Can someone help out on this ?

What is the max bullet weight that can be stabilised in the 348 Winchester with a 1 in 12" twist rate.

Thinking about having a heavyweight mould made up and need some guidance.

Thanks

Joe

It depends on the profile of the bullet. Based on what I've seen in 35 caliber's with a 1-12 twist, I'd expect a 300 gr bullet to stabalize at say 2000 fps at the muzzle.

I've never had trouble with Barnes's 250 grain original. Hawk makes bullets in 250 and 270 grains as well, but I haven't tried them yet.
With a stout charge of RL 19 and 250s, the recoil in my model 71 is enough to make your eyes cross.

Thanks for the replies.

I have a feeling that 300 gr should be OK too, but would like to know what the limit is so that I make sure there is enough margin for error.

Is there some kind of formula to calculate this ?

If this helps at all, the bullet will be a flat nose for a tube mag. I figure if I am limited to +/- 2000 fps due to it being a cast bullet, it may as well be the heaviest I can accurately drive down the bbl.

Joe

Hello JFE,
Good questions and would suggest you send Sierra an e mail concerning your questions for believe you will find them most helpfull and quite learned on the twist rates, velocities, and bullets in general. They have a very complete understanding of this type of data, probably second to none, other than our good folks at Aberdeen Proving Grounds!! Good Luck

JFE:
The NEI #108 mould throws a bullet that weighs around 270 grs. checked and lubed. This bullet seats well below the neck/shoulder case junction. I use a load of 56/Re19 topped by 7 grains PSB to give a compressed load and ensure the gas check comes out with the bullet. This load exceeds 2000 fps and shoots quite well. Any bullet heavier than this would surely seat deeper unless you have a modified throat in your rifle. It does have a snappy recoil and a load of 39 IMR4895 is a bit slower and easier to shoot. I have been taking my chances on the gas check not coming off with this load but want to do some more tinkering with the load or variation using PSB to ensure it.

My M71 is a 1-14 if I recall. Is yours a different barrel?

I would think you could use a bullet that weighed at least 280 grains, if not 300 grains, as long as the design was a blunt flatnose or round-nose shape.

(Whether a given twist will stabilize a particular bullet or not depends on the LENGTH, not the WEIGHT, of the bullet. For example, my pre-64 Win. M70 Featherweight .308 Win. is very accurate with both Hornady and Remington 220-grain blunt roundnose bullets. And it has a 1/12" twist.)

Gents - thanks for the replies.

I didnt measure the twist in my Browning, I simply assumed it was the same as the Win M71. Both Lyman & Hornady quote their Win test rifles as having 1 in 12" twists. I will try to measure it and check. Good point though - dont assume anything!

I understand that the Browning M-71's have virtually nil throats, but I wont be messing with this. My preference is to have a mould made up in a two diameter design, ie nose forward of the crimp groove of bore diameter. This way I can max the bullet forward of the case to OAL possible.

FWIW I use a cast design in my 375 H&H and that bullet is seated well below the neck juncture - the 375 has a pretty short neck anyway. This bullet has a wide band ahead of the gas check shank and gives very reasonable accuracy without having to resort to fillers etc (ie all the grease grooves contained in the neck area). I load it with 45 gr of 3031 and get 1.5 moa when I am picky with the bullets. The bullet length protruding below the neck is approx 0.25". I know this goes against all the accepted
theories but it works in this application.

Will check with Sierra and see if they can help out.

Joe

JFE: 1/12 is correct. My memory was not. The NEI #108 has long nose much of which has near parallel sides and is just a nice slip fit between the lands. Very similar to an old Lyman mould (350482??) except for a small platen. The remainder of the nose is a blunt round. A friend is making up soft nose hard shank bullets from his 108 and reports great killing effect with very large exit holes on deer and black bear. Also as much blood shot meat as normally seen with jacketed bullets which doesn't appeal to me very much. However he has not found any nose particles in the meat.Both he and I are finding that once velocity goes beyond 1900-1950 bullets must be oven hardened to maintain accuracy from our barrels.
Perhaps 1/12 is just a bit fast for unhardened but the pure lead nose does not harden appreciably when oven treating while the shank will.

Twist Rate

from: http://www.loadammo.com/Topics/July01.htm

From the link above:

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. These grooves were invented a long time ago, perhaps as early as the 14th century. However, the smooth bore, using the round ball, was the choice of weapons for warfare even through the American Revolutionary war. The smooth bore musket could be loaded faster than the rifle and didn’t foul, as bad, with the combustion products of black powder.

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 1in10 or 1:10 would be one complete turn in 10 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 D^2/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 twist formulas used in the Load From a Disk program, featured at this web site, uses 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.

To measure the 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.

To see how this works out, assume you bought a .222 Remington rifle and you measured the twist rate as described above. The twist was 1 in 14. You have two .224 bullets you want to use, the 70-grain Speer SPS and the 50-grain Hornady SX. The Speer bullet measures .812 inches in length and the Hornady measures .520 inches. Using the formula above we calculate the following twist rate:

Speer 70-grain: 1 in 9
Hornady 50-grain: 1 in 14

These calculations show that the 50-grain bullet will be stabilized, but the 70-grain won’t. Sure enough, when you try these bullets out, the 50-grain shoots ¾ MOA while the 70-grain won’t group on the paper at 50 yards. Twist is important!
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BFaucett - thanks for this.

Cranking through the formula and assuming a flat nose design in 348, I figure something like a max of approx 330-340gr should be stable. This is using the 150 (1800 fps) constant. So something like 300 gr at 2000 fps should be well stabilised and the sectional density should make it quite a good hunting bullet.

Many thanks for everyone's input.

Joe