.223 twist needed for 75gr projectiles

[Willoughby]

what twist is needed for 75-80 gr projectiles in a .223 ?
will a 9 twist stabilize 75-80 gr bullets ?
or will I need to find a 7 or 8 twist ?
advise from any of you with .223 reloading exsperience greatly appreciated

[BoilerUP]

Here's a copy/paste of what I put on Sniper's Hide:

"1:9 stabilizes 75gr HPBT/77gr SMK just fine; my 26" 1:9 Savage barrel puts a 75gr HPBT out at 2860fps and has a stability factor of 1.41.

The same barrel stabilizes the 75gr A-Max @ 2950fps good enough for 0.7MOA groups at 200yd, though at 500' ASL/80°F (the environmentals for all calculations in this post) its stability factor is a razor-thin 1.01.

The 75gr Berger VLD @ 2950fps has a stability factor of 1.15.

I just had a 22" 223AI barrel arrive last week, and I ordered it with a 1:7 twist so I could have the option of 90grs if I wanted to try them, and to provide better stabilization in cold temperatures. With a 1:7 twist, the 80gr A-Max @ 2900fps has a stability factor of 1.56, 75gr A-Max @ 3000 is 1.69, and 75gr HPBT @ 2900 is 2.35."

You can do a search here and see my results with the 75gr HPBT and 75gr A-Max in a Savage 1:9 223 barrel.

[jhelmuth]

I can't agree with BoilerUP's data - but only because I do not get the same results on the 77gr SMKs. The 77gr SMKs are NOT stabil (in my 1:9 factory Savage LRPV - Bull). The 75gr VMax will work and seem stabil enough. I'd say the 77gr SMKs are border-line and will depend on the barrel. 1:9 is approx. and your barrel may vary slightly on twist rate depending (I'd measuer it to find out - maybe if it were closer to 1:8.5?).... anywho - just FYI.

PS - I don't think you'll find the 69gr SMKs lacking for most anything you want to do in the stock Savage .223 barrel. I've found that the SMKs - paired up with Varget are a very accurate combination when you find the right node for your barrel.

[kschilling]

When I started 500 yard Mid Range F Class, I used a factory 1-9 Twist .223 barrel. They held the 75 Horn BTHP at respectful groups. I switched to 70 grain Bergers and WOW!!!!!

[BoilerUP]

I can't agree with BoilerUP's data - but only because I do not get the same results on the 77gr SMKs. The 77gr SMKs are NOT stabil (in my 1:9 factory Savage LRPV - Bull). The 75gr VMax will work and seem stabil enough.

Certainly not doubting your results, but I'm very curious how the 75gr A-Max would be stable but not the 77gr SMK. In my 500' ASL/80°F atmosphere, where the 75gr HPBT @ 2860fps has a stability factor of 1.41, the 77gr SMK at the same velocity has a marginally higher stability factor of 1.42 while the 75gr A-Max @ 2950fps is right on the ragged edge (at least, in theory) at 1.01.

Guess it proves that "only way to find out is try".

[skypilot]

Sierra 69 gr SMK and Hornady 75 gr HPBT are excellent in my FLCP-K 1/9 .223, but the 75 Amax is a little squirrely @ 400+ as far as stability is concerned. I'm a try it and see reloader with regards to bullet wt/types.

[darkker]

77gr SMK's are stable in 2 FP's and a VTR, all with 9-twists.
Buddy shoots his to 1K with groups that would make the Pope jellous, Launch @ 2745fps.

[M.O.A.]

Mine shoots 68 gn bthp great haven't tried anything bigger yet but soon yo try the Berger 70gners.

[lal357]

When I the factory 26" 9t on my lrpv 75 gr amax and bthp were mo problem tried some 80 gr amax and it was a joke

[PBinWA]

I shot some 75 AMAX through my 1-9 FCP-K at 600 yards last week. Other than getting pushed by the wind horizontally they were surprisingly accurate. MV = 2870.

[Apache]

If you already have a 9 twist, try it. If not, get an 8 twist and be done with it...... or a 7 if you want to play with the 90's.

[acemisser]

77gr SMK's are stable in 2 FP's and a VTR, all with 9-twists.
Buddy shoots his to 1K with groups that would make the Pope jellous, Launch @ 2745fps.

Now what makes you think the Pope will be jealous...You need to explain this in better detail..
I doubt the Pope even has a gun,but then you never know...

[devildogandboy]

what twist is needed for 75-80 gr projectiles in a .223 ?
will a 9 twist stabilize 75-80 gr bullets ?
or will I need to find a 7 or 8 twist ?
advise from any of you with .223 reloading exsperience greatly appreciated

hope this will be useful to you.
http://www.savage-barrels.com/calibers.htm

[darkker]

Now what makes you think the Pope will be jealous...You need to explain this in better detail..
I doubt the Pope even has a gun,but then you never know...

.... If I have to explain why THE POPE would be jellous of being able to shoot groups that tight, it's isn't worth explaining....

[MZ5]

Certainly not doubting your results, but I'm very curious how the 75gr A-Max would be stable but not the 77gr SMK.

Just a guess, but I think it's because he cited the V-Max rather than the AMax.

I agree with your first post, that the 77 SMK will be adequately stabilized by the 9-twist Savages. My 26" FP-UV runs then well. The 75 AMAXes seem to work at least for a couple hundred yards. At the 300 yard mark, when I lived and shot at ~4,800 feet, wouldn't hold as tight as the SMKs. I can't be positive whether that was a stability issue showing up, or just a better load for the SMK, because I hadn't worked with the bullets that much by then. Either way, even here at ~1,200 feet and HIGH temps, the 75 Amax is doing just 'okay' for me at 500 yards. The SMK is a clearly-better choice.

[BoilerUP]

Just a guess, but I think it's because he cited the V-Max rather than the AMax.

Considering there's no such thing as a .224 75gr V-max, I think surmising he was talking about an A-Max to be a fair assumption.

[jhelmuth]

Twist rates and bullet compatibility are actually based on the bullets bering surface and NOT it's grain weight. Clearly ther is SOME relationship there, but as each bullet is not made with the same profile(s) there is an expected difference in resluts from one bullet to the next. One cannot comapre the 75gr AMax (or VMax) to the 77gr Sierra BTHP MatchKing... they are just not the same design from the start even though their grain weight is very similar. Stability is all about angular momentum and as long as one can achieve the needed rate for a given bullet design, great. For the most part, the Savage 1:9 twist factory .223 barrel will stabilize MOST bullet weights up to about 72-73gr accurately. After that, it's a crap shoot and will depend more on the bullet design and it's requirements along with the actual twist rate (which is not always identical to the factory engraved spec.)

All the best,

Jim

[jhelmuth]

Considering there's no such thing as a .224 75gr V-max, I think surmising he was talking about an A-Max to be a fair assumption.

You are correct.... there is a 55gr VMax, the 75gr is an AMax in Hornady. My bad for misposting that.

[MZ5]

I had surmised the 7 as a type-o for a 5. Thanks for clarifying.

I'm a little fuzzy on your assertion regarding how "(s)tability is all about angular momentum..."

The 'amount of' gyroscopic stability required for a given bullet is primarily a function of its length. Weight is a factor, but a minor one in comparison to length. The primary reason why the 75 Amax will _always_ require a faster twist than the 77 SMK is that it's a _much_ longer bullet. If you can shoot the 75 Amax and get adequate gyroscopic stability for a given purpose/function, then the 77 SMK will always have greater gyroscopic stability under the same conditions. Therefore, if the 75 Amax will group and the 77 SMK won't, it ain't because of stability. :)

[Westcliffe01]

My Savage 10PC in 223 (20" barrel) shoots the Hornady Superformance 75gr BTHP 0.5MOA. However the Amax and Berger bullets are a no-go. Perhaps I could shoot those if I re-chambered to 22-250 since the higher MV requires less twist to achieve a given RPM. For the moment, I will leave it as is, since it shoots very reliably and the scope on it has a custom elevation turret for the 75gr boat tail bullet. For me, 55gr is a waste of time because I too often have the wind blowing and shooting 250yd + the wind drift on the lighter bullets is a joke. Meanwhile I also have a model 12 in 243 and that will be shooting even better in the wind, so I do have alternatives.

[jhelmuth]

I had surmised the 7 as a type-o for a 5. Thanks for clarifying.

I'm a little fuzzy on your assertion regarding how "(s)tability is all about angular momentum..."

The 'amount of' gyroscopic stability required for a given bullet is primarily a function of its length. Weight is a factor, but a minor one in comparison to length. The primary reason why the 75 Amax will _always_ require a faster twist than the 77 SMK is that it's a _much_ longer bullet. If you can shoot the 75 Amax and get adequate gyroscopic stability for a given purpose/function, then the 77 SMK will always have greater gyroscopic stability under the same conditions. Therefore, if the 75 Amax will group and the 77 SMK won't, it ain't because of stability. :)

Actually... the SMK 77gr requires a faster twist than the 75gr AMax (not the other-way-around). Bullet manufacturers are your best source for twist rate requirements as they have more extensive test data and use more highly developed formulas for calculating twist rates than are generally used (such as the Greenhill formula). Bullet stability depends primarily on gyroscopic forces, the spin around the longitudinal axis of the bullet imparted by the twist of the rifling. Once the spinning bullet is pointed in the direction the shooter wants, it tends to travel in a straight line until it is influenced by outside forces such as gravity, wind and impact with the target. Rifling is the spiral or helix grooves inside the barrel of a rifle or handgun. The rifling grooves helix is expressed in a twist rate or number of complete revolutions the grooves make in one inch of barrel length. A 1-in-9 or 1:9 would be one complete turn in 9 inches of barrel length.
How important is twist rate? David Tubb, a winner of several NRA High Power Rifle Championships, was using a .243 rifle with a 1 in 8.5 twist barrel. He wasn’t able to get consistent accuracy until he changed to a rifle barrel with a 1 in 8 twist. The ½" twist change made all the difference between winning or losing the match.

A term we often hear is "overstabilization" of the bullet. This doesn’t happen. Either a bullet is stable or it isn’t. Too little twist will not stabilize the bullet, while too much twist, with a couple of exceptions, does little harm. Faster than optimum twists tend to exaggerate errors in bullet concentricity and may cause wobble. The faster twist also causes the bullet to spin at higher rpm, which can cause bullet blowup or disintegration because of the high centrifugal forces generated. For example, the .220 Swift, at 4,000 fps., spins the 50-grain bullet at 240,000 rpm.

One of the first persons to try to develop a formula for calculating the correct rate of twist for firearms, was George Greenhill, a mathematics lecturer at Emanuel College in Cambridge, England. His formula is based on the rule that the twist required in calibers equals 150 divided by the length of the bullet in calibers. This can be simplified to:

Twist = 150 X D²/L

Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant

This formula had limitations, but worked well up to and in the vicinity of about 1,800 f.p.s. For higher velocities most ballistic experts suggest substituting 180 for 150 in the formula. The latest twist formulas use a modified Greenhill formula in which the "150" constant is replaced by a series of equations that allow corrections for muzzle velocity from 1,100 to 4,000 fps. The Greenhill formula is simple and easy to apply and gives a useful approximation to the desired twist. The Greenhill formula was based on a bullet with a specific gravity of 10.9, which is about right for the jacketed lead core bullet. Notice that bullet weight does not directly enter into the equation. For a given caliber, the heavier the bullet the longer the bullet will be. So bullet weight affects bullet length and bullet length is used in the formula.



The Greenhill equation includes no term for muzzle velocity, and several sources suggest replacing the 150 with 180 for muzzle velocities over 2800 fps. Increasing muzzle velocity increases bullet spin, and spin provides the stability. An article in the 11/2001 Single Shot Exchange cites an article by Les Bowman in the 1962 Gun Digest offering an equation which includes muzzle velocity (in fps):

T = 3.5 * V^0.5 * D^2 / L


At 2800 fps, this equation is equivalent to using 185 in the Greenhill equation, and at 1840 fps, this equation is the same as Greenhill's. Ken Howell wrote about twist rate in the 07/1999 issue of Varmint Hunter magazine. He mentioned Greenhill's work began with cannons in 1879. Two quotes Howell took from the Textbook of Small Arms (published in 1929 in Britain) are notable. "In actual practice Greenhill's figure of 150 can be increased safely to 200 and still control the bullet." The classic equation is for solid, lead alloy bullets of specific gravity (SG) 10.9, and "when the density of the bullet is less than that of lead or the density of the resisting medium is greater than that of air, the spin should be increased as the square root of the ratio of the densities." As SG decreases, the gyroscopic inertia of the bullet decreases in proportion, and one needs to increase the spin to compensate.

C.E. Harris, writing in the 08/1983 issue of the American Rifleman, noted Greenhill's formula was developed before spitzer boattail bullets and high velocity cartridges. He used a more modern analysis of gyroscopic stability, in which a factor of 1.4 is minimum and 1.7 is usually good. He found that the numbers given by Greenhill's original formula ranged from 1.5 to 2.0 for military type boattail bullets and were about 2.0 for bullets with either a flat base or short boattails. However, Don Miller has shown this older equation to not be accurate over the full range of bullet shapes and muzzle velocities. Miller's formula is expressed as:


where:

• m = bullet weight in grains
• s = gyroscopic stability factor (dimensionless)
• d = bullet diameter in inches
• l = bullet length in calibers
• t = twist in calibers per turn

Given those definitions we can expand to...



where = twist in inches per turn, and



where = bullet length in inches.
Stability factor

Using Miller's formula we can also calculate the stability factor assuming we already know the twist. Simply solve for .




To measure the "actual" twist of a barrel, use a cleaning rod and a tight patch. Start the patch down the barrel and mark the rod at the muzzle. Push in the rod slowly until it has made one revolution, and then make a second mark on the rod at the muzzle. The distance between marks is the twist of your barrel. Note that this may not be consistent over the entire length of the barrel depending on the type/method and control in machining when the rifling is cut.


Note: The formulas expressed here were taken from searches on the internet and are merely "copies" and not my actual work (such as from Wikipedia, etc.). Please use your own time and energy to research and understand how tist calculations have evolved and how this relates to bullet stability.

[BoilerUP]

Actually... the SMK 77gr requires a faster twist than the 75gr AMax (not the other-way-around).

Again, I don't see how this is accurate...from either mathematics/science or experience.

The 77gr SMK is 0.994" long with a tangent ogive and a longer bearing surface than the 75gr A-Max, which is 1.11" long with a more secant/VDL design and shorter bearing surface. The Hornady 75gr HPBT is 0.987" long, and has a tangent ogive similar to the SMK.

Running all three bullets at 2600fps through a 1:9 barrel in a 500' ASL standard atmosphere in Shooter ballistic app (and JBM calculator at http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi), the 77gr SMK has a stability factor of 1.38 (JBM 1.353) while the 75gr A-Max has a stability factory of 0.97 (0.955 JBM)...or unstable. The 75gr HPBT has a stability factor of 1.37 (JBM 1.345)

[Westcliffe01]

Hornady claims 2900fps for the superformance match (75gr) ammo from a 24" barrel. My barrel is 20" so MV will be reduced, but a few hundred more fps will help improve stability. The model 12's have a 26" barrel, so MV may be slightly higher.

[helotaxi]

[SIZE=2][FONT=verdana]Actually... the SMK 77gr requires a faster twist than the 75gr AMax (not the other-way-around).

That's not correct. The Amax is a longer bullet and requires a higher RPM to be stable. Now if you're talking about the 80gn SMK. It's true that Sierra recommends a 1:8 for the 77gn but the facts don't bear out that recommendation as a requirement. The Amax on the other hand does require a 1:8 twist at least.

Also you can pretty much ignore the Greenhill formula when discussing long ogive, boat-tailed bullets since it better describes blunt bullets that are short for caliber.

If you both running the numbers for the Miller index for the bullets in question...well BoilerUP already did.

[jhelmuth]

Again, I don't see how this is accurate...from either mathematics/science or experience.

The 77gr SMK is 0.994" long with a tangent ogive and a longer bearing surface than the 75gr A-Max, which is 1.11" long with a more secant/VDL design and shorter bearing surface. The Hornady 75gr HPBT is 0.987" long, and has a tangent ogive similar to the SMK.

Running all three bullets at 2600fps through a 1:9 barrel in a 500' ASL standard atmosphere in Shooter ballistic app (and JBM calculator at http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi), the 77gr SMK has a stability factor of 1.38 (JBM 1.353) while the 75gr A-Max has a stability factory of 0.97 (0.955 JBM)...or unstable. The 75gr HPBT has a stability factor of 1.37 (JBM 1.345)

The bering surface is the diff. You will find that there is ALOT of information on twist calculations (that is why I go with the Mfg. recommendations over any formulas). But the shorter bering surface gives the 75gr AMax a significantly better stability factor. My point in all the "formulas" is to show the progression in calculating twist rates and to emphasize that as bullet technology progresses, these "generalized" formulas don't always apply. As has been stated before, the AMaxes seem to stabilize fine in a stamped 1:9 Savage barrel (which I suspect is much closer to an 8.5 - what I measured mine at).

YMMV.... there is ALOT of science to this, but there is also ALOT of information required and these forum post serve to show how we CAN'T generalize (which is my point in all this).