Path: utzoo!utgpu!watmath!clyde!att!cbnews!malloy@nprdc.navy.mil
From: malloy@nprdc.navy.mil (Sean Malloy)
Newsgroups: sci.military
Subject: Re: Tank question
Message-ID: <6156@cbnews.ATT.COM>
Date: 2 May 89 03:29:53 GMT
References: <6077@cbnews.ATT.COM>
Sender: military@cbnews.ATT.COM
Organization: Navy Personnel R&D Center, San Diego
Lines: 39
Approved: military@att.att.com



From: malloy@nprdc.navy.mil (Sean Malloy)

In article <6077@cbnews.ATT.COM> aws@vax3.iti.org (Allen W. Sherzer) writes:
>My dad has a question: Why do modern tanks use a smooth bore barrel
>on their guns? Doesn't that hurt accuracy?

The purpose of rifling a barrel is to spin the projectile so that it
is more resistant to wobbling in flight, which generates unbalanced
aerodynamic forces that reduce the accuracy of the gun.

Most of the shells that are used by front-line tanks come in two
types: APFSDS (Armor Piercing Fin Stabilized Discarding Sabot) and
HEAT (High Explosive Anti Tank). An APFSDS round is essentially a
large metal dart -- a (relatively) thin pointed rod with fins on the
back that stabilize the round the same way an arrow is stabilized, so
that the shell doesn't have to be spun for stabilization. The rifling
of the barrel places additional drag on the shell (in order to
transform some of the driving force from accelerating the shell to
spinning the shell), and also wears down the rifling. Without rifling,
the shells can be fired at higher muzzle velocities, and reduce the
barrel wear.

For HEAT rounds, the answer is a lot simpler. HEAT rounds, also called
'shaped-charge warheads', depend on the geometry of the warhead to
generate a high-speed jet of molten metal from a metal facing on a
conical depression in an explosive charge, which penetrates the
target's armor. If you spin a HEAT round, the rotation tends to force
the metal facing of the explosive away from the centerline of the round
when it detonates, so the warhead can't form the jet properly -- the
penetration goes _way_ down.


 Sean Malloy					| "The proton absorbs a photon
 Navy Personnel Research & Development Center	| and emits two morons, a
 San Diego, CA 92152-6800			| lepton, a boson, and a
 malloy@nprdc.navy.mil				| boson's mate. Why did I ever
						| take high-energy physics?"