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  #16  
Old 10-22-2019, 02:21 PM
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Quote:
Originally Posted by Ol` Joe View Post
AR’s have a bore diameter of 0.219” Grove/0.224grove while 22lr have a bore of 0.222” grove/0.217” bore diameter IIRC. This is why people see the poor accuracy they do from AR conversion units.
Twist won’t make a lot of difference if the bullet is skipping down the barrel.

My calipers only go to 0.0005, but indicate most of my box of AutoMatch is 0.2235-0.2240". A thou or two undersized bore is normal for lead/low pressure ammo. Yeah, there's manf. tolerance and stacking, but let's ignore that for now.

With a normal rimfire chamber, would the higher initial pressure obturate the bullet more than in an adapter with 1.5" of freebore? Or would the extra velocity obturate the bullet as much when it slams into the rifling?
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  #17  
Old 10-22-2019, 03:18 PM
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Some of the benchrest folks are using 1:17 twist and even 1:17.5 but there have been some issues with the 1:17.5 in cold weather stabilizing the bullets. But they are all shooting sub sonic ammo. My custom gun has a 1:16 inch twist Shillen and I couldn't be happier with the results. I have a 221 Fireball that has a Lilja 1:14 twist because it to be used for Prairie Dogs only with 40 or 50 grain bullets. With 50 grain I'm getting 1/4 to 1/2 inch groups at 100 yards, typically in the 3/8" range.
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  #18  
Old 10-22-2019, 03:28 PM
wkd
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Quote:
Originally Posted by H-R-B View Post
Oh. I wasn't aware - https://youtu.be/5AEPwOa623I?t=1154



To others, what I'm looking for is a test of a normal precision rimfire rig, but with a fast twist of 1:7-1:9 to isolate the twist as the difference. Having another one in 1:16, but with an elongated freebore or .219/.224" bore would be great, but twist is the point most folk tend to hang onto.

This isn't looking for a performance advantage, but clarifying what causes imprecision in converted ARs.

My doubt stems from a little experience with cast boolits in a well worn .30-06. A 165gr cast boolit at 1600fps would ideally prefer something like 1:15, but they were quite decent through the 1:11. Besides cast vs swaged and gaschecked vs heeled, I don't see much difference in comparison to rimfire.
I'm aware as are many others who shoot bolt guns with a fast twist, and you did pose the question as to bolt guns.
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  #19  
Old 10-22-2019, 11:19 PM
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Quote:
Originally Posted by wkd View Post
I'm aware as are many others who shoot bolt guns with a fast twist, and you did pose the question as to bolt guns.

A .22LR though.
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  #20  
Old 10-24-2019, 08:15 PM
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Propose a test; buy these two barrels:
https://www.gmriflebarrel.com/901522...ull-blued-bbl/

https://www.gmriflebarrel.com/901600...-bull-bbl-1-9/

Shoot the same lots of sub-sonic velocity ammo, std. velocity ammo, & high velocity ammo thru both mounted on the same receiver. Say 10 groups of 10 rounds for each barrel and ammo combination.
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  #21  
Old 10-24-2019, 08:50 PM
missing1944
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I shoot competitively 22lr @ both 100 and 200 yds. I shoot 1:15 and 1:14 twist barrels. It is my understanding that almost all biathlon rifles are 1:14 twist as are a lot of pistols. I have shot a .199c/c 5 shot group at 100yds and a .729c/c 5 shot group at 200yds with a 1:15 three groove Benchmark barrel which rides the wind very well. I think that you will find that the 1:17 plus twist barrels that are getting good results are being shot in hotter climates. There are a lot variables to take into consideration that the average shooter doesn not even consider or need to consider. If you are not getting into the competition game 1:16 will cover your needs very nicely. I have a 1:10 barrel I have yet to try but I think that it will be very suseptable to wind. I shoot Eley Tenex.
Respectfully to all.

Last edited by missing1944; 10-27-2019 at 10:24 PM.
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  #22  
Old 10-26-2019, 06:29 PM
Unclenick
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This topic always causes some confusion and misunderstanding. It has already been mentioned that bullet length and shape are stability factors. Weight is also a factor, but a less influential one than length is. This is because increased length provides a greater change in profile when a bullet yaws in flight. The overturning drag that tries to make it tumble is proportional to the square of the sine of the angle of yaw presented to the headwind, so overturning drag increases exponentially with yaw angle. Also, greater length with a tapered nose profile moves the center of pressure of the bullet further in front of the center of gravity, which is like providing a longer lever arm and a bigger sail which drag can act on to pry the bullet around the center of gravity. But if you increase weight without changing the bullet length or profile, the bullet actually becomes more stable for a given velocity and spin because its angular momentum (spin momentum) is increased. Thus, if you cast your bullets from something more dense than lead, like gold or tungsten, you would have the same profile and length with more mass and the gyroscopic stiffness of the spinning bullet for the same muzzle velocity and rifling pitch would be greater than for lead, making it more stable. Of course, it would need a bigger charge to reach the same velocity as the lead bullet, but I digress.

Cold air makes bullets generally less stable because it is denser than warm air, so drag at a given velocity is increased, including overturning drag that results from the initial yaw that starts in front of the muzzle. Initial yaw is increased by muzzle blast, so you can lessen it by venting your bore pressure through perpendicular holes in the bore just before the bullet exits. But with lead bullets, doing that can introduce gas cutting that unbalances bullets. So, adding muzzle brake holes will actually help or hinder lead bullets depending on the gas pressure at the vent sites. It's more likely to help with a long barrel, for which muzzle pressure is lower and can’t gas cut the lead as easily.

The other factor cold air introduces for bullets traveling near the speed of sound is it lowers that speed of sound. Bullets undergo a jump in drag coefficient near the “sound barrier”, so it is not uncommon for a twist rate that is adequate both above and below the transonic velocity range (about 950 fps to 1400 fps) to allow instability somewhere inside that velocity range. So you could have a subsonic bullet that is higher up in the transonic range (at a higher Mach number) in cold air’s speed of sound than it is in warm air’s speed of sound, thus decreasing its stability.

A qualitative value for stability is called the gyroscopic stability factor. It is variously symbolized with an italic s, the letters gs, the letters G.S., the letters S.F., or sometimes just “Stability”. It is a number that increases with rate of spin (with angular momentum or spin momentum) at any given Mach number of bullet speed and it evaluates spin rate and projectile length and weight and shape and the drag on the bullet that tries to make it tumble. It is chosen so that for any given projectile travelling at any given Mach number, a value of 1.0 is on the line that separates stable from unstable, meaning a bullet with a S.F. below 1.0 has the coning motion of its tip describe a spiral that gets bigger as the bullet goes downrange until it is so big the overturning drag exceeds gyroscopic stiffness and the bullet starts to tumble. A value greater than 1.0 sees the nose describe circles spiraling inward toward a stable flight equilibrium position (aka, the yaw of repose). Right at 1.0, the circles described by the nose neither grow nor shrink, which you might think of as stable, but a bullet doing that has more total drag and is pulled further out around the trajectory during each cycle, so group sizes are not good. Numbers greater than 1.0 but less than 1.3 are stable, but still don’t usually produce really small groups. At 1.3 you achieve what a Sierra technician I spoke with called “hunting accuracy”. At 1.4 to as much as 1.7 is where bugholes can be made. 3.0 is where the Sierra tech put the top end of “hunting accuracy”.

There is some argument as to whether the single best stability factor value for a supersonic high power rifle bullet is 1.4 or 1.5, but it’s in that vicinity. Above 1.7 the bullet is not less stable, but if the spin is faster than necessary that will exaggerate wobble and lateral drift caused by any tiny error in the colocation of the bullet center of mass (CM) with the bore axis while traversing the barrel. An off-center CM orbits the bore line eccentrically as the bullet goes down the tube, and that’s like swinging the bullet around on a tiny short string and clearing the muzzle is like letting go of the string, so the bullet not only travels forward, it also drifts off from the ideal trajectory in a direction tangent to the side of the bore the CM was closest to when the muzzle let it go, which is randomly around the clock for any given bullet unless you knew where its CM was when you chambered it. If the bullet isn’t perfect, doubling the spin rate will double that drift, opening the group up twice as much, even though the bullet is not unstable. It becomes a trajectory error, not a stability error. This is what “overstabilization” actually means; not instability but unnecessary extra RPMs that exaggerate the influence of bullet construction or bore alignment imperfections on the size of the group. Of course, extreme over-spinning can cause bullets to fly apart, but I am not referring to spinning that fast.

If you look at most pistol barrels, you find they provide more spin than their short bullets require at most velocities. This is because so many pistol bullets are flying part of the time in the transonic velocity range where they need extra spin to remain stable passing through it. Also, for a given twist, the lower muzzle velocities of pistols, as compared to rifles, produce lower bullet RPM so you don’t get into bullets disintegrating or other high RPM problems like faster lateral drift. High power rifles fired well above the speed of sound already have a lot of RPM, and typically remain stable falling through transonic velocities because their RPMs decrease much more gradually than forward velocity falls off. As a result, their gyroscopic stability factor actually increases as they go downrange, where they are carrying more spin than is needed for the reduced velocity’s lower drag until they get to that transonic tough spot.

On another board a fellow with a .223 Rem rifle was having trouble creating a subsonic load with 55-grain Hornady .224 FMJ’s using a 10” twist. That bullet is perfectly happy with a 12”-14” twist at normal .223 velocities, but the load being used was launching it at just under 1100 fps, still inside the transonic range. The plot below from Geoffrey Kolbe’s online barrel twist calculator (used here with his permission) shows the steep drop in stability factor in the transonic range. I colored in the stability factor ranges and added comments about them to show why that fellow’s subsonic loads were tumbling.
Attached Images
File Type: gif Hornady FMJ 10 inch Twist B 2019-10-21_11-05-28.gif (51.7 KB, 104 views)

Last edited by Unclenick; 10-27-2019 at 04:04 PM.
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  #23  
Old 10-26-2019, 09:50 PM
Steve Newman

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+1 Unclenick….great, informative, post and explanation of the several different codependent factors (not just barrel twist rate) that affect bullet instability/stability. Thank you!
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  #24  
Old 10-27-2019, 04:23 PM
Unclenick
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You are welcome. I probably made it as clear as mud for some, but if all it does is get folks to realize there are a number of interacting factors, they may decide to research twist rates more carefully before choosing one. Even if that research is as simple as looking at reports from others about what twist rates did or didn't do what they wanted and under what conditions, it might save folks some experiment money.

Incidentally, I meant to link to Geoffrey Kolbe's site. That page has several links. The articles section has one about rimfire barrel tuners that is pretty thorough and worth reading. The Online Ballistics selection takes you to calculators that include a drag coefficient calculator and a barrel twist calculator. The graph I put up is from the latter for which I actually had two plots: One showing what twist to choose for a stability factor of 1.5 (the most common choice for optimum; 1.4 came from Harold Vaughn whose Precision Shooting Publications book, Rifle Accuracy Facts included it; Vaughn had been head aeroballistician for Sandia National Laboratories and was (is; I don't know if he's still with us) very knowledgable on the subject). The second graph is because I plugged in the 10" twist for the fellow having the transonic stability problem, and it produced the stability factor for the atmospheric conditions I plugged in.

Last edited by Unclenick; 10-27-2019 at 04:25 PM.
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  #25  
Old 10-28-2019, 01:53 AM
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Good technical info. Got any practical info as it pertains to LRN rimfire?

Last edited by H-R-B; 10-28-2019 at 01:56 AM.
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  #26  
Old 11-05-2019, 11:46 AM
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H-R-B

I have a fair amount of it. Is there something, in particular, you are looking for?

At one point in time, the late Robert L. McCoy investigated the LRN RF drag function in some detail at the U.S. Army Ballistics Research Laboratory (he did detailed drag on all bullets the A.M.U. shot in competition). Here are some plots I made of the drag coefficient and the predicted drag on the bullet in pounds of force at different velocities in an ICAO standard atmosphere.

The first plot is the drag Coefficient which is the bullet shape factor. It changes with velocity due to the effects of the sound barrier and shockwave formation. This number is multiplied Newton's drag formula to get actual pounds of drag. Newton's formula is to multiply air density in slugs per cubic foot times the cross-sectional area the shape presents to the wind (the size of the sail) in square feet times half the square of velocity in feet per second. This gives you the drag you would have if the drag coefficient were an unchanging value of 1.0 at all velocities. You then multiply that result by the actual drag coefficient for each velocity. You can see here that in a standard atmosphere, the drag coefficient for the LRN is a very constant 0.23 until about 800 fps, above which the first hint of the effect of approaching the speed of sound is felt and the drag coefficient just barely starts to increase. At about 970 fps, the change really becomes apparent. Because the speed of sound determines where this happens, the actual drag function tables use Mach numbers instead of velocities. These velocities are only good in ICAO standard atmospheric conditions.

You can get that information in several places if it interests you. Here is one, but the Wikipedia article on Exterior Ballistics had a fairly extensive list of freeware that includes all you need to run ballistic tables base on this. Just remember that with the RA4c drag model, the imaginary reference projectile is 1 inch in diameter and 1 lb (7000 grains) in weight. In that system, a 40-grain projectile will have a RA4c drag model BC of 0.115946, so you want to use that number for trajectory tables based on the RA4c drag function.
Attached Images
File Type: gif LRN RF Drag.gif (34.6 KB, 2 views)

Last edited by Unclenick; 11-05-2019 at 08:15 PM.
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  #27  
Old 11-09-2019, 05:22 PM
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As a bit of added information, I tried for RN drag on the Kolbe site calculator, but getting a match to the BRL measurements for a non-pointed bullet took some fiddling. In the end, it looks like the standard 16" twist keeps bullets stable in a standard atmosphere (we knew that), but the stability factor does drop near the speed of sound. If you want to stay at or above 1.5, you would need about a 14" twist for a bullet MV right at the speed of sound (about 1117 fps in the ICAO standard atmosphere), but that means your spin is greater than necessary for the rest of the flight. If you want to experiment, a 15" twist or a 15.5" may benefit you slightly, particularly in a gusting or fishtailing cross-wind. But overall, it looks like the 16" standard became standard for a reason.
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  #28  
Old 11-09-2019, 06:01 PM
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Brother Unclenick,. I think cold air raises the speed of sound, hense biathlon ammo is faster than summer ammo. BuT I'm probably wrong as cold guns and ammo are slower so faster ammo is needed. Yes dense air slower.

Last edited by thesandman; 11-09-2019 at 07:45 PM.
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  #29  
Old 11-14-2019, 01:51 PM
Unclenick
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The Biathlon ammo I find is mostly slower. Lapua is 1066 fps (337 m/s). Fiocchi Super Match Rifle Exacta Winter (Biathlon) claims to be 1070 fps in their 2019 catalog (I've seen a seller, Ammoland, claim 1120 fps, but the catalog has no rounds at that velocity so it seems to be a cut-and-paste error). SK is 1106 fps. Only the Eley ammo is closer to standard velocity at 1060 fps, but it has special shortened-nose bullets to increase stability. So I think they are all worried about instability in the transonic region, which is worse in the cold weather, and are therefore starting them out on the slow end.

Presumably, those velocity numbers are tested under standard conditions and they will be a bit slower in actual winter cold.

Last edited by Unclenick; 11-16-2019 at 05:47 PM.
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  #30  
Old 11-14-2019, 04:11 PM
David Valdina
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Some thoughts on this topic...

I will share a couple of thoughts on this topic. First, the long thread here on RFC about accuracy shooting 50 rounds at 200 yards has posts which brings into question the claim that bullets going from supersonic to subsonic suffer accuracy as a result of doing so. The other thing I will contribute to this thread is from my own experimenting with the 60 gr. Aguila SSS cartridges. In every handgun and every rifle I owned, except one, the bullet holes in the paper were out of round, indicating the bullet was not flying fore and aft straight line when it went into the target. This testing was done at close range so I am unable to comment on accuracy. I did make the assumption, for my own decision I needed to make, that this performance meant degraded accuracy. I had a gunsmith get a 1:8 liner and had it installed in a revolver and a rifle. The revolver has since been sold. I still own the rifle. It is in Massachusetts and I won't be back there until next summer. My memory is that rifle shoots very well with the 60 gr. bullets. I did not do a side by side accuracy test with 40 gr. bullets. Why did I do this ? Because I could and in the very unlikely event I would need to be in the woods for a while traveling light, this rifle and ammo combo would be perfect (my opinion) as a minimalist gun for gathering food and self protection. The bullet does penetrate very well. The rifle is a Remington 521T.
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