# Spin stabilized

#### endlessace

Hi all,

I'm a student at a comunity college in Maryland. I'm always interest in weaponry. As to my knowledge, the M16 rifle's barrel has a twist which mean to increase the velocity of the projectile. The projectile would come out the barrel spinning. Just like how quarterback throws a football. I believe that it's because of the "spin stabilized." Tell me if I'm right. Is there a formula for spin stabilized? (Velocity = ????) I need all the information I can get to write a research essay. SO PLEASE HELP.

Thank you
Minh,

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#### damgo

Welcome to our little board! Yeah, that's the definition of a rifle -- the barrel is 'rifled,' or grooved, to spin the projectile. :)

The initial velocity has little to do with the rifling AFAIK. There is something called Greenhill's Rule, which gives you the level of twisting needed to stabilize a lead bullet -- it's like

(distance to go through one full revolution) = 150 * diameter^2 / length

where the diam&length are of the projectile.

#### LURCH

Although rifling does increase range and accuracy, it actually decreases muzzle velocity. As the grooves on the inside of the barrel "grip" the projectile to spin it, they also slowe it down. Additionally, the grooves allow propellant expanding gases to escape around the projectile rather than pushing on it. However, these disadvantages are more than made up for in the greater stability of flight that rifling gives to the projectile. In addition to the gyroscopic effect, spinning the projectile also imparts a self-correcting feature to any minor aerodynamic flaws that may exist.

The great advantage of the military's new advance referred to as "smooth bore" guns is that they achieve this stability a flight without rifling.

#### climbhi

How do these smooth bore guns work?

#### russ_watters

Mentor
Originally posted by climbhi
How do these smooth bore guns work?
Better aerodynamics for the projectile, I think.

#### ObsessiveMathsFreak

Tisk, tisk. Talking about guns. Guns kill people.

Anyway. The reason the bullut is more stable when it spins is the same reason why spinnging tops and gyroscopes stay upright.

It has to do with angular momentum so i assume your formula will have to use that.

#### LURCH

Originally posted by russ_watters
Better aerodynamics for the projectile, I think.
Exactly. Smooth-bore guns are usually found on tanks, and they fire a projectile with fins (like a rocket). This projectile is encased in a shell that makes it form an air-tight seal with the inside of the gun barrell. Since these casings were originally made of wood, these projectiles were known as "Sabo" rounds, after the wooden shoes once worn in Holland. Once the projectile has cleared the end of the barrell, the wooden shell peels off, and the projectile continues onward, stabalised by its fins.

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