Barrel twist rate:
Hi can someone comment on the relation
between twist rate and bullet weight for
the .308? I understand that in the .223
faster rate is recommended for the heavier,
longer bullets where 62-80 gr can use 1-8
or faster rate. Is there a similar relation
for the .308? What’s a good weight for 1:12?
Thank you, everyone, and have a nice new year.
Hello Mr. Nosack!
S. Fla USA – Friday, January 01, 1999 at 18:06:08 (EST)
Lou: Gunsmiths generally use the Greenhill Formula to determine the optimal twist for a given bullet. The formula is T=150(d/r) for velocities from about 1500 to 2800 fps. Substitute 180 for the 150 value for velocities exceeding 2800 fps. “T” is the twist rate. “d” is the bullet diameter. “r” is the bullet length to diameter ratio (length of bullet divided by diameter). In .308, it works out fairly well. Sierra 168 grain Matchking is 0.308 inches in diameter. Bullet length is about 1.210 inches so we have a length to diameter ratio of 3.929. Plugging this value into the formula and using the 150 constant (the proven best velocity for the 168 grain MK is 2550 – 2600 fps), we get T=150 x (.308/3.929); T=150 x 0.078; T=11.76 inches or 11 3/4 inch twist …. One rotation of the bullet for every 11.76 inches of barrel traveled. This is very, very close to the “standard” 12 twist barrels on our PSS’s and on my Chandler and one of the reasons they shoot the 168s so well. The 175 Matchkings do not perform as well in the 12 twist barrels. Again diameter is 0.308 inches. Bullet length is 1.257 inches, resulting in an ld ratio of 4.081. … plugging into formula (optimal velocity is 2500 to 2550 fps so we will use the 150 constant again) T = 150 x (.308/4.081); T = 150 x 0.0755; T = 11.320. That approximate half inch twist difference makes a big difference in my guns. 12 twist barrels tolerate 175s but do not excel with them. My Hart 26″ 10 twist barreled .308 tactical rig, built by John Eckenrode on a Remington 700 SA in a McMillan A2 stock with Ross guard and Leupold 3.5 x 10 M3 LR, eats 168s, 175s, 180s and 190s alive. I’ll never go back to slower barrels. The general rule of thumb is to use the fastest twist you can tolerate and match to the longest bullet (usually heaviest) you intend to use for the life of the barrel. I don’t remember the exact source of the Greenhill formula. He was a Brit and developed it back in the late 1920s. I obtained the formula from a friend while working at Navy Intel.
Clearwater, FL USA – Friday, January 01, 1999 at 23:40:30 (EST)
Lou… While there is a fair amount of “Art” and “Disagreement” in this science of ballistics, I think your “Greenhill” formula is probably very close to being “Right on”.
I am shooting a bunch of .308’s with 12″ twists, three are custom “Select Match” grade, and three are very good factory bbls. My normal procedure with a new gun, is to make a slew of loads starting fairly low, and work up. With 155 Palmas, and 168 Matchkings, it was typical that the groups would all be fairly good, and a few would really shine. I starting shooting the 175 Matchkings this fall, to match the BDC of my Leupold M3 LR, and found an interesting change… at the starting loads the groups were not good for the guns in question… groups were large-ish. But, as the loads got faster, the groups got smaller. To me, this indicated that the twist rate was marginal. At the velocities that I wanted, the groups were within reason for the guns, but I feel that if I drove them faster, they might go tighter.
I’m having a gun barreled by Shilen, and asked them for a 10″ twist, and Gary Huntsman of Shilen Said “… Unless you are shooting bullets heavier than 168 grains, I’d use the 12 twist…” which supports the Greenhill formula’s results. Also, the M24 bbls have a 11.25″ twist, and were designed for the 175gr M118-LR load which uses the 175 Sierra Matchking.
How-some-ever, to throw some confusion into the pot for discussion…
Berger Bullets, says, on their web sight…
- .308 CAL 155gr Very Low Drag Recommended twist rate is 1/14
- .308 CAL 168gr Very Low Drag Recommended twist rate is 1/13
- .308 CAL 175gr Very Low Drag Recommended twist rate is 1/13
- .308 CAL 185gr Very Low Drag Recommended twist rate is 1/12
- .308 CAL 190gr Very Low Drag Recommended twist rate is 1/12
- .308 CAL 210gr Very Low Drag Recommended twist rate is 1/11
… and the Berger VLD’s are longer, for a given weight, than the Sierra’s???
Would be interested in results (not theories) of shooters that have shot the heavier Sierras and/or Bergers in 12″ twist barrels at long range.
It’s 2 damn degrees here!!!, in Connecticut! USA – Saturday, January 02, 1999 at 12:04:51 (EST)
I have no qualms with the Greenhill formula, and it has serve us well for century or so. I suggest you visit this site and enter the data that you have at hand and it will calculate the stability factor of the bullet. This is from the McGyro program developed by Bill Davis and Robert McCoy. This program takes into consideration:
O.A.L. of projectile
A stability factor of between 1.25 and 1.75 or so will serve you well.
Although Bill from Clearwater uses the fastest twist consistent with several parameters, conventional thought it to use the slowest that will get the job done. This way bullet balance problems are not accentuated, but minimized. Now one can say, “I always use Bergers, etc. and my bullets ARE in balance.” OK,…I use them too. But when bullets feed into the lead they are usually not concentric and on-axis with the bore and therefore take on a shape that is conducive to perfect balance.
I am just picking nits here, but thought you would like to hear the other side of the coin. Tactical people and target shooters SHOULD be picking nits. More accuracy never hurts and may do some good. No sense in giving it away for no benefit.
In Champions Choice catalog they mentioned stocking 1:11 twist Kreiger’s for people who can not make up their mind. Sounds like a reasonable solution to me.
As an aside, back in the ’60s and early ’70s when 168s were used in benchrest competition, 1:14″ twist ruled.
My Rem. 40-XC has a 1:12 ¼” twist barrel and uses 190 gr. Sierra’s just fine.
USA – Saturday, January 02, 1999 at 15:07:51 (EST)
Lou, barrel twist rate.
Everyone else has thrown in their 2cents worth so I will too.
When I had my first M1A built I went thru the same dilema and settled on a 1\12 twist because I planned to shoot mostly 168’s. It shot these bullets very well. The rifle also shot the lake city match 172 gr. ammo well also, at 1000 yards. These bullets actually weigh about 174.5 grains by the way. Then the Gov. started issuing something called Special Ball about 1984. This stuff was special all right. This ammo would not hold a 6 foot target at 600 yards in my rifle. And I am not the only one who noticed this. As far as I could tell the bullets were the same as the old Lake City Match but they sure didnt group as well. I built another rifle with a 1/10 twist just to be able to compete again, but as soon as I finished the rifle, the Gov. stopped issuing Special ball and started issuing M852 again for the National matches. What I’m trying to say here is if you shoot enough target matches where you have to use the ammo provided, you will eventually question the twist rate of your barrel if it is on the slow side.
Also, there is alot more to setting up a rifle to shoot so called VLD bullets than selecting the right twist rate. The longer the bullet, the more critical it is to get the bullet started right in the throat of the barrel, This calls for tight neck chambers, concentric case necks, special throating, seating depth adjustments, and so on. All of this extra trouble does not go hand in hand with a tactical rig.
Just my 2 cents worth.
S.C.D.H., Ohio USA – Saturday, January 02, 1999 at 16:40:16 (EST)
I hope I am right, didn’t somebody not long ago post the formula for bullet stabalization vs twist rate? If so could it be posted again. I lost my copy and it doesn’t seem to be in Hot Tips and Cold Shots. It probably should be, hint ,hint.
Central, Ny, USA – Thursday, January 21, 1999 at 05:21:39 (ZULU)
The formula for bullet stabilization vs. twist rate is called the Greenhill Formula. Someone else with their copy of Hatcher’s Notebook handy will have to give the details.
New Orleans, LA, USA – Thursday, January 21, 1999 at 15:18:07 (ZULU)
Somebody ( Bill R.? ) asked for the Greenhill Formula. Look in the “Compendium of Terminology” under “G” and you will find it. I know the terminology is common knowledge for most of you, but look at it from time to time – I did not do all the work in compiling that for nothing 🙂