Physics-type question: what's the longest bullet that can be stabilized?

[PercyShelley]

I was reading about projectiles earlier, and I came across a bold statement that projectiles longer than six calibers cannot be spin-stabilized. Why is that?

 

[mc223]

I do not know for sure, but my initial thought would be that the twist required, would be so fast that the projectile either would not be able to be accelerated through the barrel. Or the projectile would "strip" and not rotate at all. Or the projectile may dissassemble itself from the excessive rotational force upon exit from the muzzle.
Try the link below and plug in some numbers for fun.
I used a .243 at just over 6 calibers long. Came up with a 1 in 5.9 twist for velocity under 2800fps.


http://www.netrifle.com/shortmags/ref_data/TwistRateCalc.asp

 

[Afy]

Artillery Shells are routinely longer than 6 inches as were those fired from the 21 inch guns.
Both are spin stabilized...

Whoops... read inches...

 

[PercyShelley]

21 inch guns shot 126 inch long projectiles?

Really?

Because the shells I've seen at museums were reather stubby affairs.

 

[Walkalong]

longer than six calibers

longer than 6 inches......Whoops... read inches

Artillary shells were longer than 6", but not longer than 6 times the caliber.

I don't know the answer either, but twist rate, like mc223 said, would have to be to fast to be feasable.

 

[Darth Muffin]

My bet is that once the projectile gets too long (in proportion to it's width) it starts to precess from the spin. Sort of like how a top will start to wobble. If you increase the width, then there's more angular momentum and it's stable. If you decrease the length, then you decrease the tendency to precess and it's stable.

You can fin-stabilize longer projectiles though. A good small-caliber example is shotgun flechette rounds.

 

[cracked butt]

A good small-caliber example is shotgun flechette rounds

Or an arrow or an antitank round also.

In my little bit of experience in this matter that might be analogous: fast twist barrels in small calibers are very difficult to shoot long cast bullets through with anything other than moderate to slow velocities. The quiticential example of this is the swede mausers- they have roughly a 1:7.7" twist rate. If you try pushing a heavy bullet much over 1700 fps, everything goes to hell. Every flaw in the bullet it greatly magnified with the type of spin put on them. Jacketed bullets are much more 'perfect' than cast bullets and can be driven to higher velocities with more spin, but like lead, they have their limit also.

 

[Grump]

Depends on what medium you're shooting them through, too. I'm no physics dude, but I know there's some fluid dynamics stuff going on with stabilizing flying objects which have a center of pressure ahead of their center of gravity.

 

[Idaho Sharpshooter]

I see, from personal experience, that any bullet much over 4.5 calibers long is difficult to stabilize. I shoot a 22-284 with a 9" twist and the 1.114" 80gr JLK bullets really need the speed to stabilize them, like 3500+fps. The budget version is a Savage 12SS in .223 rechambered and the .223 bolthead switched for a .308 bolthead . It makes a 22CF a legitimate 600 yard Rockchuck rifle here in Idaho.

Rich

 

[tasco 74]

Interesting Thread!!

i think i've learned something today!

 

[Cosmoline]

It's an excellent question. AFAIK the longest bullets in proportion to caliber that can be fired from a normal centerfire rifle are the heavy handloads in the big fifty, with sectional densities over .4. In contrast, a 195 grain 7mm bullet has a sectional density of just .345.

 

[BAGTIC]

The general rule of thumb is that maximum bullet length is 6 times bullet diameter.

Sectional density has nothing to do with it. The basic rule applies to aluminum bullets as well as tungsten carbide/uranium bullets.

All spin stabilized spitzer bullets tend to tumble when fired into water. Some RN designs are a more stable.

Drag stabilied projectiles will continue straight without tumbling under water.

A common myth is that firing a long bullet faster will stabilize it better because the higher rpm generates more centrifugal force. It is generally not so. Just as centrifugal force incrases as the square of the spin rate so does the air pressure on the nose, which is what causes the bullet to tumble, so much so that in the practical world the two effects counteract each other. The whole thing becomes a wash.

 

[Mal H]

A few questions BAGTIC - you say air pressure on the nose of the bullet increases as the square of the spin rate. How is the spin rate a factor at all in figuring nose air pressure?

You mention centrifugal force (a fictitious force) as a stabilizing force, do you mean gyroscopic inertia?