Twist Rates

[rsterne]

Could anyone who has accurate information please post the twist rates for any .45 cal barrels they have or know about.... also the actual bore size (groove diam.).... Are they .452 or .458.... or other?....

What would be your preferred diameter and twist rate if you could start from a clean slate in .45 cal?....

Bob

[melloroadman]

Good read on barrels . Starts with explaining that most people consider the land to be the bore diameter .Marvin
http://www.chuckhawks.com/rifle_barrel.htm

[rsterne]

I've read that before... excellent article....

Bob

[lloyd-ss]

I was looking at the article Marvin referenced and I see the 444 Marlin has a twist of only 1 in 38.  If that slow a rate works, are we typically spinning bullets faster than necessary.  I know a long bullet needs a faster spin, and a fast bullet needs a faster spin. But BB airguns are certainly a lot slower than a 444 Marlin.  I don't understand.
Lloyd

[rsterne]

and a fast bullet needs a faster spin

Every twist calculator I have seen says the opposite.... higher velocity can use a slower twist rate.... I know for a fact that when doing "squib" loads for PBs that if you drop the velocity too low the bullets can start to tumble.... I've had it happen.... Bump the velocity back up and they're fine....

One thing I can't figure, though.... is that the twist calculators I have seen with a velocity correction tell you to use 1120 fps for any subsonic load.... ie the calculators don't increase the twist as the velocity decreases below Mach 1.... No explanation given, however....

Bob

[Butcher45]

Good read on barrels . Starts with explaining that most people consider the land to be the bore diameter .Marvin
http://www.chuckhawks.com/rifle_barrel.htm

I know Chuck Hawks......nice guy.  I've met up with him and some of his crew for coffee when I was living in Eugene.  As you can guess, I introduced them to the concept of bigbore airguns, and also the more powerful PCP smallbores.  You guys should have seen all their eyes light up when I showed them the slugs I was shooting from my Samyang, and explained the potential for increased nutria hunting opportunities with shrouded .25 PCP's (they are using suppressed .22's).

A few things about Chuck: 1) is he is NOT what one would call a fan of cast boolits, viewing them as vastly inferior to jacketed.  2) He may very well be Jeff Coopers' biggest fan, and  3) If he has any sort of opinion about anything, it is likely to be a very strong one that is he quite certain about.

I've been in regular contact with Chuck periodically to keep him up-to-date on the latest developments in bigbore airguns.  Even though bigbore airguns will likely never be Chucks' cup of tea, I think my updates of the power and accuracy of the .45Ranger, Crosmans' new Rogue, along with my latest predator hunting success has really got his attention.  He will for certain be shooting/evaluating my future .45Ranger whenever I actually buy one.

I hope to attend one of the "Great Oregon Varmint Massacres" with the G@S Online crew one day, and demolish ground sqwerls with them when I once again have a powerful .25

[lloyd-ss]

and a fast bullet needs a faster spin

Every twist calculator I have seen says the opposite.... higher velocity can use a slower twist rate.... I know for a fact that when doing "squib" loads for PBs that if you drop the velocity too low the bullets can start to tumble.... I've had it happen.... Bump the velocity back up and they're fine....

One thing I can't figure, though.... is that the twist calculators I have seen with a velocity correction tell you to use 1120 fps for any subsonic load.... ie the calculators don't increase the twist as the velocity decreases below Mach 1.... No explanation given, however....

Bob

Yes, got it on the higher vel needs  faster spin.   Seems like the twist calculators are reasonable approximations to get you in the ball-park, and they all tend to err on the safe (stable) side.

[Butcher45]

What would be your preferred diameter and twist rate if you could start from a clean slate in .45 cal?....

Bob

That would depend on what I was setting it up to do (as in what weight slugs I would be shooting the most out of it).

Large caliber muzzleloader roundball barrels have VERY slow twist rates......I suppose a roundball stabilizes pretty easily, and to much twist would be bad.  A common twist rate for .45-70Gov. (read heavy slugs) is 1:20.  SamYang 909 barrels typically shoot much lighter slugs than a .45-70 does, and have barrels that are either 1:20, or 1:20 1/2 twist I can't recall for sure which is correct.

Some guys are having luck with TOW liners that are a little bit faster twist than the SamYang barrels at around 1:18 I believe.

[rsterne]

Perhaps I should also be asking what people shoot in .45 cal?.... What weight bullets, the diameter, and the velocity?....

Bob

[woodland]

Good read on barrels . Starts with explaining that most people consider the land to be the bore diameter .Marvin
http://www.chuckhawks.com/rifle_barrel.htm

thx Marvin
very interesting site for rainy days....
which we have a lot this moment.

regards

[Rescue35]

All the old information I have relates to rifles with a bore of .45 and grove depth of .458. Twist varies 1:12-18 except for the 45-70 which is 1:22

[lloyd-ss]

Then there is always barrel length to consider which directly impacts velocity.
Shorter barrel = lower velocity = more twist
Longer barrel = higher velocity = less twist

An oversimplification, I know, but still something to consider.

Here is a very nice, short, but analytical article about barrel twist and the science behind it.  Even though it is about pistol barrels, it tied a bunch of loose ends together in my mind.  It gives some of the logic behind choosing a twist rate.

http://www.schuemann.com/LinkClick.aspx?fileticket=N0S0LsAu1RQ%3D&tabid=67&mid=445

Lloyd

[rsterne]

Then there is always barrel length to consider which directly impacts velocity.
Shorter barrel = lower velocity = more twist
Longer barrel = higher velocity = less twist

I realize that is true for supersonic bullets.... However, very few (no?) twist rules seem to address subsonic stability.... at least none I have found.... The most sophisticated one I have found (and one that has been tested against several others) is the Miller Twist Rule....

http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_2.pdf

http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf

This has a velocity correction, but you will note that it states:

Below the velocity of sound, 1120 ft/sec, use the velocity correction fv for 1120 ft/sec.

NEWS FLASH !!!
While writing this post I found a twist rate calculator that actually has SUBSONIC values.... It calculates the minimum (slowest) twist for a stability of 1.5.... You probably already know that <1.0 is unstable, 1.3 is what the "benchrest guys" aim for, and the Military use 1.5-2.0, so 1.5 at sea level and normal temperatures is a practical minimum to use....

This program is basically a slick version of Robert McCoy's "McGyro" DOS program, written in the late 1980's when he worked for the U.S. Army Ballistic Research Laboratory at the Aberdeen Proving Grounds. It was later improved by William Davis Jr. of Tioga Engineering and the claim was an accuracy of 5% for super and subsonic velocities, and 10% for trans-sonic velocities.

http://www.border-barrels.com/barrel_twist.htm

If you put in some values you will note that it produces a graph which has a discontinuity around the speed of sound.... Below that, MORE twist is required, and not just a little bit.... 

I'm gonna go play with this for a while.... 

Bob

[rsterne]

I thought I would run the numbers for a Lee .457 cal 405 gr. flat nose through the "Border Barrels" twist calculator.... Here are the results....







I scaled the dimensions from the drawing of the bullet on the Lee website, so the dimensions may not be perfect.... I used Standard Atmospheric Conditions for the calculator.... I assumed that the bullet was cast from pure lead.... If from Linotype (SG 10.4) it would need a touch more twist (worst case @1100 fps a 26" twist instead of 27").... Of particular interest to us are the muzzle velocities to the left of the discontinuity on the graph.... ie subsonic from 500 fps to 1100 fps.... Note that even with a 405 gr. bullet a 26" twist should be no problem....

Good thing, because I have one on order for my Hayabusa.... 

Bob

[lloyd-ss]

Very neat stuff !  I'll check it out.
Lloyd

[Butcher45]

Great stuff!  I've been trying to find such a twist rate calculator for awhile....it does make sense that additional twist would be beneficial at "low" velocities.  Probably why you see faster twist rates in .45 pistols than you do rifles.

[rsterne]

Interesting how the stability drops drastically in the transonic region.... Here is an example of a bullet that is normal stability when supersonic, but goes unstable as it drops through the speed of sound.... However, at under 1000 fps it is stable again....



Minimum stability is 1.0, and that example drops to about 0.93 at 1100 fps.... I used a .22 cal round nose bullet 0.5" long and an 18" twist to get that graph....

Bob

[Pete]

One thing I have wondered is...
dose the pellet , bullet fully follow the twist
or dose/can it jump it...

[rsterne]

I've never seen a pellet with smeared rifling marks, so as long as you're not talking a very loose fit I'd say no.... I've never read about it, either.... For it to "jump" grooves it would have to completely peel off the lead down to the size of the lands....

Bob

[melloroadman]

If it does you have a problem .As mentioned too lose a fit or to soft of lead for the speed of the bullet known as stripping . Marvin