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Military Guns and Ammunition

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This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.

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PDW again   Small Arms <20mm

Started 20/12/20 by DavidPawley; 40657 views.
roguetechie

From: roguetechie

12/3/21

You quickly run into issues with both length to diameter ratio and aerodynamic issues.

Bullets are like aircraft In that their exact shaping determines how much drag they will experience across the various velocity regimes.

What's ideal at 3300 feet per second isn't the same as what's ideal at 1500 feet per second, so you wind up making compromises to get acceptable and stable flight characteristics across the entire range of velocities your projectile could be traveling at when it impacts the target.

As you've pointed out though, you also have to consider it's impact characteristics too.

This puts you in a position where you need to have acceptable terminal performance and flight characteristics throughout a pretty large velocity window.

Because of this, no matter what you're doing it will be a compromise.

In the case of making the bullet tip needle fine, this can create drag and or stability issues that you don't want.

This is why bullet tips are shapes as they are. Unfortunately I can't remember the name of the term specifically for the bullet tip shape and fineness ratio at this time though.

I'm sure someone else will come in and expand on what I've said though.

graylion

From: graylion

12/3/21

This is my current bullet shape. full spitzer, no meplat. 

geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=mm.&weight_unit=grains&bullet_name=GLS+PDW+III&re_calculate=yes&boundary_layer=L%2FT&diameter=6.5&length=20&nose=14&meplat=0&drive_band=6.72&base_diameter=5.5&angle=8.9&boat_tail=3&secant_radius=5.12&weight=34.5&density=5.25

And yes, I agree that CTA would make it possible to have the spitzer actually be 'spitz' (pointy in German).


Hardened Steel penetrator with polymer rest of bullet. My current v0 is 4100 fps, Which I am sure could even be improved upon with CTA and higher pressures. Mind you, I am at ca 560 MPa.

  • Edited 12 March 2021 13:30  by  graylion
poliorcetes

From: poliorcetes

12/3/21

I would like to understand that aerodinamic issues: a really pointy bullet would not be so much different compared with a pitot tube on a mach 2 fighter...

Msg 7776.312 deleted
renatohm

From: renatohm

12/3/21

The Pitot tube isn't expected to keep its shape upon impact...

I seem to recall that pointy projectiles tend to break apart even at very shallow impact angles, which means that their usefulness may actually backfire if the impact angle isn't perfectly aligned.

EmericD

From: EmericD

13/3/21

poliorcetes said:

Looking that old 6.5 CT cutaway, I had a simple and possibly stupid idea: since bullet tip is totally surrounded by plastic case... why don't make it as pointy as possible, in order to improve slightly aerodinamics and armor penetration?

Depends of you definition of "as pointy as possible".

We tried bullets with "needle shape" ogive with Sears-Haack profile. They should be babied during the whole manufacturing process, babied during the packaging process, babied during the cartridge loading process, they don't bring any exterior ballistic advantage compared with bullets with a meplat of 0.8-1.2 mm, and the tip is so fragile that there is no terminal ballistics advantage.

If you can damage the bullet tip just by dropping the bullet on hard ground with an impact velocity of a few m/s, this tip will not bring you any advantage at impact velocity of several hundreds of m/s.

It's a known fact that the best penetration results are achieved with pretty blunt cores with a ballistic cap.

graylion

From: graylion

13/3/21

EmericD said:

If you can damage the bullet tip just by dropping the bullet on hard ground with an impact velocity of a few m/s, this tip will not bring you any advantage at impact velocity of several hundreds of m/s.

But the point @poliorcetes made was that the CT case protects against that. 

stancrist

From: stancrist

13/3/21

graylion said:

EmericD said: If you can damage the bullet tip just by dropping the bullet on hard ground with an impact velocity of a few m/s, this tip will not bring you any advantage at impact velocity of several hundreds of m/s.

But the point @poliorcetes made was that the CT case protects against that.

Even so, there are still the other issues noted by Emeric:  "They should be babied during the whole manufacturing process, babied during the packaging process, babied during the cartridge loading process, they don't bring any exterior ballistic advantage compared with bullets with a meplat of 0.8-1.2 mm, and the tip is so fragile that there is no terminal ballistics advantage."

poliorcetes

From: poliorcetes

13/3/21

Thanks as always, Emeric

However, I would like to insist a little bit: given that point is protected inside telescopic part of the case, a meplat decreased by an order of magnitude would not affect precision? given that some sport shooters do preciselly that... I mean, it's quite clear that conventional auto rifles don't use sharp-pointed bullets because if point is damaged during chamber insertion, grouping will be worsened

roguetechie

From: roguetechie

14/3/21

https://en.m.wikipedia.org/wiki/Meplat 

This was the term I was thinking of and in the wikinit clearly states that a correct meplat configuration can be consistently calculated that will give you your best supersonic performance.

You can go full smooth pointy but it will induce higher drag and etc. As I understand this you are essentially separating the boundary layer to keep more of the projectile out of direct contact and thus reduce aerodynamic resistance and velocity loss.

That and boat tail angle can and do have a pronounced effect on projectile performance.

And even you consider the fact that bullets just flat perform better terminally and more consistently the more velocity they have at impact v is squared in the relevant energy equation which means that additional velocity adds up way faster than adding weight to the projectile when it comes to your impact energy budget.

Hopefully this answers your question. 

Note: citation 6 in the wiki is the relevant NACA paper dealing with optimal meplat design.

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