From: bercov@bevsun.bev.lbl.gov (John Bercovitz)
Newsgroups: rec.guns
Subject: Re: aluminum bullets
Date: 14 Aug 92 19:11:42 GMT
Organization: Lawrence Berkeley Laboratory, California

In article <1992Aug14.043707.16254@news.eng.convex.com> cash@convex.com 
(Peter Cash) writes:

#In the latest issue of the Atlanta Discount Ammo flier, there's some weird
#.38 special ammo listed: a 33 grain HP _aluminum_ bullet. The manufacturer
#(American Ballistics) claims a muzzle velocity 2,750 fps. This stuff is +P.

#Weird. Can anybody figure out what the terminal ballistics of such a round
#might be like? Would you get any penetration? 

Let's see, 33*2750*2750*2.22*10^-6 = 554 ft-lb.  That sounds like it would
be a little hard on a standard 38 revolver.  Wonder what sort of pressures
they're producing.  Well, one thing's for sure, it wouldn't travel very
far if it's a full-diameter 38 bullet: that light it's going to shed
velocity like crazy.  If it hits something close (while it's still got
some velocity) it ought to blow up on the surface.  Might give good 
penetration on a mouse.

As far as the ballistics go, one way of finding ballistic coefficient
is to calculate it, which you can do if you know the form factor, i.
The formula is C = m/(i*d*d) (Nope, they ain't no pi in there). m in lbs.
Let's assume this aluminum bullet looks just like a standard old 125 gn
38 hollow point.  (This is reasonable because aluminum has one-fourth
the densitry of lead.)  Then we can find the ballistic coefficient of 
this bullet simply by multiplying the coefficient of a regular 38 hp by
the mass ratio (33/125).  'Cept I can't remember what the coefficient
of a 125 gn 38 hollow point is.  Must be .0 something or other.  Well,
the form factor for a cavitied* bullet like that must be at least 1.5
so for the 33 gn bullet, C = m/(i*d*d) = (33/7000)/(1.5*.357*.357) = 
.025 or less.  That's pretty pathetic.  As I mentioned in an earlier
post, you can scale how far a bullet goes in dropping from one velocity
to another by the ballistic coefficient.  (This is because the deceleration
is inversely proportional to the ballistic coefficient.)  Well, a 165
gn 308 bullet has about twenty times as high a coefficient so the 33
gn bullet will only travel 1/20th as far in dropping from one velocity
to another.  I guess a useful figure would be how much velocity this
spit wad has at 50 yards.  That would be 1000 yards to a 165 gn 308
bullet.  I think a 165 gn 308 bullet would have about 1200 fps left
at 1000 yards therefore our 33 grainer would have the same at 50 yards.
The remaining energy at 50 yards would be KE = 33*1200*1200*2.22*10^-6
= 105 ft-lbs.  That surprises me; I would expect less.  You might 
could hurt somebody with this at 50 yards; not bad, but a little.
It's more than a 22 long rifle fer sure.  Kinda in the p*s-em-off
range of energies, I'd say.

Maybe it would do better at 25 yards which would be 500 yards for
the 308 bullet.  The velocity would therefore be down around 1900 or
2000 fps and the energy around 265 ft-lbs.  This might be useful if 
the bullet doesn't blow up.  It all comes down to how well the bullet
holds up.  If it's lightly built (thin in places) it's going to
blow up and not be very useful.  

Another thing to consider is that the way this thing sheds velocity
presents the bullet designer with a tremendous problem: he has to
design it to behave properly over an extravagant range of velocities.
He can't just design it to work at 25 yards, obviously.

John Bercovitz     (JHBercovitz@lbl.gov)

* "cavitied" - an ad hoc word meaning having a hollow point.  8-)