6br 70gr sierra match king twist

[Wellington]

I have searched all over the web and not found an answer, will a 1 in 13 handle the 70gr HPBT match king?

 

[Bronsin]

We shoot 68's out of 1:14, so I don't see any reason it wouldn't, unless your rifle just doesn't like that bullet.

 

[Bill Norris]

Should work well.

 

[freebird22/250]

We shoot 68's out of 1:14, so I don't see any reason it wouldn't, unless your rifle just doesn't like that bullet.

They work great in my 6mm BR 14 twist Krieger bbl.

 

[Laurie]

I have searched all over the web and not found an answer, will a 1 in 13 handle the 70gr HPBT match king?

Using the Miller Stability Rule formula / tools based on the Miller formula (eg JBM and Berger stability calculators), the 6mm 70gn Sierra MK is under-stabilised in a 13-inch pitch rate.

The 70 SMK is 0.834" OAL (Litz: Performance of Rifle Bullets 3rd edition). Feed that into Miller and you get an Sg value of 1.22 at 3,200 fps MV at sea level / 59-deg F.

If your shooting venue is hot & high (with thinner air), things get better. Sg rises to 1.44 at 90-deg F / 3,000 ft. ASL - not optimal (1.5 and above is optimal), but plenty good enough.

In practice, Miller / Berger etc may be a tad pessimistic in their results here as the 70 SMK has a very short boat-tail. Nevertheless, 13-twist is really marginal in most environments. Note that in such conditions, bullet holes are often clean and round looking at short distances, but groups are rarely up to the standards of the same kit but using an inch-faster twist barrel.

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

We shoot 68's out of 1:14, so I don't see any reason it wouldn't, unless your rifle just doesn't like that bullet.

It's not the weight per se that matters most, rather bullet OAL in relation to calibre and the existence or absence of a boat-tail form. Are your 68s flat-base? If so, you can happily use a 1-inch or so 'slower' twist rate than that needed for a same weight and OAL BT model.

That's why Berger has the following note in its twist rate calculator resource and a table listing minimum twist rates needed for individual FB bullets in its range.

"Important Note: The Miller Twist Rate Formula isn’t accurate with flat based bullets, so we recommend you view our flat based bullet twist rate chart to identify what the minimum twist rate is for all of our flat based bullets."

https://bergerbullets.com/flat-based-bullet-recommended-twist-rates/

 

[Wellington]

Using the Miller Stability Rule formula / tools based on the Miller formula (eg JBM and Berger stability calculators), the 6mm 70gn Sierra MK is under-stabilised in a 13-inch pitch rate.

The 70 SMK is 0.834" OAL (Litz: Performance of Rifle Bullets 3rd edition). Feed that into Miller and you get an Sg value of 1.22 at 3,200 fps MV at sea level / 59-deg F.

If your shooting venue is hot & high (with thinner air), things get better. Sg rises to 1.44 at 90-deg F / 3,000 ft. ASL - not optimal (1.5 and above is optimal), but plenty good enough.

In practice, Miller / Berger etc may be a tad pessimistic in their results here as the 70 SMK has a very short boat-tail. Nevertheless, 13-twist is really marginal in most environments. Note that in such conditions, bullet holes are often clean and round looking at short distances, but groups are rarely up to the standards of the same kit but using an inch-faster twist barrel.

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



It's not the weight per se that matters most, rather bullet OAL in relation to calibre and the existence or absence of a boat-tail form. Are your 68s flat-base? If so, you can happily use a 1-inch or so 'slower' twist rate than that needed for a same weight and OAL BT model.

That's why Berger has the following note in its twist rate calculator resource and a table listing minimum twist rates needed for individual FB bullets in its range.

"Important Note: The Miller Twist Rate Formula isn’t accurate with flat based bullets, so we recommend you view our flat based bullet twist rate chart to identify what the minimum twist rate is for all of our flat based bullets."

https://bergerbullets.com/flat-based-bullet-recommended-twist-rates/

I ran it through their calculator as well, just thought someone had some real world experience. It is interesting to note that the Berger boat tail comes in at 65 gr., the AOL on it is .833 and they say that bullet will work in a 13 twist.

 

[Murray Brook]

@Laurie Always wondered why BT bullets require a faster twist than FB. Could you shed some light. TKS

 

[RGRobinett]

@Laurie Always wondered why BT bullets require a faster twist than FB. Could you shed some light. TKS

When using JBM, never trust the Miller formula - use the DRAG/Twist calculator! Drag creates/contributes to stability . . . AND, a BT version (which reduces drag) of a given bullet is always LONGER, which [further] reduces stability - a "double whammy" . . . Drag is both our friend and our enemy - but mostly, the former! RG

 

[dgeesaman]

I have searched all over the web and not found an answer, will a 1 in 13 handle the 70gr HPBT match king?

My 1:14 6PPC-USA stablizes that bullet just fine. So definitely yes with faster twist and more velocity.