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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; 38660 views.
poliorcetes

From: poliorcetes

13-Mar

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-Mar

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-Mar

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-Mar

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-Mar

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-Mar

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-Mar

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

JPeelen

From: JPeelen

14-Mar

If you look at figures 5 and 6 of the NACA Report 1306 (which is reference 6 mentioned in roguetechie's message; the report can easily be found by searching for "Eggers" "Resnikoff" "Dennis") the graph in figure 6 shows how small the drag gain is that can be achieved by going from projectiles having a 3 caliber long forebody to 5 caliber. 

Figure 5b shows in my view quite well how fragile these shapes in bullet manufacturing and handling would be.  

I think it is too often overlooked that drag coefficients shown are valid for "zero lift" condition (see fig. 6).  That means, the bullet must have exactly(!) zero yaw. That cannot be achieved with a real world, spin stabilized projectile, in particular if its very pointed. The already very small advantage (compared to the mechanical expenditure) then even gets smaller.

P.S.: When searching, NACA TN 3666 (same authors, same title) will also show up. It was superceded by Report 1306, but figures 5 and 6 are practically the same in both.            

ZailC

From: ZailC

14-Mar

Agree, the problem with an overly long forebody is its reaction to yaw on launch. The tip has a long moment arm that delays stabilization resulting in early velocity/energy losses. These loses are amplified by the relatively poorer rotational moment of a long projectile compared with a shorter projectile of the same total mass. Similar losses are to be expected from instabilities on transition to subsonic flight, possibly worse depending on boat-tail design details. Also note that a "good" boat tail can help to minimize yaw on launch amplified by interactions with the reverse flow (out to as much as 10-15 calibers) when compared with square-butted projectiles. 

roguetechie

From: roguetechie

14-Mar

I wanted to say something yes but I'm not convinced that I understand the mechanical situation well enough for my input to be valid...

I've been consulting my reference library and etc as I get time but I've been splitting my attention quite a bit. I just got a new glock kit in and have been working on some other things too so distractions are a part of my issue currently.

As soon as I get this gun built and my new night sights on it (it shipped with just straight up black fronts and rears!) I'll be able to focus more.

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