rifling and twist rate

[rsterne]

I agree.... We're looking at a 14" twist for the new 41.5 gr bullet in .224 caliber.... should be ideal from 850-1050 fps....

Bob

[airpuffhunter]

Hi bob and all, I have been playing with the barrel twist calculator with the EPP-UG in .45
here is the link to its dimensions
http://www.biglube.com/BulletImage.aspx?ItemID=d3fb4508-70e1-4d9b-87bd-6d9c2d504d33&ImageUrl=Images/Epp-ug-45-150-Big.gif&Quantity=1
with a barrel twist of 1:16 I get crazy results, can you please chech to see what you get?

thank you

[airpuffhunter]

the Sam yang 909 is supposed to have a 16 to 1 barrel twist, and we all have heard of excellent accuracy with the EPP-UG, but I get over stability with this combination.
 

[herx77]

I believe John Whiscombe did some work on twist rate,and it might be found on "Shoot the Breeze" or "BBS" forum.( English shooting forums).
I think his reasoning was, a high rate of spin of the pellet did not allow the pellet to follow the decaying flight curve so that the head of the pellet would impact at point of impact.The pellet profile almost keeping parallel to the ground.
A slower rate of spin would give a tight following of the flight decay curve,(impact at the head of pellet); As the pellet profile would dip as the further out it got and the curve decayed.
Paper would show the variation of impact impressions.
 Slow spin rate clean entry,high spin rate torn entry.
It would follow that sufficient twist to give just enough stability to ensure accuracy at the ranges you shoot competitions, I am assuming we are talking air guns?
HERX77

[Cliff]

Overstabilization causes problems too (vertical stringing due to increased magnus effect in crosswinds, emphasizing pellet imbalance, etc.)

Wouldn't magnus forces cause  random  mis-groupings to the upper right  as opposed to the rather well defined vet-string?   I say this because the magnus wouldn't necessarily be linear  it'd push the projectile in whatever direction was opposite the pressure bulge and on a spinning object which might be unpredictable -  more or less - given cross winds - - yes?   The asymmetric air molecule adhesion to the bullet   (& assuming a right hand twist ) would cause a yaw  of the projectile nose upward and off to the right - - yes?  So the drift would to the right of the plane of the trajectory?
I could be wrong.  The calculation, which includes rate of change math, for  Magnus forces is way too high falutin' for the likes of me.


 There's so many things that are attributed to ver-string in powder burners the list is  too long to even contemplate.  I wonder  how many of the attributions are  more myth  than real.  I'm sure I read somewhere that the main cause of vert-string is  magical dwarfs pushing the bullet up.

However  in an air rifle, I'd think that breathing  improperly, and gun balance ( too heavy a barrel) would be the main culprits for vert-string.

[Muppit]

i'm going to have to play with the twist rate caculator now and find out the twist rate of my air guns and maybe narrow down what pellets it might like even better not that i'm unhappy with the jsb's but its something to do. i wonder how much r&d  air gun companies are putting into twist rate with the evolution of pcp and big bore stuff it would be nice to order what ever barrel you felt was right. we know how barrel lenght effects velocity and accuracy to a point but how much better would say a .22 cal 2240 shoot with the right pellet barrel lenght and twist rate. most people seem to know to look for whatever pellet soots best in their gun but it would be nice to choose what ammo you want to shoot at a certain target and build the gun to shoot that pellet accuratlly......dave

[rsterne]

According to the Border Barrels Twist Calculator, the EPP-UG .45 is wayyyyyyy overstabilzed.... A twist of about 68" would be about right at 900 fps.... I don't have a weight to use the Miller Rule, but the old Greenhill Formula also gives 68" as the proper twist for a .45 cal bullet of that length.... Your twist is about 4 times that fast....

Twist is actually calculated in "calibers" and then converted to inches.... A .22 cal barrel with an 18" twist is 1 turn in 82 calibers.... For the same twist rate (in calibers) with a .45 cal, the barrel length for 1 turn would be 82 x .45 = 37".... Larger barrels, with a bullet of the same proportions, need a slower twist rate.... Note that the EPP-UG is, in proportion, shorter than virtually any pellet we use.... Shorter bullets need less spin....

RE: Magnus effect, as I understand it, the higher the spin rate, the greater the vertical deflection in a crosswind.... so you want the slowest spin compatible with stability.... That is one reason PB benchrest guys use SF's as low as 1.2....

Bob

[Muppit]

you know bob i 'm begining to think you been playing with guns to long, LOL. You are a wealth of knowledge and thanks for your imput. i knew a little about twist but you have put it at a whole new level ........dave

[rsterne]

I suffer from trivia retention and verbal diahorrea....

Bob