Effect of twrist rate on ballistics questions

[Silas7458]

Ok here goes. I got two questions for the Guru's

1)I was wondering what the ballistic effect would be between a 24" HB with 1:12 vs the same round shot out of 24" HB in a 1:10 twist rate?
For example, take the Hornady 178 AMAX with 41.5gr of IMR 4064 shot from a Remy 700sps @2645fps... What would be the effect(if any) if it had been shot from a 1:10 in the same barrel?

2) using the rifle data above, what relationship/Effect is there between identical rifle barrels of differing Lengths?

Thank you for your patients if these are Noob questions

 

[taseal]

Re: Effect of twrist rate on ballistics questions

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

QL says 178 bullet length is 1.240 (it's probably not, but we'll go with it)
same everything except twist rate is 1:12 vs 1:10

Stability out of 1:12 - 1.753
Stability out of a 1:10 - 2.525

now I'm not sure on the formula behind it, because the higher the stability number the better. so if I do a 1:2 twist, it becomes 63.114 which is crazy...

so this thing says, any bullet is more stable if the the rifling is faster. which sounds pretty flawed to me.

but to get small differences, those are the results you get

 

[armorpl8chikn]

Re: Effect of twrist rate on ballistics questions

Taseal I don't think he is looking for a stability number. I think the question in laymans terms is "How much faster will the 178gr bullet be out of a 1:12 twist as opposed to a 1:10 twist, all other factors being equal."
My answer to this question would be "Not enough difference to say grace over."
Too much hoopla is made over velocity. Worry more about accuracy and sizing the cartridge to that task. In other words, don't try to make a 308 acheive 30-06 performance.
In simple terms. The longer the barrel , the more velocity. Exact numbers are only derived from actual firing. Every gun is different.

 

[Aimsmall55]

Re: Effect of twrist rate on ballistics questions

Hornady say's it on the box... recommended twist 1:10. I shoot a .300 win mag with a 1:10 and it eats them up. can easily shoot moa up to 500 yds or so. Don't have anywhere i can shoot farther than that.

 

[Aimsmall55]

Re: Effect of twrist rate on ballistics questions

One more thing, I agree with armorpl8chikn, who cares how fast it is going. The key to properly reloading is to find a load that works and stick with it. Continuity and Stability... those are the two most important factors to take into consideration once you have found what works for your weapon. Velocity can be compensated for with a jbm ballistics program and a few more turns on the turret..just my thoughts

 

[MitchAlsup]

Re: Effect of twrist rate on ballistics questions

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: GH05T</div><div class="ubbcode-body">1)I was wondering what the ballistic effect would be between a 24" HB with 1:12 vs the same round shot out of 24" HB in a 1:10 twist rate? </div></div>

Inside 800 yards: barely noticible if at all.

Beyond 900 yards: the bullet from the slower twist barrel will fly with ever so slightly better BC because the lack of overstability will allow the nose to point downward and decrease drag. This effect might account for all of 0.1 MoA at 1000 yards (that is barely noticible).

Another effect from twist is called spin drift. At distances under 800 yards it, too, is essentially negligible. At 338 LM distances (1700 yards) it shows up as a measurable annoyance.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">2) using the rifle data above, what relationship/Effect is there between identical rifle barrels of differing Lengths?</div></div>

to the 98%-ile: the longer barrel will simply shoot the bullets faster.

There is almost no MV degredation/change between a 1:10 barrel and a 1:12 barrel of equal quality.