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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; 40102 views.
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?

JPeelen

From: JPeelen

14/3/21

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 It was superceded by Report 1306, but figures 5 and 6 are practically the same in both.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

14/3/21

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

14/3/21

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.

In reply toRe: msg 326
graylion

From: graylion

16/3/21

Also thinking that the gun should be issued with a shmuzzle, so that if you have time you can attach the can and make the whole thing less of a nuisance to yourself an others.

graylion

From: graylion

24/3/21

roguetechie said:

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.

BC is atrocious though because of the low density

Red7272

From: Red7272

24/3/21

graylion said:

BC is atrocious though because of the low density

You don't need a PDW you can just use a assault rifle like the AK-74u with a thigh holster.

gearmrk

From: gearmrk

30-Mar

The calculator on that website massively miscalculates Meplats.  Can't use it. It miscalculates another parameter as well, but I can't remember at the moment.  It will make you think pointy bullets are great when the optimum low drag Meplat range is 0.08 - 0.12 like someone said earlier.  

Years ago I asked people to stop using that drag calculator due to its issues.  

gearmrk

From: gearmrk

30-Mar

0.15 to 0.18 calibers actually.

graylion

From: graylion

30-Mar

Good to know, thanks. Not sure how relevant that is for this design due to short range and brute force approach. Also pointy being useful here because armour penetration.

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