optimizing stabilization for barrel length

[johnfkingmatrix]

Hi,

ive seen a lot of data and stabilization #s and maybe im reinventing the wheel, but:
Shooting 18" 5.56 1:8 i naturally started with 75g+ bullets thinking heavier was better, but,
given the somewhat shorter barrel and tendency to not push velocity to the max (using 2520 fwiw)
im wondering if im actually shooting too heavy and would get better long range stabilization/accuracy if i went down to maybe ~69g bullets? I really struggle past 500yd with consistent impacts

Am I onto something here with going down in bullet weight being the sweet spot for my gun/situation, or am I pissing in the wind lol.



looking forward to advice or suggestions. thanks

 

[6BRA]

Twist...not length. Powder burn rate is dependent upon bbl length.

 

[johnfkingmatrix]

yes sorry, edited 1:8, but the velocity im getting if its verging on the low end of 2200-2300, maybe I'm just not pushing it hard enough?

which is my broader question of, would a 14" barrel with identical twist, for instance, benefit from shooting lighter bullets since the necessary velocity would not be achieved with the heavier bullet.. even tho in theory the heavier is better for LR

ie..does shorter barrel with identical twist require lighter bullets to achieve optimal velocity for stability at range. and, if so, is 18" better with lighter than 75g

 

[dellet]

Depends on your current bullet and twist rate.

There are two different stability numbers, one where the bullet does not wobble, and one where the bullet flies at maximum efficiency, highest BC. So if your current bullet is marginal for twist and velocity, switching bullets could help. Since length more often than weight determines stability, you may or may not have a weight change.

A good place to start is Berger Bullets stability calculator.
As an example the 75 grain VLD at 2300 in a 1/8 twist needs to be appears marginally stable and is losing 2% of its BC.
Same bullet, same velocity has reached full potential in a 1/7.5 twist.
To get the same SG and BC numbers in a 1/8 twist as the 1/7.5, you would need to increase the velocity about 1000 fps.

It’s worth the time to put in a few bullets and see what different speeds and twists do to the numbers. Not many calculators have both Stability and BC so readily and easily available.

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.

bergerbullets.com

 

[6BRA]

What powder? Use a faster burning powder possibly. Is it a flame thrower?

 

[Paddyd]

I think velocity helps with stabilization. I'm told I need a 1/7 or 1/8 twist to stabilize the 77gn Sierra match in my 223, yet I seem to have no issue with that in my Savage 12 LRPV which has a Criterion 1/9 twist and is 26" long. I think barrel length helps add velocity, especially with slower powders. No doubt I'm on the edge using the 1/9 twist, but the groups tell me to not worry. They shoot 1/2 minute out to as far as the range here allows which is 200 yds. Try pushing your velocity. A bit over 2k fps is too slow to keep stability at the 500 yds you talk about. By the time it's out that far it is getting pretty wonky in my opinion.

 

[chromatica]

One measure of stability is the Miller Stability Index. You want the Miller index to be greater than 1.0 to prevent tumbling. If you launch a 77 gr Sierra Matchking out of a 1:8 twist barrel at 2520 fps, the Miller index is 1.694. That is very stable value. However, between 500 and 600 yards the bullet velocity drops into the transition zone - that is it falls below 1.2 times the speed of sound. The 77 SMK is known to have a dynamic stability problem at this range.

If you launch a 69 grain Sierra Matchking out of a 1:8 twist barrel at 2650 fps, the Miller index is 2.058. This is a very stable value. The velocity also drops into the transition zone at 550 yards, but the 69 grain SMK doesn't have dynamic instability in the range of Mach 1.2 to 1.0.