Twist Rates and Reasons
We are in a discussion at the range and I wish to run my version of 'understanding' past 'experts' for comment.
We often hear 'it may require a 1:10 twist to stabilize the heavier bullets' (when speaking of 5.56 centerfire for example).
Some folks call it voodoo because your smoke pole is 1:30 or so and you are shooting 500 grain pills.
I have searched through references at home (where I got my viewpoint), and I am coming up dry. We need a competent '3rd party' voice.
IT IS MY UNDERSTANDING that the physical characteristic of focus is the length of projectile, and the spin needed to prevent tumbling. Or more, the length/caliber or length/front width. An increase of 50gr in a 30 cal makes a longer pill than a 50gr increase in a 50 cal. This length/caliber or length/width ratio is small for smoke poles, so less spin is needed. (a round ball needing the least ?)
Any 'sound' responses would be appreciated.
We often hear 'it may require a 1:10 twist to stabilize the heavier bullets' (when speaking of 5.56 centerfire for example).
Some folks call it voodoo because your smoke pole is 1:30 or so and you are shooting 500 grain pills.
I have searched through references at home (where I got my viewpoint), and I am coming up dry. We need a competent '3rd party' voice.
IT IS MY UNDERSTANDING that the physical characteristic of focus is the length of projectile, and the spin needed to prevent tumbling. Or more, the length/caliber or length/front width. An increase of 50gr in a 30 cal makes a longer pill than a 50gr increase in a 50 cal. This length/caliber or length/width ratio is small for smoke poles, so less spin is needed. (a round ball needing the least ?)
Any 'sound' responses would be appreciated.
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You're pretty well on track from what I've been told. It has to do with the center of gravity relative to the diameter so the round ball example is a good one. Patched round balls do very well in a 1:60-1:48 twist rate. No matter how you look at a round ball the center of gravity is the same distance from the outside. We used to shoot consistent x's with 32 lb. bench rifles and iron sights at 50 yards in 1:60 twist barrels.
As far as the "voodoo" is concerned, velocity is a factor as well. The 1:30 smokepole (inline?) is travelling around 2,200 fps at best. The .223 is capable of another 1300 fps.
The barrel I'll be putting on my .45 rolling block wll be a 1:24 or so for bullet weights exceding 500 gr and lengths over 1". Bear in mind that a cartridge rifle will generate higher pressures and velocities than a sidelock.
Here's a forum for BP enthusiasts, most of whom know more than I do on the subject. http://www.muzzleloadingforum.com/ubbthreads/login.php?Cat=
These guys are all sidelock enthusiasts with the exception of a few BP cartridge shooters.
"No taxation without representation"0 -
Length to diameter ratio, the longer the bullet (relitive to it's diameter) the faster it needs to spin inorder to stabilize. Note that weight does NOT come into play (a 30 cal RN can be stabilized at a slower twist rate than a VLD design of the same weight). A roundball has a ratio of 1:1 and can be stabilized in a 1-72 twist barrel.
A basic formulia is the Greenhill Formulia. I say basic, because having been created for early rifled cannons, it does not account for muzzle velocity changes (MV is one of the factors used to to determine the constant 150), and needs to be tweaked if MV goes over 3000fps (use 180) or below 2000 (use 125 down to 1000fps and 100 below that). The "tweeks" should get you close to the correct twist ratio.
There is a great explaination on Lost River Ballistics website, but you had better like higher math.
Some guys like a mag full of lead, I still prefer one round to the head.0 -
[8D]Yo gotstolefrom,try http://stevespages.com/page8e.htm-he's got all the answers and ratios.[:p][:p][:p][:D]
I'm only wearing Black untill they make something darker0 -
Thanks for the responses....very much appreciated.
The //stevespages.com... has a great deal of information, and is supported by other references. I'm just going to leave one table for two very different bores in the 'bull pen' at the range.
Now we will have to find something else to chew on.
Thanks again.0 -
65gto
If the design is similar than a heavier bullet will also be longer (your example), however if the design is dissimilar (say a wadcutter style vs a VLD style, my example) it's the longer bullet that requires the faster twist rate. If for example you had a 220gr 30cal wadcutter style and a 125gr VLD style that is the same LENGTH than they would both require the same twist rate.
Search out the Greenhill formulia, and you will see that there is no consideration of bullet weight given when selecting twist rates.
The # of grooves (or the shape of the grooves) has nothing to do with the twist rate, bore sealing and bullet deformations yes, but not the spin rate as measured in RPM comming out of the muzzle.
Some guys like a mag full of lead, I still prefer one round to the head.0 -
weight is the RESULT of an increase in length, given a diameter that remains constant. Therefore is is the length and not the weight that dictates twist rate needed. The "tumble" or "keyhole" is a rare issue and can come from a few variables (one being a defect in the rifling) as well as not enough spin (properly known as RPM's). The symptom most often presenting from a pill that was not stabilized correctly will be "yaw". The bullets will actually wobble in flight. Many VLD style pills will shoot a group at 100yds that is say 1/2moa (poor for competition) BUT, as they get some ground underneath them, they will stabilize in flight and actually can often shoot that same 1/2" (not 1/2moa which would be 1.5") at 300yds after they "go to sleep". These are long range bullets made for long range competition, so 1/2moa at 100yds is overlooked since the target boards will most often be at 500-1000yds.
Point and case, lets look at VLD pills for both the 6.5mm and the 7mm bore dia. The 6.5mm vld pills are only 140-147gr offerings, but the VLD pills for the 7mm are 175-180gr offerings. The increase in bore dia of only .5mm allows for the increase in weight to be very large compared to the 6.5mm pill, but both will be close in length. One would say that you could push the 6.5mm pills up to say 155gr (the sierra match king specialty bullet) to keep it in the same ratio of increase as the 7mm pill. That was tried, and most folks find that even with an 1:8 twist barrel, they still can't get the 155gr 6.5mm SMK to work well (maybe a 1:7 or 1:6 barrel could fix that). That is the result of the Bearing surface having to be sooo long to make that pill weigh 155gr, that is just doesn't stabilize without a HUGE rpm number. That is proof that length of the bearing surface is the key to stabilization and weigth is simply the result of increasing the length. The longer the bearing surface is, the more twist needed to stop the "yaw", and THERE WILL BE a point at which the length is too long and in order to stabilize it, the twist will have to be so great, that the jacket will be seperated from the core of the bullet as the cintrifugal (sp) forces become too great at that RPM needed for that length bearing surface, and the bullet will be litteraly ripped apart from inside out. This can be helped some by dropping back to a 3 or 4 groove barrel instead of a 5 or 6 groove barrel,..but still, there is the point at which any diameter pill is too long for the bore diameter.
That's my speech on twist rate as related to weight and/or length. I don't bother with patch and ball smokepoles so I have no input on them. I switched to the Savage Smokeless muzzle loader, and it don't smoke at all[:D] unless you call a .980" 3 shot group at 100yds with a 250gr pill... "smokin"[;)][:D]
why chase the game when the bullet can get em from here?....
Got Balistics?0 -
Yes, as far a a 223 goes..
IF you are shooting a hyper velocity 45 grain bullet out of a 223, you need a 1/12 twist. Because you will strip the jacket off of the bullet with too much twist.
1/12 is for 45-55grain super fast varmint loads
1/9 is for 55-69grain projectile MOST POPULAR
1/7 is for 62grain projectile
1/8 is for the heavy slower 69-75grain projectiles0
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