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Fri, 21 Sep 2001 19:09:00 +0800
To: sci-military-moderated@moderators.isc.org
From: Paul Saccani
Newsgroups: sci.military.moderated
Subject: Re: Infantrymen's weapon
Date: Fri, 21 Sep 2001 19:09:13 +0800
Reply-To: safer.roads@iname.com
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On 20 Sep 2001 13:11:20 -0700, gherbert@gw.retro.com (George William
Herbert) wrote:
>I'm not familiar in detail with subsonic bullet ballistics,
>but from my knowledge of aerodynamics, drag will drop off a lot
>once they go subsonic, which is around 1,000 meters out
>for standard 7.62 rounds. It's worth noting that most boattail
>bullets are nearly ideally the perfect shape for transonic and
>subsonic low drag flight.
>
>If fired such that the bullet will be headed up at 45 degrees
>flight path at the moment it goes subsonic, and assuming that the
>bullet's drag goes to zero at that point, the bullet would travel
>something like 11 km. It goes subsonic about 750m alt, 660m downrange,
>two seconds out, then does a ballistic arc going about 220 m/s horizontally
>for about 45 seconds down to 750m altitude again, then continues about
>another 750 meters downrange as it descends to initial firing level
>again. That works out to a total range of 660 + (44.2 * 221) + 750 meters
>or 660 + 9770 + 750 = 11,180 m. If you assume fired straight downwind
>with a 20 m/s wind (72 kph / 45 mph) times 48 seconds that's another
>nearly kilometer of range.
For someone not familiar in detail, that is a damn good effort!
>All of that said, I don't have a handle on how good the "and then drag
>goes to zero" maps to actual subsonic aerodynamics of rifle projectiles
>at extreme ranges.
The Cd changes at mid arc, typically. The bullet spins down until it
is unstable, goes through a transient and dramatic increase in Cd (but
at a relatively ow speed, with high potential energy, so little energy
is lost) as the bullet tumbles and then stabilises travelling base
first. This reduces the Cd dramatically, but of course makes it
essentially impossible to predict where the bullet will land, giving a
beaten zone about 1200 m by 500 m, which is rather large.
>I do however find that a bullet / bore forensics match to a particular
>Australian service rifle
Australian made, not necessarily being used by Australians. At least
three other countries were using Australian made L1A1, PNG, NZ and
Malaysia.
> and a bullet removed from the shooting victim,
>and the geography involved, are somewhat indicative of a relatively high
>degree of certainty that the event actually happened very closely as
>described, particularly as Paul knows enough about the incident to have
>remembered the serial number of the rifle involved...
The serial number had a personal significance which made it
unforgettable (I hope).
>a pointer to the
>report would be better, but I find the basic report credible.
I don't know where you would find that (at least in a publicly
available form, the autopsy report would be at GH Ipoh. It happened
near Ipoh in Perak State, Malaysia in the early eighties. Three or
four years ago, it came up in conversation with a friend, and to my
surprise, it turned out that he had been next to the girl, who was his
niece, when this happened!