Is stability for a given twist a function of bullet length or weight?

[CamoWhamo]

I have a .357 Maximum rifle with a 22" barrel and 1:18.7" twist.

The 180gn cast bullets i have tried in the rifle have proven to be inaccurate and many people more experienced with this caliber are of the opinion that the twist is too slow for the heavy bullets.

The rifle shoots 1 hole clusters with 158gn Jacketed so that kind of proves their point.

So i'm looking for a mold around the 158gn weight but i have seen a couple of molds designed for powder coating where the lack of a lube groove means a 170gn PC bullet is the same cross section as a 158gn bullet with lube grooves.

So my question is.
Would the stability be the same with a heavier bullet that is the same length as a lighter one?

[Cast_outlaw]

That’s a good question I’m going to keep my eye on this thread

[megasupermagnum]

A 180 grain bullet is stable from a 1:18" twist 357 magnum at less than 1200 fps. Bullets in excess of 220 grains can also be stable from a 357 magnum handgun. Your twist rate is not the problem.

Stability is a function of bullet length, but also effected by velocity. The faster it's launched, the higher the RPM's.

[Hannibal]

More a function of twist and velocity. Sometimes a longer barrel can increase the velocity enough to aid stability, but not always. A short bullet generally weighs less and requires less twist because it will attain higher velocity without excessive pressure.
Think RPM. That's where the magic is.

[5Shot]

Length and velocity. Bullets like the Barnes are longer for their weight, so they need a faster twist than a standard bullet of the same weight. Cast bullets are the shortest for a given weight, so you can shoot a heavier bullet than you can with jacketed. When you are right on the edge, pushing a long bullet faster can keep it stable until it slows down below a certain threshold.

[5Shot]

According to the Berger Stability Calculator, with that twist, your bullet should be stable even at subsonic velocity. Even a 200 grn cast bullet is stable below 800 fps.

[bmortell]

on stability calculators if you leave everything the same and move weight up and down stability goes up and down with it. which is new to me I just thought length and rpm was all you needed. but also whats missing is im pretty sure center of mass matters even if its not on the calculators. or maybe that's the only reason why they ask weight is to shift center of gravity?

[country gent]

I have been told by a few length is what you stabilizing but in reality you cant make a longer lighter bullet so weight does play a part. AS mentioned above the solid coppers are longer for a given weigh. In my 300 win mag 180 partitions shoot exceptional the 180 barnes is barely acceptable with slightly oval holes at longer ranges. Same weights but different lengths. The 150 barnes is a stellar performer though. Velocity does play a big role in stabilizing a bullet

[missionary5155]

Greetings
The calculators are not perfect. There are many factors they do not take into the formulas neither can they. With all the variations they are simple guidelines. Much like the "Greenhill" formula... it is a nice guide but is not exact in all applications.
RPM's for your slug from your rifle will differ a bit from my combination when we approach that "instability threshold".
When it happens it can go south fast. But boost the velocity some and maybe it will fly straight another 20 yards further.

[NSB]

First, your 1:18 twist should stabilize the 180g bullet. I have two .357max guns and both have a 1:20 twist barrel (not the ideal twist I might add). They can stabilize a 180g bullet reasonably well. I can get jacketed 180g bullets to shoot five shots around 1.25-1.5" groups at 100 yards. That's stable enough for deer hunting. Being an accuracy nut, it isn't what I want it to do. Shooting 158g jacketed bullets the guns will shoot five shots into an inch or just a bit less at 100 yards. I have pages of data developed with many powders and bullets over the last several years. The last barrel I bought, an MGM barrel for my Encore, is a 1:20 twist. MGM insisted that a 1:20 would stabilize any bullet, even the heaviest ones. Their "experts" said so. I've had several custom built .357 handguns and rifles in the past, really high end guns. The master gunsmiths who built them told me that the best twist for a .357 bullet is 1:16 or a 1:14, with the 1:16 being the most common. I know that from experience they are correct. However, I deferred to MGM's suggestion and it's been an exercise in futility to get great groups from their 1:20 twist with any bullet or load when the bullet is over 158g. Cast bullets are the hardest to get to shoot really well, not just because of the bullets being heavier, but you can't drive them fast enough to stabilize well with a 1:20 twist. Also, as you probably know, every feature of a cast bullet can effect accuracy.....dia of base and bands, lubes, nose profile, etc, etc, etc. Summary: the 180g bullets I did get to perform "well enough" were Hornady 180g XTP's using Rel7 powder. Again, they're very adequate for deer hunting. Don't listen to the seller when buying a Max barrel, they'll sell you what ever they have on the shelf they want to get rid of. Aside from all of this, the MGM barrel does shoot the 158g very well and I shot three deer with it this year, two complete passthroughs and one recovered bullet with perfect expansion and weight retention. Good luck with your gun.

[sharps4590]

You can dance around with all sorts of things or just use the Greenhill formula and be both right and done. I haven't used it in thousands of calculations but for 20 or so rifles it's been on the money.

http://kwk.us/twist.html

[GARD72977]

It is length that you are stabilizing. An example of lighter long bullets would be tracers.

[beagle]

And, I'll throw another variable in the equation. Now, this is my own humble opinion. Bullet design also affects stability. Cases in point are the 148 grain wadcutter in the .38 Special. Loses velocity quickly past 50 yards. This is primarily due to velocity loss. The old touted 200 grain "Police Load" in the .38 Special has been accurate for me out to 100 yards for plinking. One of the attributes is that once it hits something, it goes crazy tumbling through whatever it hits. That's why it was so popular as a man stopper in police use. Another factor is "Where is the center of gravity". I've shot bullets with so-so accuracy, hollow pointed them and accuracy is enhanced. The .30 Ness no ricochet design is a real mystery. Big old long 170 grain lead cylinder with a huge open cavity. I tested these with George XXX for several months one summer. They work as I was firing into a flat looking for ricochets into a berm at 100 yards. None detected after 40 rounds. I fired 5 for group at 100 yards. Went into less than an inch at 100 yards. I like to have had a heart attack at that result. Fired the last 5 rounds and again a sub 1" group at 100 yards. Probably the best I've ever fired with a .30/06.

This matter has been discussed at length for years and will be for many more years. We just don't know at this time.

I do know that rotation plays a big part. You can use velocity to overcome instability as long as your pressure envelope is safe. The design and center of gravity make a difference as well.

All you can do is keep trying and find what works for you./beagle

[Larry Gibson]

CamoWhamo

"The 180gn cast bullets i have tried in the rifle have proven to be inaccurate

As beagle mentions there are many things that affect "accuracy" besides stability. Both jacketed and cast bullets can be stable in flight yet wildly inaccurate. If the bullet holes are round in the 100 yard target they are "stable".

What is the 180 gr cast bullet in question?

What is the length of the bullet?

To specifically answer the question; "Would the stability be the same with a heavier bullet that is the same length as a lighter one It would be very close as there is a constant/factor in stability formulas (including Greenhill's) that accounts for the mass of a bullet. Most lead based cast bullets have a close enough mass to fall within that constant/factor. However, if we seriously change the mass of a given size bullet, say one cast of #2 alloy and one cast of aluminum, then the stability will be much different.

[popper]

Stability has 2 factors. Rotational momentum and CF/CG relationship. Momentum is based on mass (which is related to length) for a given material. So what does momentum do? Keeps the boolit going in a strait line. Air velocity forces act on center of gravity and center of force to cause the straight line path to change to a nutating path (helical spiral). That spiral cone can be decreasing or increasing diameter.It is caused by gyro precession. So 'stability' is controlled by twist (momentum) and boolit design. Then there is the transonic region where shock waves act on the CG/CF.