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Ballistics etc

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This came up in one of the threads, and I was asked to copy it here for reference purposes..

It is an EXTREMELY simplified explanation of external and terminal ballistics, leaving out the maths and twiddly bits.. If any ballisticians wish to wade in and embroider, then please feel free...

There are broadly two approaches to achieving projectile stability: gyroscopic stabilisation and drag stabilisation...

Gyroscopic stabilisation relies on the property of a spinning object to maintain its position along its axis of rotation. In free flight, the object will resist deflection and remain on the axis it was initially spun on. How stable the projectile stays is dependent on two main factors - the length/width ratio of the projectile and the rate of spin. All things being equal, the longer and thinner the projectile the faster you have to spin it to keep it stable. Thus, a sphere is the easiest to stabilised by spinning and a long rod the hardest. In practice the maximum ratio you can spin stabilise a long projectile is about 1:5 (width to length). In a rifled gun, the twist of rifling has to increase the longer you make your projectile. Hence the change in rifling between the original (short bullet) 5.56 M16, and the replacement (long bullet) NATO 5.56 - this also explains why you cannot fire conventional bullets out of a rifle designed to fire balls...

Drag stabilisation relies on having the centre of drag in a projectile behind the centre of mass as you would do with an arrow. As the projectile moves through the air, pressure on the rear part of the body(e.g the feathers) will tend to drag back the rear part of the projectile, keeping the point aligned. Whilst it is advantageous to give drag stabilised projectiles a slow spin, this is to even out errors and does not in itself give stability. This method of stabilisation was used on the 84mm Carl Gustav and all finned projectiles such as mortars etc.

The choice as to which method of stabilisation you use, depends mainly on the characteristics of the projectile you want to launch. Having said this, rifling a barrel does cause increased friction and the barrels are more susceptible to the effects of wear at the commencement of the rifling. (both smooth and rifled guns wear, but has a greater effect on rifled systems)...

Spin stabilised projectiles are generally more compact and robust and are more reliable in battlefield conditions. Drag stabilisation is inherently more difficult to make work, smooth bore barrels are difficult to make accurately (apparently) and the projectiles are less robust. The two main reasons in the past for using drag stabilisation is that either you cannot spin the projectile (because it has a shaped charge in it...) or you need to get more weight/mass into it by making it long and thin to punch through armour.. ( e.g APFSDS).

Manufacturing drag stabilised projectiles (e.g darts) and launching them from small calibre systems has proven to be extremely difficult in practice. The required tolerances and variabilities are much greater than you need for the equivalent rifled/spun systems.

Terminal ballistics are about what happens when a projectile hits a target. The key to good terminal ballistics is to cause a reliable transfer of energy from the projectile to the target. The easiest way to achieve this is to cause the diameter, and hence the impact area to increase on impact (or better still inside) the target. This is generally achieved in hunting ammunition by using hydraulic shock to expand the bullet by entering a hollow nose or by deforming a soft tip.

As you are aware, this approach is not considered "cricket" on the battlefield, and so full metal jacket ammunition (FMJ) is used, which is not Designed to expand. The terminal effect with FMJ is triggered by a reduction in the spin rate. When the spin rate falls below the critical level for the bullet size, the bullet "tumbles". This tumble motion is not immediately "tip over base"!, but is caused by the tip yawing and then spinning in an ever increasing spiral ( watch a spinning top running out of steam to get the idea). The drag increases rapidly and the whole projectile loses dimensional stability. Some projectiles (in particular the German 7.62) have a habit of breaking up across the cannelure at this point, although I am sure this is not the intended effect. The Russian 5.45 has a very high critical spin rate and is particularly good (or bad?) at performing this manoeuvre. Remember that in a spun projectile, much of the available energy to do damage is derived from the rotational momentum of the projectile and not just from the linear velocity...

Spin decay is usually the factor that limits the maximum effective range of a projectile. NATO 7.62 starts to tumble at around 1100 -1200m. The bullets will fly a bit further but they are much less capable of doing damage at this distance...

With regard to tolerances, remember that economics are a factor in good weapon design. High tolerances cost money, and whilst you may be prepared to pay this for a specialist anti armour projectile such as an APFSDS costing hundreds of pounds a pop, this is not realistic for small arms ammo at less than a pound a shot...

... and finally let me repeat my warning about the lethality of tracer! Most of a tracer bullet is made up from a pyro pellet which gets burned up in flight. By the time it reaches the target there is not much mass left to do damage. Tracer will bounce off a wet T shirt (well almost..) watch the fireworks at any night shoot if you don't belive me. 1B1T is prettier than 4B1T but is about half as lethal.

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