458 twist

[elmerkeithclone]

I've mentioned somewhere on this forum that I am sending a ss77 Ruger off to be rebarreled in 458 mag. I have decided to go 22 inches (no muzzle brake..thanks Murphy) but I am unsure about rate of twist. What twist would be best suited for 350 grain bullets? 16?18?

 

[Murphy]

Twistin' n' Turnin'

Twistin' n' Turnin'

EKC,

Which 350 grain bullet?

Some mathatician years ago came up with a formula for calculating proper twist rate. His name was George Greenhill, buit who cares right.

The Greenhill formula, as it is known, is : 150 X D squared/L

D=Diameter we then square this (.458x.458 in this case)
L=length of the bullet in inches. (Divide this into the quantity)
150= a constant- which will change as exit velocity changes but is considered good for velocity up to about 2800 fps. The number would actually go up with velocity.
The answer will come out with the number of inches in which the bullet must make one revolution. 1 turn in ( ) inches.

What determines how much spin that's needed is the length of the bullet, not the weight. Long ogive spitzers require more spin than round nose, of the same weight. And a 200 grain 30 cal copper bullet will require more spin than a 200 grain lead bullet of the same caliber.

The stabilization is surface per minute, not RPM. So, a 45 caliber that is the same length as a 35 caliber bullet, will not need as many rpm to stabilize because of it's greater diameter and thus covers more area as the bullet spins.

The Greenhill formula is considered a good one for target accuracy, where the bullet never runs into anything except a piece of paper at the end of it's journey and it assumes a certain type of construction. Hunting bullets have different needs because of internal differences such as partitions, bonding material and crimp grooves which can alter an otherwise predictable center of gravity. The "easiest" bullet to stabilize (after a round ball) is the round nose design of just swaged lead with soft copper jacket (Barnes Original, Hawk.). And it is certainly the most stable at impact, compared with all the other modern bullet designs. It also stays point on through brush and other vegetation better than spitzer types. A big factor for stability at impact, of course, is a bullets ability to retain it's shape and not deform. The soft lead doesn't do that well. So many factors come into play for a good hunting bullet.

Now on to your question. I have two 350 grain, .458 diameter bullets on my bench, a Speer hot core, for 458 Win velocity and Hornady 350 grain flat point, for the 45-70 velocity. The Speer is just short of an inch at .990" and has a longer ogive and is really more like a semi-spitzer with a flat point. It won't work through a Marlin action because it can't be seated short enough.
The Hornady measures .900" and has a short stubby look and works fine in the Marlin. So they would have slightly different needs. But the Greenhill formula would show a need of about 1 in 35" twist for the Hornady and 1 in 32" for the Speer. Not really standard. This same formula shows a twist rate of 1 in 8.3 for a .224" bullet of .9" of length (a 69 grain .224" bullet) (Now this bullet, stable in flight is not stable at impact. Hmmmmmm!)

The truth is that most .458 diameter guns will stabilize much heavier bullets, such as 500 grain for the 458 Win and will stabilize them even if spitzers.
For the 458, it has always been "over-stabilized" with a 1 in 14" twist. This may also be the reason it works so well and we recover very few bullets from this old cannon. They ding the countryside behind even the biggest and toughest animals, after passing through. (Stabilization means Penetration)

Now old George was a helluva mathmetician but so am I. And, I have something he didn;t have. A whole lot of experience with animals being hit with lots of different bullets. I like his formula but when hunting I use my own constant of 100, not 150, for these velocities.

With Murphy's formula, the correct twist for the .458" 350 grain is 1 in 20". With the .458" 500 grain the twist will be 1 in 14". I think some body else who designed cartridges had this same idea, way back in the '50's.

I got up early this morning. Was going to go test the stability of some long bullets from the 308 Norma. As I look out over the Tanana valley, I see nothing but smoke. I can see birch trees out to about 120 yards, beyond that I see smoke. The town of Fairbanks is totally obscured (Not all together a bad thing) and Murphy's mountain is not there to inspire my day. So you don't know how much I appreciate this opportunity to ramble on about ...well......shootin', good shootin'.

Murphy

 

[BrownBear]

Interesting point about "over stabilization" and penetration. I've taken quite a few deer with the 175 grain Hornady RN at modest velocities from a 7x57, and those things penetrate out of all proportion to their light construction- even end to end on good sized deer. Years ago I experimented with Remington 220 grain RNs from an 06 for close range shooting in tight cover, and had very similar results.

Don't quite know how it relates to your point, but even in smaller calibers long bullets at moderate velocities (not that much more than the 458 Win 500 grainers, as a matter of fact) sure ventilate a lot of countryside after passing through game.

 

[Murphy]

More Twistin'

More Twistin'

BB,

To attempt to further my point. You're right about penetration with long heavy bullets. Of course that is sectional density at work and is the greatest benefactor in penetration. Also, the lower velocity is beneficial after impact because the expansion is slower. When a soft point bullet impacts it is shortened by expansion and if this expansion is fairly uniform it helps the bullet to remain stable and "on course" (larger diameter, weight forward and shorter will stablize with less spin) so we have balancing factors at work here. An FMJ spitzer, which can't shorten and is difficult to stabilize anyway, will veer off course and swap ends before coming to rest, butt first, in a totally unpredictable location. Sierra's HP Match bullets are not reccomended for hunting for this reason, the tiny hole in the hollow point closes up and prevents expansion and it behaves as an FMJ.

Another note, by using round nose bullets for hunting, such as the 175 gr .284 or 220 gr .308, we add weight and sectional density without adding length, such as a heavier spitzer would do, and it will stabilize like a much lighter spitzer. The very best TERMINAL bullet is the worst EXTERIOR bullet and vice versa. The hunting bullet is a compromise and understanding the differences is kind of important sometimes.

Thanks for the space, good shootin'.

Murphy

 

[7STW]

Murphy:as you know, bullets can do strange things when they impact animals.

Two cases come to mind that seemingly defy the "laws" of bullets, but I witnessed then first hand.Interestingly, both were out of TCs with SSK barrels.

The first was back in 1986 on a "goat" hunt out of Rawlins.At that time, I was all hot and lathered about handgun hunting and was toting a 30-40 Ackley with a ported 15 inch Shilen barrel.Load was 54 grains of IMR 4350 over a 165 Partition.I know, because I loaded it.

Anyway, after popping a nice SCI book buck, we had a bunch of doe/fawn tags to burn.The first one I tried was a 336 pace one shotter on an average size doe.I can tell you he looked like a prairie dog through a 2X EER Leupold at that distance.

When we got up to him (he staggered maybe 10 yards from where I shot him), there was an exit hole in his side that looked like you had blasted him from 5 feet with a 12 guage shotgun.Literally a foot of intestine was hanging out of this dude...looked like he had swallowed a LAWS rocket.

When we gutted him later, we found just the heel end jacket of the Partition intact...nothing else.I weighed it later and , if memory serves me correctly, it weighted about 10 grains.

Keep in mind that impact velocity at around 350 yards was under 2000 FPS.Nosler suggests around 1700.

I sent this to Gail Root at Nosler, wondering how they could explain the complete disintegration of a Partition on a 65lb antelope.

They couldn't...

Second scenario was in 88 in South Africa.On that hunt, I had a 12.5" 375JDJ along.Load was 45 grs H322 and a 270 Spirepoint.

Shot a blesbuck with a finisher at about 15 feet (he was dead but didn't know it).This was a pure texas heart shot.That load chronoed about 1900 instrumental.

The skinners found the Hornady lodged against the skin of his chest...and gave me the bullet.Other than the land engraving and the tiniest of smear of the lead spitzer tip, it looked like it had just come out of the box.Weighed 269.6 on my scale back home.Of course, it acted just like an FMJ, but did drill through a good sized male that went around 165lbs on the hoof.At least 40" penetration in a straight line...perhaps more.

Bullets do funny things.

 

[elmerkeithclone]

Good stuff Murphy

Good stuff Murphy

I had heard of the Greenhill formula and even messed with it some several years ago in regards to 6mm Rem/1 in 9" vs 244/ Rem 1 in 12" twists. It did lead me to the conclusion that long skinny bullets do need to spin faster than short ones of the same diameter. Thats kinda common sense though. Changing the constant to 100 as opposed to 150 gives new and warranted perspective to the formula. Who was it in the 50's that messed with doing that? Was it EK?

I never even gave a thought to the fact that the rate of spin could have an effect on damage inflicted because it passed through without loosing stability. It makes perfectly good sense that slowing the rate of spin down a little will inflict more damage on the quary......never heard that before, never concidered it.

Another thing is that I have always tailored my loads to to the gun. However, when I ask what twist and you answer by asking me what 350 gr bullet will I be using. Now all of a sudden I'm building the gun around the bullet that I intend to shoot. Thats bassackwards to the norm for me but it makes good sense and I like it. Live and learn!

I have been reading lots of stuff on the different .458 bullets and was strongly leaning towards the Speer 350 gr Hot core. Knowing that it is one of your bullets of choice is encouraging. Since I have several guns I shoot only one "pet" load in each of them once I figure out what that load is. The 350 grain bullet will be the only one I use in this gun. Just wondering how the 350 hot core does on big bruins?

 

[Paul H]

I've yet to have a problem with a faster twist barrel not shooting the lighter bullets accurately. So I'd opt for something in the 1-12 or 1-14 range. That way if for some reason you decide to go heavier than 350 gr in the future, you have the option. Think 500 gr cast bullets at black powder velocity, ie 1300 fps. Relatively dosile to shoot, and so long as you have a fast enough twist, you're good to go. Speaking of cast, faster twists, ie 1-10 or faster can be harsh on the bullets and strip them at higher velocities, so too much twist isn't good for cast.

 

[Murphy]

A new Twist'''

A new Twist'''

EKC,

I would say that a 20" twist will be good for up to 500 grain bullets at 2000 fps if shots are taken close.

Most of the NE calibers have a 20"-26" twist and stabilize bullets out to 100 yds. Also you should consider the effect of recoil reduction by going to a slower twist (slower than 14") Recoil is reduced as twist rate is relaxed, though not easily measured, it is noticeable. The 450 Alaskan builders found this to be the case. The 45-70 with its' 20" twist, will stabilize a 540 grain cast bullet and shoot quite well at 500 yds. Also cast bullets at 1500 fps will not shoot well in fast twist barrels. You do not want faster than 20" for cast bullets in 45 caliber rifle and slower may be better. The cast bullet technology is well proven, there are a lot of long range shooters who use 45-70 up to 45-120 cartridge guns at long range (1000 yds.) with lubed cast bullets. Cast bullets stabilize more easily than the copper jacketed bullets.

However, if you consider using this gun with 500 grain jacketed at 2000 fps as a dangerous game rifle, maybe the faster 14-16" twist would be in order. I thought your original quest was for a down loaded 458 Win or an up loaded 450 Marlin with bullets of 350-450 grains at 1800-2000 fps, and not the super smasher of the 458-500-2000fps.

The 350 grain Speer is a very good bullet, tough and kind of a semi spitzer. Speer makes it for 458 WM velocities. Also the supply of 400-450 grain jacketed and/or bonded bullets is huge and as a North American rifle, these ballistics will do all we need. The 400-450 grain bonded bullets will exit if impact velocity is kept down below 2000 fps. The expansion will be slowed enough to allow good penetration. I once did a penetration test using a 540 cast in a 45-120 (2 7/8 Sharps) at 1500fps compared with the 460 WBY 510 grain softpoint at 2500 fps, the Sharps always won.

What you will find that works the best is to find a velocity that you can live with that works, such as 2000 fps and shoot every bullet at that velocity. When more power is needed, go to 450 grain, when less is needed go to the 300 or 350 grain. The trajectory will be the same out to reasonable hunting ranges and it will be a more mild mannered rifle. Your 458WM/450Marlin/450EKC will get 'er done. Of course if you shoot cast they will need to be kept below 1800 fps but just go up in weight until you get your buffalo gun. I think your project has a lot of merit, likely an ideal coastal bear gun, more shootable than the big guns but more than enough power, and can be made in a lighter rifle. Just some more thoughts, good shootin'.

Murphy

Post Script:
I build about all of my guns around one bullet.

 

[Paul H]

I would beg to differ on cast needing to be kept under 1800 fps, I'd say good cast bullets with a good lube and properly sized for the gun can be driven 2200 fps with fine accuracy and terminal performance.

 

[Murphy]

Cast Bullet Velocity

Cast Bullet Velocity

Paul,

Yes, you're right. Cast can be driven to higher velocity, I've shot many at 2600 fps. I just meant as a hunting caliber on tough guys the higher velocity of heavy hard cast slugs will damage them at impact and reduce their effectiveness. On deer sized critters and/or with softer bullets which will expand, higher velocity is ok. With softer cast with gas checks, they can be driven as fast as desired and still work very well on game, if no leading problem. A gas check will also allow a harder bullet to be driven at lower pressure, which won't obturate the bullet in the bore, without leading.

You're also right about the size and lube, important points. But at impact with big bones, heavy hard (BHN 21) cast 45 caliber bullets will deform if velocity is too high. There is a lot of force on them. So I would keep them under 1800 fps. Thanks, for bringing that out.

Good shootin'.

Murphy

 

[elmerkeithclone]

On the same thought

On the same thought

Murphy, One of my original posts on this forum was bashing the Nosler Partition bullet. It was to do with a 400 gr Partition being driven over 2500fps in a Rigby. A bear was shot and not recovered because the slug "went off" on impact. Knowing what I knew then it was the bullets fault. Whith what I know now I'm guessing that if that slug would have been running around 2200fps then that bear would be dead.

That is a big part of my new found thought process and has a lot to do with my 458 project. I used to be a speed freak when it came to muzzle velocity. You guys have convinced me that speed is seldom the answer with big bores and penetration! I have been using Elmer's handle but now I'm starting to understand his way of thinking!

 

[Paul H]

Call me a sceptic, but when an animal isn't found I find it very hard to buy the bullet failure argument. Even if the nose of a partition blew up on impact, the rear section of the bullet would have kept on going. With proper shot placement, I'd think a 400 gr partition at 2500 fps would be about the most devastating bear killer you could find. Any failure to perform I'd put squarely on the shoulder of the hunter.

 

[BrownBear]

There's something to what you say Paul, but unless the animal is recovered it's all speculation.

I can recount three instances of "bullet failure" from guide friends who eventually managed to recover the bears. In each case Joe Hunter swore up and down he had made a perfect shot "and that ****ed (pick your brand) Bullet blew up on me!" In each of the three recovered bears, the bullets performed fine, but the gut shot was slow to kill. Those same guides say that even if a bullet performs poorly on a good shot it will kill better and more quickly than a premium bullet in the guts.

What are the guide and packers doing when the client fires? Looking hard for any sign of where the bullet lands so they can make their own assessment of the shot. Joe Hunter aint that reliable a witness.

 

[elmerkeithclone]

Witness was reliable!

Witness was reliable!

Kenny Schoonover out of Hoonah was the guide. He witnessed the hit from 50 yds. He said the shot placement was on the money.You guys know Kenny?