View Full Version : The proper twist rate for bullet weight?
June 21, 1999, 04:53 PM
I've read many times that the proper weight bullet for my AR-15 is dependant on the twist rate of the barrel. Like:
1x7 - for 87 gr
1x8 - for 69-80 gr
1x9 - for 52-69 gr
My question is, My .308 has a 1x12 twist in a 23" barrel. Is there an optimum bullet weight for this twist rate or should I stick with the 168gr BTHPs.
June 22, 1999, 03:51 AM
I'm from the old school. I don't know if there is an optimum weight etc for any twist configuration. If there is I probably would't use it. I have always "Married" my ammo, and if I couldn't find it mfg'd I'd have my friend load different kinds to make it work.
Yes! it is tedious and sometimes expensive. But, depending upon what you are going to use the gun for, will depend upon how far you want to carry it. (The testing)
There are several other variables to be considered. If Charley has a gun with 1X10", using a 165gr bullet, Sierra Boat Tail and he is from MN, and you are from GA this may make a difference. Smoothness of the bore can make a difference too.
If you are looking for "Surgical" precision, marrying is, in my opinion, the best road to go. You could of course use the standard for that caliber and bullet, to begin with, but many times I have found trying different bullets, pwdr etc does enhance the accuracy.
The length of the bullet can also be a factor. Sometime back the Army won the Long Range rifle match with a .223, H-BAR, modified and they increased the bullet weight and length.
[This message has been edited by Harley Nolden (edited June 22, 1999).]
June 22, 1999, 06:05 AM
OK, Thank you for the advise.
I'm a target shooter and looking to get the tightest groups from distances of 100 - 600 yards. I know thats quite a spread.
I'm a creature of habit and usually stick to Federal Ammo, but I'm open to other options. I'm not a reloader. I know.... I should be.... So I'm stuck with factory loads.
I'm currently using the:
Gold Medal 168gr Sierra MatchKing BTHP (Federal Load #GM308M)
I'll planning on picking up some:
Gold Medal 175gr Sierra MatchKing BTHP (Federal Load #GM308M2)
Gold Medal 155gr Sierra MatchKing BTHP (Federal Load #GM308M3)
What other manufacturers have a reputation for making accurate and consistant ammo and what loads would make a good place to start.
I also understand that I'm going to have to re-zero the rifle everytime I get different lot numbers of the same ammo, due to variances in manufacturing tolorances and components.
June 22, 1999, 10:00 AM
By keeping "dope" on the loads, range, weather, temp, humidity etc. Re zeroing should be nill. It also takes time and patience.
You will probably notice that the lighter the bullet, will produce better @ short distances, where the heavier at the greater distances. This is only rule of thumb, not the standard cut and dried.
You must dope the ammo too. "Dope" (keep records of each firing.) What grain, lot number, did you use to get a good group on a day when it was 100 degrees, sun left, humidity 85% wind L front quarter @ 8 mph and what sight setting did you have?
Did you pre-foul?
Are you cold shooting?
What power on the scope, for what distance?
Sand Bag or Sling? (Leather, Canvas)
Shooting Glasses? (Perscription?) I don't use my glasses when using a scope.
Standing, kneeling, prone?
Did you zero for each range on that day?
These are just some of the things that must be taken into consideration. There may ever be something special you do that someone esle may not. As long as it is consistant, use it. Add it to your "Dope"
June 22, 1999, 11:26 AM
No particular disagreement with anybody's responses, but it's not an EXACT correlation of twist and bullet length.
For any twist, there is a RANGE of bullet lengths which are best stabilized. It will vary somewhat with caliber, so a "proper" twist in a .22 might not work in .30 or up.
Over-simplified and generalized, faster twists for heavier (longer) bullets; slower twists for lighter bullets.
In .30 caliber, if you get a twist for your primary shooting with, say, 180- to 220-grain bullets, you might not get good groups with 110-grain bullets.
Since the majority of shooters use bullets around 150-165 grains in the various .30s, a 1 in 10 or 1 in 12 twist works fine. You might find you just can't get as tight a group with a 220-grain bullet in your gun...
June 29, 1999, 10:15 PM
Here's the formula for finding the correct twist rate for any particular bullet you may want to use. I found it in an article in Shooting Times (I think) a long time ago. It will give you the rate that will best stabilize any particular bullet.
You can get all of the specific measurements required from a good reloading manual.
(1) Bullet length / Diameter (1000's of an inch)
(2) Divide 150 by result of (1) above
(3) Multiply result of (2) by bullet diam.
When the world is at peace, a gentleman keeps his sword by his side....... Sun-Tzu 400 B.C.
June 30, 1999, 05:03 AM
I'll give it a try.
So bullet weight doesn't factor into it?
[This message has been edited by chucko (edited June 30, 1999).]
June 30, 1999, 05:42 AM
I will have to disagree. The bullet weight, in my opinion, does make a difference.
Example: During military testing of the 30-06 round. Using the marrying system, the 150 grn bullet would only group @ 600 yds, M-i Grand, and the same for the Springfield 1903. The 180 Grn was good to 800 yrds.
The 308 was a similar story. 150 grn good to 600, the 168grn to 1000yds.
Colt H-Bar. The 55gn bullets, opinion, good to 400 yrds. Now I don't remember the bullet weight, however, it was greater than 55grn, won the 600 yrd match at Camp Perry twice. I am positive that the bullet weight was instrumental in that. The 55gr wouldn't make it.
P.S. Have you checked out: http://members.tripod.com/stock4broker/guns/index.html
I was able to get it to print on one page and clear up the slides. Opinion. Printing has to be @ "best" setting to contrast the text.
June 30, 1999, 04:22 PM
Gentlemen. May I put in my humble .02. let me blow a long held axiom that heavy 30 calibre bullets must be fired in a 1-10 inch twist barrel. I have a 30-06, custom barreled with a 1 in 12 inch twist. I had it built to experiment with cast lead bullets. For kicks,no pun intended, I loaded up some 220 gr. bullets. Now all the egg-spurts in the gun mags say they won't stablize. Baloney. .75 to 1 inch groups at 100 yards. Bullet was the Sierra 220 gr. RN.
I enjoy taking these axiomatic pronouncements from the gun rags and proving otherwise.
When I get the time, I am going to work on this further, and see if I can write up an article on the subject.
June 30, 1999, 05:43 PM
Harley: I don't remember which gun magazine, but in an article on long-range shooting, the comment was made that an apparent limit of effective accuracy or group size is that distance at which the bullet drops below the speed of sound.
Now, I had never thought about this in relation to bullets. I remember it well from the early days of Mach 1 airplane design and testing, as it was a problem "coming and going". Tremendous shock waves.
It seems to me that a lighter weight bullet could be upset more than the heavier bullets.
A point to ponder...
Paul B: You're proving what I said: There's no exact correlation. Fer instance, and in line with your own test results, my first "training rifle" in big bore centerfire was an old Enfield Model 1917 '06. K2.5, Bishop stock, Dayton-Traister trigger so it would cock on opening. This was in 1950. I loaded 80-grain .32-20 bullets; 110- and 150-grain Hornady; and 169- and 220-grain gas-check bullets. Everything went into about 1-1/2 to two inches at 100 yards. All in all, for that era it wasn't bad. (That 80-gr. load was definitely rough on rabbits, ahead of 55 gr of 3031.)
Maybe it's cause I didn't even know there was more than one rate of twist?
Find a sponsor (magazine?), and try 1-8, 1-10, 1-12 & 1-14 twists with all the various weights, and write a helluvan article! I don't recall ever reading such a comparison test...
July 1, 1999, 01:54 AM
I know that the deceleration back down through the sonic barrier has quite an effect on a little 22LR bullet. The resulting shock wave causes the bullet to start wobbling.
Though the forces involved during acceleration and decelleration are the same, the effect is not as bad when accelerating up through the sonic barrier because bullet has just left the barrel and still has much of its original energy and rotation to help it to re-stabilize.
With 22LR, it is important to know at what range your bullet goes sub-sonic, because this is the maximum range that you will be able to get tight groups. Some shooters opt to shoot sub-sonic or near sub-sonic ammo in order to get around this problem. With near sub-sonic ammo the bullets decelleration down through the sonic barrier happens soon after it has left the barrel and it is still able to re-stabilize itself due to its retained rotation and energy. If you notice, match grade 22LR ammo is not a hot load. It usually hovers right around the sonic barrier.
I would ASSUME that at least part of this principal is aplicable to larger bullets, though probably to a lesser extent.
Though I can think of no way to measure it, I can see how the rotational rate of a bullet would also be critical in calculating its maximum effective/reliable range. If a bullet of "L" Lenght, "D" Diameter, and "W" Weight, needs to rotate a minimum of "R" RPMs in order to maintain its stability, the distance at which its rotation fell below that level would be its maximum effective/reliable range.
Intresting side note:
To test the formula shown above, I pulled the bullets from a Portugese NATO 7.62x51mm, and a UMC .308win. The Portugese appeared to have a longer bullet than the UMC before I pulled them. I discovered that both we identical in length ( about 1127 thousanths). The UMC is seated much deeper in the brass. 425 thousanths for the UMC and 365 thousanths for the Portugese. I don't have a scale to weigh them right now.
[This message has been edited by chucko (edited July 02, 1999).]
July 2, 1999, 08:20 PM
You Da Man!
The Email you sent me tied it all together perfectly. (Its too much to post here) Bullet Diameter, Bullet Length, and Bullet Weight all actually contribute to the formula.
July 3, 1999, 10:45 AM
As the great Greek philosopher Trajectidees once said "Knowledge is power and power is the velocity squared, times the weight of the trigger pull" (or something to that effect).
When the world is at peace, a gentleman keeps his sword by his side....... Sun-Tzu 400 B.C.
July 3, 1999, 11:00 AM
I think John Wayne said it with a simplicity and humoristic manner.
"The Harder You Pull The Trigger,
The Farther The Bullet Goes."
July 3, 1999, 11:41 PM
While teaching pistol technique, I'll often slip a snap cap in the magazine to demostrate how they are indeed anticipating the recoil or yanking the trigger. Its real easy to notice when they expect the pistol to go off and it doesn't.
I tell them, " The bullets don't go any faster if you push them" :)
July 4, 1999, 06:30 AM
We call that "Ball and Dummie" best technique ever devised.
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