, and a 12-inch twist rate was regarded as optimal until recently for this bullet. In fact in its very early days, Bryan Litz won US Rules Palma matches with it in a 13-inch twist barrel rifle.

What is now known is that any significant drop below 1.5 sees the bullet create more drag, so its effective BC value is degraded. Shooting the 185 Juggernaut in standard conditions with a 12-inch twist barrel produces a calculated Sg of 1.43 which Berger says degrades the BC by 2%. Drop to a 13-inch twist and the Sg falls to 1.21 which although still stable reduces the BC by an estimated 9%. Get down to around those values and group dispersion often increases too, especially at long ranges, even if the bullets still make round holes in the paper. You can work all these things out for Berger Bullets using the Berger calculator that shtnrise has provided the link for, and other makes using the simple Millers Twist Rule spreadsheet that can be downloaded - just Google the name for places where it's available - but you need to know the bullet's overall length as well as calibre and weight.

Personally, I'm with M14AMU is saying you only occasionally go wrong specifying a 1 in 10-inch rate for the 308 for everything other than the longest Berger Hybrids which need faster twists. 155s shoot very well at 1.000 yards in the 10 rate even if some people (but not Bryan Litz whose books are WELL worth buying and reading) believe that bullets are 'over-stabilised' in such conditions.

 

[Hammer47]

Laurie... one of the finest responses with which I have ever been favored. Many thanx
to all. Gary

 

[Bart B.]

There's a small rpm band that a given bullet needs to spin at to stabilize throughout its flight to the target. Most bullet rpm rates slow down about 10 percent through 1000 yards. Ideally, the bullet's rpm spin rate is as slow as possible. Any faster means the more they'll jump off the bore axis fron centrifugal forces due to their unbalance.

150 and 155 grain 308 bullets will stabilize well shot from 1:14" twist 30" barrels in hot weather. They may need a 1:12" twist in freezing temps; 32F, 0C.

People have shot Sierra 190 HPMK's from 24" 1:12" twist barrels winning 1000 yard matches in cold weather, in the 40'sF. 44 grains of IMR4320 in new 7.62 M118 primed cases. Load tested MOA at 1000 shot from Garands.

42 grains of IMR4064 under 185 and 190 grain match bullets was a favorite in 308's.