Twist rate

[Bruce W]

twist rate for 30 cal up to sierra 168 match #2200

thanks, Bruce

 

[1shot]

11.25 will serve you well.

Lloyd
Edited to correct what I meant to type.

 

[Sandstorm]

12 twist works well for me with the 168's and 175's

 

[Laurie]

There is unfortunately no weight-determined 'up to' formula or values. The primary factor (assuming we're talking gilding metal jacket / lead core boattail-form designs as flat-bases and all-copper bullets dance to different tunes) is the bullet length as measured in calibres.

The Sierra 168gn MatchKing is a relatively short bullet for its weight at around 1.215" OAL, so manages with a slowish twist. I happily used this bullet on a range at ~1,000 ft ASL in an old UK 'Target Rifle' rifle with a one turn in 14" twist Lothar Walther barrel. 13" twists are better and nearly optimal.

Berger's 168s though are longer at 1.288" for the Hybrid and 1.261" for the old Target VLD. The former needs a twist of at least one turn in 12" in 'standard ballistic conditions' for full stabilisation.

Again, the 'up to' designation is likely to confuse in that lighter Sierra MKs can require faster twists due to their higher OAL to weight ratios. The original 155gn SMK (p/n 2155) is the shorter of the pair at 1.131" OAL and works very well with the 13" twist that most TR / Fullbore / Palma Rifle competitors use. The current later 155gn 'Palma' model (p/n 2156) is the same weight but longer at 1.202" and needs a faster pitch for optimal stabilisation at one turn in 12.5", in practical terms a one in 12" twist barrel.

Then, there is the environment - altitude and temperature. 'Standard' conditions are those appertaining to 59-deg F air temperature at sea level on a 'standard' meteorological day, ie 29.92-inches of mercury air pressure or whatever that is in newfangled millibars. Go higher and warmer and slower rotation works. So, if your shooting is at Raton (7,500 ft ASL) in high summer, you don't need as 'quick' a twist.

Berger does a very good twist-rate solver on its website with its own bullets in the database. Click onto the nearest model to your Sierras and overwrite the bullet OAL and if needed the weight, then input your MV, altitude and likely temperature etc. Full stabilisation is achieved at Sg (coefficient of stability) values of 1.5 and above, but lower values at around 1.4 will stabilise bullets well, albeit with a small loss of effective BC value.

https://bergerbullets.com/twist-rate-calculator/

The up to 168gn Sierras lengths are:

150gn MK ......... 1.123"

155gn MK (2155) ..... 1.131"

155gn MK (2156) ..... 1.202"

168gn MK .......... 1.215"

(Source: Bryan Litz's Ballistic Performance of Rifle Bullets 3rd edition)

Alternatively get a good 12-twist barrel and they'll all shoot well in it, something a tad 'faster' if you're likely to adopt longer designs such as Berger's at any point. If erring, it's always better to do so towards faster twists.

 

[Liven The Dream]

In the old M1A (M-14) Days of shooting Service Rifle 1-10 was the most common ?
If yo could get a Boots Obermeyer 1-11.5 you where Hit ten the Big Time .
I remember Uncle re- barreling some State M-14 with 1-12 Barrels they shot Match Ammo (168 Gr.) well.

 

[Bill Norris]

1 - 11.25 works well with bullets up to 190 gr. Maybe even a little heavier.

 

[Bruce W]

thanks for help fella's. regards, Bruce

 

[martyj]

1-10 works well up to about 210 gr bullets in my experience.