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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; 40559 views.
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.

roguetechie

From: roguetechie

14/3/21

There's also another issue though. Bringing something to a very incredibly sharp point will tend to result in a high Length to diameter ratio bullet that's light for a given L:D.

If you think about a case telescoped round and the way the nose caps for them work it also means that you must extend the nose cap back further resulting in less space for propellant because modern CT rounds aren't actually telescoped so your nose cap would have to extend back far enough to help stabilize the projectile from primer ignition until after it jumps through the freebore in the barrel throat.

graylion

From: graylion

14/3/21

This is the shape I am currently working with. I'd defo call that pointy enough. 6.5 mm, with a 3.5 mm penetrator. Bullet is 20mm long, might be possible to make longer with CTA

  • Edited 14 March 2021 4:20  by  graylion
EmericD

From: EmericD

14/3/21

poliorcetes said:

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

During our last radar shooting session, we fired bullets as pointy as mechanically possible, loaded one at a time in the rifle chamber, and there was no particular improvement of the BC.

What some sport shooter (or manufacturers, now) are doing, is reducing meplat diameters that were generally in the 1.6-1.7 mm vicinity, down to 1.0-1.2 mm, but up to now I didn't see any benefits of decreasing the meplat diameter below 0.8 mm.

roguetechie

From: roguetechie

14/3/21

Yeah for what you're doing it should be fine. The solution space you're working in is going to be brute force over absolute aerodynamic refinement anyway and what you've got there is substantially better shaped than most other pdw rounds by quite a bit.

I was mostly answering poliocretes questions with what I wrote last night not criticizing what you've come up with. I'm not certain I 100% agree with the approach you're taking but it's workable for sure.

graylion

From: graylion

14/3/21

wasn't feeling attacked, no worry; sorry if that accidentally sounded tetchy. BTW were you going to say something about bringing RoF down?

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