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tubular bullets   General Military Discussion

Started 7-Jan by smg762; 2506 views.
nincomp

From: nincomp

9-Jan

A great deal of what is generally called barrel wear is actually throat erosion from propellant and hot gasses rather than friction from the bullet.  For example, it is not uncommon for benchrest shooters to take a "shot out" barrel, cut a short section of the base off, then rechamber it, cutting out the first couple of cm of damaged rifling, then continue to compete with it.   Throat erosion tends to be worse with overbore cartridges.   Particles of unburnt and partially burned propellant are moving at high speed, mixed with the high temperature, high pressure gasses erode the throat and first section of barrel.  

A bullet with deep enough striations to significantly reduce the frontal area will have an unusually high surface area.  Most of my knowledge is for incompressible (subsonic) flow, where this would dramatically increase surface drag.  My guess is that each projection would create its own shockwave, increasing drag, but there are others here who know more.

ZailC

From: ZailC

9-Jan

Striations on a bullet will be inside the tip (or shoulder) shock cone until the bullet goes subsonic. Subsonic they will interfere with smooth flow and act to decelerate the projectile. As for adding spin, it's not necessary and could as easily decrease the spin rate. Striations would also take mass away from the perimeter and adversely effect yaw moments; there is no free lunch. The idea seems akin to adding dimples like a golf ball's. 

Red7272

From: Red7272

9-Jan

smg762 said:

Well as my posts often show I'm a proponent of calibers around. 17 or so.  With very heavy-for-caliber bullets,  you get extreme range yet very low recoil and weight. 

How are they supposed to be heavy for weight? The 2 gram, 4 mm saboted tungsten projectile has been done as a pistol round. No takers because it is not economically viable and of dubious lethality. 

https://en.wikipedia.org/wiki/6.5%C3%9725_mm_CBJ

ZailC

From: ZailC

9-Jan

The actual diameter of a tubular projectile isn't the limiting factor - it's the ratio of inside diameter to length that determines when "choking" becomes a problem. The Mach angle off the projectile's leading edge is the inverse-sine of the local speed of sound divided by the projectile velocity. Choking begins when the interior shock cone is shorter than the interior passage. 

Conducted experiments in the early 1990s with tubular brass projectiles: outside diameter 0.338", Inside diameter 0.275", length 0.375", weight about 4g IIRC. Launched stacks of three from a sandbag-rested 24" .338 Win Mag rifle using an aluminum pusher plug with a Lexan obturator, at about 3,400 feet per second IIRC (M 2.9). They were stable (round holes) at 200 yards and all within a 2" bullseye, but into the dirt by 400 yards.

In reply toRe: msg 24
smg762

From: smg762

10-Jan

I should have been more specific,  a problem with. 17 cal is more to do with fouling than barrel wear. I don't know if a polymer coated bullet could reduce this fouling.  Combine that with the fact that my velocities would be way below the usual 4000fps. 

Again this 'perfect' 17 projo would get it's weight from being very long,  not a pricey tungsten core. 

The striation idea was to emulate all this without the fouling. 

You could have,  say,  three half-moon cutaways for your striations.  The opposite extreme would be two sharp v-shape cuts.  could either of these work I wonder. 

All this stuff would only run along the bearing surface,  not the nose.

My last idea was to use a 'foul friendly' caliber of. 19 cal.  Then a squeezebore takes it fractionally down to. 17/. 18.  HKMR556 rifles have a very tiny squeezebore.  I suspect it could ruin the B.C. and create inconsistent shapes with each shot. 

Regarding tubular ammo,  if a tank round is stable to 1500m,  then in theory 600m is doable.  Perhaps you'd need a minimum of 762 power levels to achieve this

EmericD

From: EmericD

10-Jan

smg762 said:

To get a visual idea,  Google bullet striations and look at the 9mm rounds.  Imagine that but Much deeper.  They could either be straight,  or angled which would add spin to bullet. 

Bullet striation will increase the Clp of the bullet if they are straight, or will increase the drag if they are angled (in order to keep the RPM high).

They are already used on some 12 Ga. slugs, like the Brenneke or the Foster, so it's a pretty "short range" solution.

A more interesting variant are found on extended range 155 mm shells, but that will increase the price of your bullet!

smg762

From: smg762

10-Jan

Presumably the 155mm must be manually loaded in the breech to slot the Finns in the rifling. 

Any thoughts on that guys CD ROM shaped bullet? 

https://patents.google.com/patent/US20140150319A1/en

autogun

From: autogun

10-Jan

smg762 said:

Presumably the 155mm must be manually loaded in the breech to slot the Finns in the rifling. 

As I understand it, the canted fins are of bore diameter, not groove - they do not engage with the rifling at all. Their purpose is to position the shell centrally as it passes down the bore; without them, the length of the ogive means that the shell would be unstable in the bore. I believe that the fins also have the incidental advantage of increasing the range, as they provide some aerodynamic lift.

smg762

From: smg762

10-Jan

Tony are u saying that the projectiles only contact with the barrel is via those fins.... wouldn't that be placing an awful lot of weight on those fins.. how do they even obturate into the rifling. 

Also surely the canted fins would remove the need for rifling. 

Lastly,  It seems like a costly but effective solution to the dilemmas described above? 

autogun

From: autogun

10-Jan

No, the pale grey ring is a driving band of groove diameter so it provides obturation and takes the rifling. The shell body immediately above the driving band is of bore diameter, so the shell is supported in the barrel in two places.

The fins are not remotely enough to provide adequate spin-stabilisation - that's the job of the barrel rifling.

These shells are obviously more expensive to make, but the design does provide a considerably greater range than the standard 155mm M107 without any loss of HE capacity (in fact, it has more). The M107 still gets used a lot at shorter ranges. 

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