Barrel Length - Twist Rate - Bullet Weight

Frak · Rookie Member Joined: May 03, 2019 Posts: 18

Hello All!

Using the .308 as a calibre for discussion and probably as it is my go to for ungulates, 26 inch heavy hammer forged 1/12 twist barrel and bear protection in a semi-auto. I carry two rifles to my hide and two rifles when hunting grouse. Too many bears and I won't give up the area of Alberta's NW foothills as my area of choice.

First off a statement and PLEASE correct if incorrect. This conclusion is from reading many posts.

The heavier/longer the bullet the lower the twist rate for flight stability. Example: 1/12 for 168 to 175. 1/10 for 180 - 220.

Is this FACT based on a specific barrel length? 20 -22? I'm not able to locate an answer/statement. If it is based on a standard barrel length less than 26" would a 26 inch barrel improve the stability of a heavier bullet jumping from a 168 Barnes to a 178 Hornady ELD-X?

My latest experience, fall of 2018, while fine tuning my accuracy shooting off my day pack is as follows.

Two 3 shot groups were fired allowing the barrel to cool completely between different manufacturers cartridges. Didn't take long @ 0 degrees centigrade.

Rifle and scope:
.308 CZ S1M1 - Zeiss 6 X 24 - 56mm objective - Rapid Z-1000 reticle

168 Barnes Cartridge - dead centre @ 100 yards. .6 MOA

168 Hornady Cartridge - 1 inch high dead centre @ 100 yards. .5 MOA

178 Hornady Cartridge - 3 inch high 1 inch to right @ 100 yards. Different barrel harmonics I'm guessing.
Once this cartridge was dialed in to centre the repeat-ability @ 100 yard was less than .4 MOA. A couple of shots almost in the same hole.

The Barnes was the shortest of the three cartridges with the 168 and 178 increasing in total length. The 178 just fits the magazine.

In theory the 178 should not be as accurate.

Now Hornady has issued the A-Tip in .308 @ 230 and 250 with a BC of .823 and .878.

I'm waiting for a Cartridge in the A-Tip from Hornady.

Will Scratch

it be accurate with a 1/12 twist rate?

SingleShotLover

First, barrel length isn't overly important to your question. What is important , as you suggest, is the twist rate. This imparts the same rate of revolution to the individual bullets regardless of the barrel length. Barrel length does affect muzzle velocity to some extent (roughly 25 fps per inch), but not bullet revolution.
Where it gets interesting is when the twist rate for a certain bullet length is on the edge of stability unless pushed to a certain speed. This can also be affected by elevation (higher elevation affects speed/stability less than lower ones).
Not sure if this helps your situation, but the best option is to choose a twist rate compatible to your heaviest chosen bullet weight. Lighter bullets will also stabilize just fine in faster twist rates within normal hunting distances. Remember that there is a difference between "optimal" and "adequate" twist rates that may well just plain depend on your particular rifle too.
Hope this helps rather than confuses! Luck to you.

inthedark

Read up on the Greenhill formula. Google it. It will explain the twist to calibre theory. Also the Miller formula and the twist -gain formula.

PaulS

In 30 caliber 1 in 10 inches is more or less standard for 3006. I don't know why the 308 has a 1 in 12 twist. As long as your bullets are not going through the target at an angle you are fine. The twist rate has less to do with accuracy as long as the bullet is at least minimally stabilized than the load for the gun. It is not really weight that matters as much as length. For instance a 170 grain spitzer cup and core bullet may be fine but a solid copper 170 spitzer may not stabilize - the copper bullet has to be longer as it is less dense. A tipped bullet is similar in that it is longer for the weight and so are the Very Low Drag bullets. You will have to try them to find out for yourself. The math doesn't always work either. My 03A3 has 1 in 10 twist but it will not stabilize a 200 grain spitzer or a 220 grain round nose bullet. Also temperature and humidity plays a roll in stabilization. Cold temps and low humidity tend to lessen the stability while high humidity and warm temps tend to be more stable. If your ammo is stable at low temps you shouldn't have any problems in higher temps.

Frak · Rookie Member Joined: May 03, 2019 Posts: 18

Rather interesting that the longer and heavier 178 provides the best MOA with a 1/12 twist than the 168s.

That was last autumn.

Hmm maybe it was weather but no , it was getting colder as I fired the 178s.

Temp drops fast when the sun disappears. Best recall for temp was 0C to -5C and very dry air.

Best to go out and fire off another series to hone accuracy/shooting repeat-ability skills. Been waiting and waiting and waiting but everyday we are being dumped on with another few inches of snow that melts and then it snows again and it remains mud at the range.

Three years ago though it was hot and dry. Fort McMurray was burning and 80,000 people were evacuating on one thin band of black asphalt.

en.wikipedia.org/wiki/...y_wildfire

Frak · Rookie Member Joined: May 03, 2019 Posts: 18

Greenhill and Miller formulas are interesting Math/Science better described as PHYSICS?

Located this info:

"higher velocities also spin the bullet faster and may enable a slower-than-recommended twist to stabilize a marginally overweight bullet. However, this is tricky stuff best left to those who are
highly experienced in operating at the ragged edge of stability and sanity."

Possibly the 26 inch barrel may contribute to a slightly higher velocity. The 178 is a very sleek bullet when compared to the 168s. Possibly as the 178 cartridge is longer than the 168s the bullet jump to the lands is decreased further stabilizing time in flight.

Many possibilities . Haha

.308
- 7" * for heavy VLD bullets and/or subsonic ammo.
- 8" for bullets heavier than 220 gr.
- 10" for bullets up to 220 gr.
- 12" for bullets up to 170 gr.
- 13" * Ratchet rifled 4 groove
- 14" * for bullets up to 168gr.
- 15" * for bullets up to 150 gr.
- 17" * for bullets up to 125 gr.highly experienced in operating at the ragged edge of stability and sanity.

Hopefully Hornady will manufacture a .308 cartridge in the A-Tip and further experimenting will be possible. A BC of .823 and .878. WOW!

Frak · Rookie Member Joined: May 03, 2019 Posts: 18

Comparing twist rates for the .308 @ 178gr.

ELD-M --- ELD-X --- A-Max

ELDs a 1/12 is acceptable but the A-Max is a 1/10.

That is the science.

Very Interesting!

Frak · Rookie Member Joined: May 03, 2019 Posts: 18

The physics and time to understand and develop the new bullets designs is remarkable.

Located an in depth discussion that states:

when comparing a plastic tipped bullet with an identically shaped bullet with constant density. In addition to moving the center of gravity slightly backwards, the effect of the plastic tip increases the axial moment of inertia and decreases the tumbling moment of inertia. Both of these effects on the moments of inertia tend to increase the gyroscopic stability and they more than compensate the change in the center of gravity.

[ IMG]

The fundamental equation for gyroscopic stability is shown above, taken from

www.nennstiel-ruprecht...rocond.htm

Ix is the axial moment of inertia; it appears twice in the numerator, so increasing Ix, increases stability strongly. Iy is the tumbling moment of inertia; it appears in the denominator, so decreasing Iy also increases stability. Moving the center of mass backward slightly increases CMa, the derivative of the overturning moment coefficient.

So the plastic tip changes three terms in the stability equation. The change in one term tends to decrease stability, but the changes in the other two terms tends to increase the stability, so the net change is an increase in stability.

If you don't believe the theoretical explanation, then believe the experiments reported in the Precision Shooting papers. We've also conducted additional experiments showing that the original twist formula is accurate for metal bullets, but it underestimates the stability for plastic tipped and open tipped bullets. The effect of a plastic tip or an empty space in the tip of a bullet is to increase stability relative to a constant density bullet of the same dimensions.

This explains why firing .308 180gr soft points had MOA all over the map but the ELD-X 178 being a tack driver with the CZ.

OK, I'm done.

Thanks for the replies.

Pumpkinslinger

Almost 40 years ago I loaded some Speer 70 grain bullets for a friend's .22-250 because he wanted to deer hunt with it. I can't remember the manufacturer or twist rate of the rifle. At the starting charges those things key-holed pretty good! They did a little better as they approached the max load but he'd lost confidence in them so we never did find a good load. It was an enlightening experience for a relatively rooky handloader!

slimjim

Frak wrote:

Hopefully Hornady will manufacture a .308 cartridge in the A-Tip and further experimenting will be possible. A BC of .823 and .878. WOW!

First, welcome to HuntingNut, Frak!

Second, don't wast your money or time. The slickest A-Tips are designed for 1:8.5 twist. Or hold your breath as Hornady will not make a round they cannot sell in .308 Win.

Third, hunting in cold weather especially in your area requires more twist to stablize the bullets in the denser air. 1:10 is the standard for .308 caliber rifles but 1:12 is common in 308 Win because they expected folks to shoot lighter bullets, e.g., 150 to 165 grain. If you are shooting copper bullets like the Barnes, their expansion is measurably increased with faster twist (another advantage for 1:10).

Fourth, why are you lugging around a 26" barrel in Alaska? Trying to stay warm? What length gas system do you have? Consider cutting the barrel down to 18" (open up gas port if necessary). It will serve you much better and you will not lose that much MV in .308 Win.

Frak · Rookie Member Joined: May 03, 2019 Posts: 18

Pumpkinslinger wrote:

Almost 40 years ago I loaded some Speer 70 grain bullets for a friend's .22-250 because he wanted to deer hunt with it. I can't remember the manufacturer or twist rate of the rifle. At the starting charges those things key-holed pretty good! They did a little better as they approached the max load but he'd lost confidence in them so we never did find a good load. It was an enlightening experience for a relatively rooky handloader!

Developing a cartridge from available bullets is rewarding. Back in the day before global travel interrupted my life I did reload with my primary choice being the 30-06 I favoured. I still remember the 54.1 grains of powder for a 180 spitzer boat tail. It was my top pick as the .303 always required two shots to harvest a moose whereas the woman that used the .306 only required ONE. She always considered the plight of the animal and called her husband an asshole for not upgrading to a better calibre. Army surplus .303 were $10 in 1965. Cartridges were $.01 a piece.

Who knew much of anything back in the day except what was talked about at sporting good stores and then how much of that was truth. LOL

To bad about your buddy losing confidence. Too much bang for a buck?

Frak · Rookie Member Joined: May 03, 2019 Posts: 18

slimjim wrote:

Frak wrote:

Hopefully Hornady will manufacture a .308 cartridge in the A-Tip and further experimenting will be possible. A BC of .823 and .878. WOW!

First, welcome to HuntingNut, Frak!

Second, don't wast your money or time. The slickest A-Tips are designed for 1:8.5 twist. Or hold your breath as Hornady will not make a round they cannot sell in .308 Win.

Third, hunting in cold weather especially in your area requires more twist to stablize the bullets in the denser air. 1:10 is the standard for .308 caliber rifles but 1:12 is common in 308 Win because they expected folks to shoot lighter bullets, e.g., 150 to 165 grain. If you are shooting copper bullets like the Barnes, their expansion is measurably increased with faster twist (another advantage for 1:10).

Fourth, why are you lugging around a 26" barrel in Alaska? Trying to stay warm? What length gas system do you have? Consider cutting the barrel down to 18" (open up gas port if necessary). It will serve you much better and you will not lose that much MV in .308 Win.

Yeah Eh! Haha.

I don't lug that rifle too far. Possibly the longest is crossing a creek to climb a ridge to accessing a sight/hide. A couple of kilometers. I'm OK with it as a workout a day keeps the doctor away. At about 13.5 lbs with a muzzle break when I'm on my game that firearm is a tack driver with 168s, relatively no recoil and with a set trigger it is: acquire target and bang, watching the hole appear on paper or watching the hit on the critter. I was surprised that the 178 ELD-X worked well. The rifle may be a 1/10 as the twist rate was changed somewhere in time. Didn't much care as the proff is in the shot placement. I was pleased with the accuracy. I'll check it out.

The R1, do believe it is 22 inch barrel, open sights, is my bear and less than a 100 yards ungulate rifle. The A-Tip looks promising for the semi Having a heavier penetrating bullet means the most to my safety. You don't get to do a careful acquisition of bullet placement when a rolling mass of flesh charges you at less than 50 yards. Bears run at 13/yards a second. Not much time with a bolt action rifle.

Hunting around the Whitecourt, Alberta area. When I step off the quad my day pack has water, food, binocs, knives, game wrap, rope, pulley, first aid
kit, change of clothes in case I shyte my pants, fire starter, etc. The 26" barrel travels on a rig attached to the day pack and the R1 is in my hands.

It is the bush. No excuses and no mercy for those unprepared.

Look up on Utube "Whitecourt Grizzly".

And please don't try to convince me that bear spray will suffice. I'm not brave enough to use bear spray. My muscle memory advocates SHOOT not SPRAY.

If the general public knew the truth about bears fewer would hike in bear country. Ever witness someone using a fire extinguisher for the first time?
Now imagine trying to use bear spray when a foul smelling hairy killer is attacking. Good night. See you on the other side.

slimjim

Frak, the A-Tip will only put you in danger. It is a target bullet made for 1:8.5 twist. I've been looking at them to build a 300 PRC. If you want a lethal .308 bullet with great penetration and expansion, consider the 168 Tipped MKZ from CavityBackBullets.com. I do terminal testing of bullets for a hobby. This bullet creates a deep tri-star wound cavity with massive hemorrhaging.

Frak · Rookie Member Joined: May 03, 2019 Posts: 18

slimjim wrote:

Frak, the A-Tip will only put you in danger. It is a target bullet made for 1:8.5 twist. I've been looking at them to build a 300 PRC. If you want a lethal .308 bullet with great penetration and expansion, consider the 168 Tipped MKZ from CavityBackBullets.com. I do terminal testing of bullets for a hobby. This bullet creates a deep tri-star wound cavity with massive hemorrhaging.

Slimjim: Thanks! Appears to be a Kick A$$ bullet. Any manufacturer loading these and selling cartridges? Long ago I had the equipment but no longer.

Testing bullets. Good gosh, there must be an array of calibre weapons in your hold.

So glad to read that you are of the opinion that MATCH bullets are not to be used on animals as tests. I stated this on another site and dealt with the comments from no less than 8 opponents. I'm about ethical kills/hunting. Slow but steady won the race with the foes. Keyboard warriors.

Cheers!

Elvis

Frak.......the bog standard 180grn winchester power point loads are the ducks nuts at sub 100 yards..the recoil is minimal,feels LESS than the same load in 150grn.....more of a shove than a boot.
I looked and looked to find a projectile I could buy to duplicate the terminal preformance and have settled on another bog standard load with 180grn round nosed hornadies with plenty big lead exposed at tip..results on game so far are very impressive..the bonus with these is rifle pretty much bang on at hundy when 3" high with faster/lighter load....
here in NZ there are quite a few people who use the AMax-eldm-eldx type loads for game...they got a following with long range people and SOME people swear by them for an all round load...others swear AT them....the 200grn of that type has been suggested to use along similar lines to what I do...an up close n personal soft n slow loading.