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Rifling Question

This is a discussion on Rifling Question within the Pistols & Revolvers forums, part of the Pistol & Revolver Forum category; I know different bullets require different amounts of twist to stabilize them. That's about all I know. Why do some guns that fire the same ...


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Old March 20th, 2011, 05:00 PM   #1
 
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Rifling Question

I know different bullets require different amounts of twist to stabilize them. That's about all I know.

Why do some guns that fire the same ammo have different rates of twist and a different number of groves? Seems especially odd when the examples I've selected below are all from Ruger and fire the same .357 ammo. So shouldn't they all have the same number of groves and same twist to stabilize the same bullets?

-GP100 & SP101: 5 groves 1:18.75" RH

-Blackhawk & Vanquero: 8 grooves 1:16" RH



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Old March 21st, 2011, 04:08 PM   #2
 
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I think it has to do with barrel length and bullet velocity to get the bullet to spin at the right rotational speed.

There are so many details that all just have to add up- it's a wonder that there are so many different gun designs.
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Old March 21st, 2011, 06:51 PM   #3
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KJS, As noted above, the basic concept for rifling is to spin the bullet in order to keep it stabilized down range. Bullet spin rate decays at about the same rate as velocity due to air friction. At some point down range, bullet spin will slow down enough where it will begin to yaw and soon after it will begin to tumble. Of course this affects long range accuracy and because there is more air friction on a tumbling bullet, they drop pretty fast.

Handguns are quite different from rifles when it comes to rifling. First, handguns are not designed to shoot at long distances like rifles so they are intended to maintain bullet stability for at least 50 yards. There are exceptions such as hollow base wad cutters where the nose is way heavier than the base and are typically launched at low velocities. This will make a typical 38 Special HBWC start to tumble at about 35 yards. A better balanced handgun bullet such as a lead dual ended wad cutter (DEWC), which is a basic cylinder, will maintain stability for several hundred yards.

There are several issues with bullet stability. The first is diameter ... the larger the diameter, the less spin is required to maintain stability. This is because a larger diameter bullet has a better gyro effect and will remain stable for a longer distance. Next is length, which also correlates with weight. Longer (heavier) bullets require a faster twist rate to stay stable down range. Actually it is not just length but a combination of length and diameter so a long skinny rifle bullet, such as a 6.5x55mm will require a very fast twist rate. A handgun bullet with the same length will have a much larger diameter so it will stabilize with a slower twist rate and because its intended max range is much shorter, the twist rate can be way slower.

Another issue that affects bullet stability and long range accuracy is what's called "front-to-back ratio", meaning the weight of the bullet's nose versus the weight of the base, which affects the balance point and center of gravity. In all bullets except DEWCs and HBWCs, the nose is always lighter than the base, which makes the balance of the bullet base heavy. When a bullet is fired, the natural tendency (think of a dart) is the heavy end wants to go first. By spinning the bullet fast enough, the gyro effect (think of a spinning toy top) will keep the light end in front ... at least for a ways down range. At some point the gyro effect will diminish and just like a toy top, the bullet will start to wobble and finally tumble in its quest to go heavy end first. So ... bullet design has a huge effect on bullet stability. The best bullet for stability is a DEWC because it is a near perfect cylinder with a 1:1 front to back ratio. The worst bullets for stability are hollow point bullets where not only the taper of the nose reduces weight, so does the hollow point. These bullets can have a front to back ratio as high as 1:2, which is pretty grim. You can test bullets by finding their balance point (center of gravity) then measuring from that point to the tip and again to the base. The ratio of these lengths is the bullet's front to back ratio.

Twist rate is measured in inches per 360 deg of bullet rotation. As an example, it takes 16" to rotate the bullet 360 degrees in 1:16 barrel. The twist rate combined with the velocity will determine the bullet's spin rate. Here's where it gets interesting .... a bullet being pushed at 1000 fps in a barrel with a 1:12 twist rate will result in a bullet spin rate of 60,000 rpm (12"=1 foot X 60 seconds X 1000 = 60,000). It is not unusual for rifle bullet spin rates to exceed 250,000 rpm. So .... even with a slow twist rate in a handgun, the bullet is still spinning very fast.

Barrel wear ... more specifically, bore wear is very related to twist rate. The faster the twist rate, the faster the bore will wear. This is very obvious in high velocity target and varmint rifles where barrel life is measured in hundreds of rounds rather than tens of thousands of rounds in a handgun with a very slow twist rate.

About the turn of the last century, S&W experimented with 38 Specials with 158 gr lead bullets and found the odd ball twist rate of 1:18.75 kept the bullet stable 50 yards down range, which was considered plenty good for a DA revolver. Since then, the 1:18.75 has become a standard for many DA revolvers, even if they are chambered for the much faster 357 Magnum. So that particular twist rate is based more on tradition than design. Many people are misinformed about twist rate thinking faster twist rates means better accuracy. Not only is this not true, it often results in premature bore wear, which will degrade accuracy. The only connection between accuracy and twist rate is range distance. With high power rifles, the maximum clean kill range is about 250 yards so twist rates are designed to keep an average bullet stabilized to at least that distance. A faster twist rate will not improve accuracy until you get beyond the maximum usable distance, however for long distance target shooters, their max range may be as far as 1000 yards so they will need a much faster twist rate to maintain bullet stability. If your handgun was designed for a 50 yard max range, the slowest twist that will stabilize the bullet will be the optimum.

The number of lands and grooves makes no difference in accuracy nor does the direction of twist. These are used in crime labs to identify what gun may have been used in a crime but beyond that ... it bears no relevance for function or accuracy. There are different types of rifling that do make a difference in accuracy ... especially with lead bullets. Common "Ballard" rifling is used in most handguns. It has fairly deep grooves ... typically .006" from land to groove or .012" for the complete bullet diameter so they get a nice bite on the bullet and prevent the bullet from skipping across the lands. If lands/grooves are much deeper than .006", they will loose the seal between the bore and bullet and will result in excessive fouling and a loss of accuracy. If land/grooves are less than .006", they won't get a good bite on the bullet, which will result in shaving and a loss of accuracy. Although not a good example for handguns, Marlin uses Micro-Groove rifling in most of their rifles. This type of rifling uses many shallow grooves. This works well with 22 LRs and any rifle that shoots jacketed bullets but works very poorly with larger diameter lead bullets. Some of Marlin's straight case rifles (357 Mag, 44 Mag, 45 Colt, and 45-70) are also available with Ballard rifling that work far superior to Micro-Groove with lead bullets. Glock pistol barrels don't have rifling at all ... rather they have a twisting pentagon shape. This works exceptionally well with jacketed bullets but not very well with lead bullets ... in fact lead bullets are not recommended for Glocks.

So in conclusion, the twist rate for handguns is not critical like it is in rifles. In essence, the bullet only needs to stay stable for the shooting distance it was designed for. The faster the velocity, the faster the bullet will spin and extend the max usable range. This is notable for 44 Mags and 357 Mags where velocities are nearly double those of a 44 Special or 38 Special. There is a twist rate chart in the Library but it is only for rifles. It is interesting to note the difference in twist rates within the same caliber for different weight (length) bullets. See: http://rugerforum.net/library/8959-r...ist-rates.html
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Old March 22nd, 2011, 09:08 AM   #4
 
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Thanks, Iowegan. As always, that's a stunning amount of great information.

I never would have thought of bullet RPM and how it goes way up with velocity.

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Originally Posted by Iowegan View Post
Barrel wear ... more specifically, bore wear is very related to twist rate. The faster the twist rate, the faster the bore will wear. This is very obvious in high velocity target and varmint rifles where barrel life is measured in hundreds of rounds rather than tens of thousands of rounds in a handgun with a very slow twist rate.
Interesting. Before I thought barrel wear was simply an issue of velocity, never even thinking of twist rate. Though I guess high velocity & fast twist rate go hand in hand. Someone who's really into guns told me that .460 Mag barrels can be worn out after only 1,000 rounds, though I can't imagine many folks fire a .460 Mag much and if they do, well, they clearly have huge piles of money to burn anyhow so no problem.

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Glock pistol barrels don't have rifling at all ... rather they have a twisting pentagon shape. This works exceptionally well with jacketed bullets but not very well with lead bullets ... in fact lead bullets are not recommended for Glocks.
Doesn't Glock officially call what they have 'hexagonal rifling', which basically looks like standard rifling that's been rounded off? I know a local range where they make you buy factory ammo for use in the Glocks they rent, while they'll let you use their less costly cast lead reloads in any of their other guns. Until I read of Glock's unique design last year and how it hates lead I had no idea why that was. Apparently, a Glock can get fouled so quickly & severely with lead as to produce pressure levels that destroy the gun.

I've read that hand-loaders who love Glocks can get a conventionally rifled barrel for use with lead bullets that cost half as much as jacketed. I know Glock justifies their design by saying it's easy to clean and lasts longer. I've heard the easy to clean part is accurate, though read Patrick Sweeney question Glock's claim of lasting longer when he has numerous conventional handgun barrels that show no signs of blow by even after more than 100,000 rounds. I'm thinking very few people will ever fire 100K rounds through any gun to test that out, dropping dead long before their gun does.

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So in conclusion, the twist rate for handguns is not critical like it is in rifles. In essence, the bullet only needs to stay stable for the shooting distance it was designed for.
Makes sense. This probably explains why I've never seen twist rate listed in any other handgun catalog the way Ruger does.

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The faster the velocity, the faster the bullet will spin and extend the max usable range. This is notable for 44 Mags and 357 Mags where velocities are nearly double those of a 44 Special or 38 Special.
And before I thought these Magnums had a greater usable range only because with faster velocity they didn't drop as much. Never considered the faster RPM spin being a factor. I'm assuming the fasting something moves the less time gravity will have to act upon it and drop it by the time it reaches any given distance.

I remember on Mythbusters they put theory into practice, showing that a dropped bullet and fired bullet hit the ground the same time. They used a .45 ACP, no doubt picking it as it has the ballistics of a bowling ball such that the experiment can be done in an airplane hanger with it dropping three feet to plow the ground after only 360 feet of flight. Lots of drop, but then nobody is shooting bad guys at such distances with a 1911 so no problem.
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Old March 22nd, 2011, 11:00 AM   #5
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KJS, My bad .... Glocks have "polygon" type rifling (not pentagon like I said above), which simply means "multiple sides". I'm not a Glock expert so I will defer the number of sides in different calibers to someone else. Of course there are exceptions ... my Glock 35 (Tactical/Practical) came from the factory with a Ballard style 5.32" rifled barrel ... go figure!

Yes, gravity pulls a bullet down at the rate of 32 feet per second squared no matter what velocity .... even if it is just dropped. The issue with down range bullet drop is based strictly on "time of flight", which is the time it takes a bullet to reach the target. The faster the velocity, the less time gravity has to pull the bullet down. Of course the ballistic coefficient of the bullet (air friction resistance factor) plays a roll too. The more aerodynamic the bullet, the more velocity it will retain so it gets to the target faster, thus less effect from gravity. Ballistic coefficient is not a consideration with handguns ... again because of the limited shooting distances and because most handgun bullets have terrible ballistic coefficients. With rifles where long range shooting is way more critical, BC is very important. A typical handgun bullet will have a BC between .1 and .2 (wad cutters are only .05) whereas a good rifle bullet will have a BC of .5 to .6 The BC does three things ... the higher it is the more aerodynamic the bullet will be so it will resist air friction better. It also reduces wind drift at about the same rate and of course it reduces time of flight.

At normal handgun shooting distances (out to 50 yards) the rifling twist rate has very little influence on accuracy because the bullet maintains stability. At longer distances, the bullet may become unstable, start to wobble, then tumble. This is very evident when you look at your bullet holes in a paper target. A nice round hole indicates the bullet is still stable in flight. An oval hole tells you the bullet is starting to yaw (wobble) and several more yards down range you will see a "key hole" in the target, which is the sideways shape of the bullet. This tells you the bullet is tumbling.

Assuming the same exact bullet in a 38 Special at 800 fps and a 357 Mag at 1600 fps, the bullet will spin exactly twice as fast in the 357. That means the max stability distance doubles too. With a normal 357 Mag, you can expect the bullet to maintain stability to at least 100 yards ... same for a 44 Mag. With the right bullet (closest to a 1:1 front-to-back ratio), the stability range can extend way past 200 yards. With a poor front-to-back ratio bullet (ie hollow point) the stability range will be considerably less. The front-to-back ratio conflicts with BC because pointed nose bullets are more aerodynamic than flat nose bullets. So ... even though a bullet may maintain stability longer in flight, they aren't as good at longer distances because the BC is so poor it will slow the bullet down and make it drop much more. So ... there are always trade offs and until recently, there wasn't any way to have a good BC and a good F-T-B ratio in the same bullet. Walla ... along comes Hornady with their "LEVERevolution" ammo. It has a pointed plastic tip that provides a much higher BC yet because the tip is so light, it also has an excellent F-T-B ratio. This is a win-win for long distance revolver shooters.

Last edited by Iowegan; March 22nd, 2011 at 11:03 AM.
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Old March 23rd, 2011, 01:48 PM   #6
 
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Quote:
Originally Posted by Iowegan View Post
Walla ... along comes Hornady with their "LEVERevolution" ammo. It has a pointed plastic tip that provides a much higher BC yet because the tip is so light, it also has an excellent F-T-B ratio. This is a win-win for long distance revolver shooters.
That's the ammo I always see ads for with the red tip, right?

I didn't know that had anything to do with making it more aerodynamic. I don't recall the ads ever mentioning that, only saying how that tip is supposed to produce reliable expansion of their hollow point unlike lesser ammo that can fail to expand due to being clogged up by heavy clothing.
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Old March 23rd, 2011, 04:42 PM   #7
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KJS, Hornady first introduced LEVERevolution ammo for 30-30 lever action rifles. Seems during recoil, pointed metal bullets have a tendency to detonate primers in tubular fed magazines. So for over 100 years, all 30-30 (30 WCF) ammo had flat tip bullets. These flat tip bullets were not very aerodynamic and limited the shooting distance. The pointed flexible tip bullets will not detonate primers, are way more aerodynamic, extend the shooting distance by 1.5X, plus terminal ballistics (expansion characteristics) are a vast improvement.

Hornady now makes LEVERevolution ammo for several other bottle neck cartridges used in lever guns plus several straight wall case cartridges, which are also popular in lever action rifles. The 357 Mag and 44 Mag LEVERevolution ammo uses pointed bullets that have much higher Ballistic coefficients than normal bullets. Like in lever guns, this ammo will extend revolver shooting distances by 1.5X, it expands well, and the bullets stay stable in flight for a long ways. See: Hornady Manufacturing Company :: Ammunition :: Rifle :: Choose by Product Line :: LEVERevolution®
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Old March 25th, 2011, 01:25 AM   #8
 
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Seems during recoil, pointed metal bullets have a tendency to detonate primers in tubular fed magazines.
Oh, yeah, I forgot about that issue. I can see why one wouldn't want a pointy metal bullet that's effectively a firing pin rammed up against a primer.
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