Rifling question…

Thanks for the plug Oli 🙂 I do have to make one correction, though: the bourrelet is at the front of the shell and is carefully machined to fit inside the bore. The bit that takes the rifling is the driving band (or, more logically, rotating band in American) at the back, which is made of a softer material (usually copper, but soft steel in some cannon and plastic in one) which both seals the bore and is spun by the rifling.

Some early large-calibre muzzle-loading rifled cannon did have studs in the shell bodies which were lined up with the grooves as the shells were loaded. On a present-day fired shell, the driving band has grooves but that’s the result of the shell being fired – they weren’t there at the start.

There is indeed a risk that the violence of firing will ‘strip’ the driving band so it is no longer gripped by the rifling. This is why some ammo uses steel bands. Some guns also have progressive rifling, which starts out parallel with the bore but then twists at a gradually increasing rate, to spin the shell up more slowly.

Tony Williams: Military gun and ammunition website and discussion forum

There’s no teeth on the shell/ bullet. The outside diameter of the round is larger than the nominal bore/ calibre (which is measured across the smaller diameter that can be seen – ie across the “tops of the teeth” in the barrel). The driving band ( bourrelet) or jacket (on small-calibre bullets) deforms is of a sufficiently large diameter to fill the gaps to the “bottom of the teeth” – which is what leaves the striations that police forensic scientists look at when they do a ballistics check.
There’s no need to line up anything, the outside diameter is “perfectly” circular.
Typical bullet actual diameters are not what they’re called eg 5.56 (NATO SS109 round) is 5.656 mm, 20 mm shells are actually about 19.9 but the bourrelet is large enough to fill the gaps (sorry I can’t provide more figures, I’m not at home and all my note books are 🙁 )
The only calibre I can remember that actually has a bullet diameter that is what it’s called is 9 mm Parabellum, which also happens to weigh ~9 grammes.
Tony Williams’ site – http://www.quarry.nildram.co.uk/ will have information of that sort, and he usually posts on here as well. I recommend his books, they have (OK I admit I’ve only got the autocannon one so far) huge data tables at the back with that sort of data.

So do the “teeth” on the bullet or shell engage the rifling on a one-to-one basis or is there some sliding going on? AFAIK the plastic driving bands on a 30mm round on the GAU-8 deform to conform to the rifling of the barrel. But with rounds that have “teeth” on the side how do they get them lined up with the rifling or do they even need to?

The seal has to be more or less high-pressure gas-tight, otherwise the propellant gasses would just scream through the gaps and leave the projectile in the barrel.

Something I’ve always wondered is how close do the grooves on the shell/bullet engage the rifling? Lead being as soft as it is I’m surprised they don’t just strip right off the bullet (on lead bullets of course). Let alone the PLASTIC bands on the rounds of a GAU-8.

Still trying to figure out why, when I was a kid, I had a smooth bore BB gun that was incredibly accurate, but I lost it….in later years, when I picked up the same model, they had rifled it, and it was always just a bit less accurate than the smooth bore had been…

Very simply put the purpose of rifling is similar to the purpose of spinning a top to make it stand on its end. Obviously with a round projectile the effect of keeping one end pointed one way is less useful, but if you think about it the purpose of tail feathers on an arrow or fins on a rocket, or for that matter a stick on a Skyrocket, or the plastic cone on a shuttlec0ck is to have a high drag item at the rear of the missile to keep the nose pointing forward. With a homogenous item like a bullet with no high drag bits like fins or feathers the only way to reliably get it to fly point forward is to spin it in flight. Without spinning the pointed nose would end up pointing backwards because it weighs less than the rear. (note on impact with a target every pointed bullet eventually tumbles whether it is designed to or not because the rear of the bullet has more mass than the front. Unless the bullet deforms (is soft nose bullet that mushrooms on impact to make a bigger hole) it will generally end up tail first. This is because the impact ruins the stabilisation of the spinning so the bullet starts to perform as it would if it wasn’t stabilised.

In other words for ball projectiles the effect of rifling is not that noticible, it is with longer bullet shaped rounds that the effect is easy to see on target. It is also quite clear that longer bullets are rather more effective than round projectiles for a given calibre.

Still trying to figure out why, when I was a kid, I had a smooth bore BB gun that was incredibly accurate, but I lost it….in later years, when I picked up the same model, they had rifled it, and it was always just a bit less accurate than the smooth bore had been…

m

Sorry, but wrong wrong wrong wrong.

I’m unclear on this concept…what is rifling in naval guns all about?

Rifling in any guns that have it are spiral grooves from the chamber to the muzzle that spin the projectile as it travels down the barrel. The projectile type determines what rate of twist will stabilise the projectile properly in flight. A very tight twist is needed for long narrow projectiles while a slower twist will stabilise a short fat round. Over stabilised rounds will wobble in flight and this will reduce their accuracy. Understabilised projectiles will also deviate from normal flight and be less accurate. As an example the 5.45mm standard Russian assault rifle round has a rate of twist of about 1 turn in 20 or so cms. At 900m/s this means the projectile is spinning at roughly 4,500 revolutions per second.

Excuse the naive viewpoint, but I always thought rifling meant the number of times the shell spun after leaving the barrel of a gun

Rifling means the barrel is not smooth (like a shotgun) and has grooves to make the bullet spin.

(and increased stabilization upon leaving the barrel)….so as in the case of the 16″ guns of .45 caliber, they would spin less than the 16″ guns of .50 caliber, the .38 caliber would spin less than the .45, yadda yadda yadda…..someone care to clarify for me?

A Naval 16″ gun, means that the gun has a calibre of 16 inches. In other words the diameter of the shell the gun fires is 16 inches across. The calibre of a gun describes the diameter of the barrel internally (ie the diameter of the projectile it fires). A .50 cal gun fires a projectile that is 0.5 of an inch in calibre, or 12.7mm in diameter.

The rate of twist of the rifling in the particular barrel determines the rate of spin of the shell. For example the D-25 gun carried by the WWII JS-2 Tank is described as the 122mm L/43 M1943 Tank Gun. This tells us the calibre of the gun is 122mms, and the L/43 tells us the length of the barrel is 43 calibres long, or 5.246m long. It tells us nothing of the rate of twist of the rifling or even if the gun is rifled at all, though because we know the 115mm gun of the T-62 was the first smoothbore gun fitted to a MBT we can assume this gun is rifled. (Smothbore guns have the advantage of lower internal drag/friction and can be made shorter and lighter than rifled equivelents. As most naval guns are artillery or anti aircraft weapons most will be rifled for accuracy rather than smoothbore for penetration).

You’re on the right track Mark, the faster the spin the more stabalised the shot and thus the further it can travel and do more damage.

Some guns have a smooth bore which decreases the friction when the shot travels down the barrel. Certain shots also are fired from non rifled guns (rocket assisted shots, fin stabalised shots ect), though I haven’t heard of these being used by a navy, US Army ravine patrol craft in Vietnam did use them and I think that quallifies here.