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From: bercov@bevsun.bev.lbl.gov (John Bercovitz)
Newsgroups: rec.guns
Subject: Re: Reloading questions: 40 S&W and 45 ACP
Message-ID: <35681@mimsy.umd.edu>
Date: 15 Jun 91 02:56:18 GMT
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Organization: Lawrence Berkeley Laboratory, Berkeley, California
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In article <35657@mimsy.umd.edu> ke4zv!gary@gatech.edu (Gary Coffman) writes:

#I'm going to tackle this and ask the real experts to straighten out any 
#mistakes I might make. I've got a couple of questions of my own.

##In article <35456@mimsy.umd.edu> n9020351@unicorn.cc.wwu.edu 
##(James D. Del Vecchio) writes:

##Per mag maximum chamber pressure is about 35,700 cup for 38 super, 40 S&W,
##and 10mm Auto, vs about 19,900 max for 45 ACP.

##Why is this and what significance of it?  Are the 45 cases weaker for some
##reason, or is it the construction of the pistols?  Is it possible to make
##a 45 withstand the higher pressures?  Thicker cases maybe?

#.45 cases are solid head and plenty strong. The 1911 is the limiting
#factor. It's delayed blowback design and lack of full cartridge head
#support limit the pressure it can tolerate. High pressure loads will
#not drop in pressure enough during the delay time when the slide and
#barrel are moving backwards together and it will unlock before pressure
#has dropped enough. Also the hotter load will cause stronger recoil
#and hammer the gun severely leading to eventual failure. You can shoot
#much hotter loads in a Contender where the case is fully supported and
#the action remains locked until you reload.

I'm no expert (Ed Harris and Gale Barrows are, though; what happened to
ya Ed, ya fall off the net?) but I'll critique your comments some:

The 45 case is pretty strong and will hold up to most abuse at least until
you "ramp" the entrance to the chamber so it will feed those good old
Hornady #4515s.  Then your case gets a little bulgey if you go too much
over nominal.  So next you have to get into cutting down 308 cases or some 
other similar foolishness.  ("pretty strong"?  What's this?  Lake Wobegone?)

The 1911 is _not_ a delayed blowback pistol.  It is a locked-breech pistol.
Delayed blowback means you've got some scheme for making a blowback pistol
delay its slide opening enough that the case doesn't blow out.  A "classical"
blowback or delayed blowback pistol will have no relative motion between
barrel and frame during the firing cycle.

Unless a 1911 is busted, the chamber pressure will have fallen very low by
the time the barrel/slide unlocks because the bullet has left the barrel by 
the time the barrel unlocks from the slide.  This is so because of basic
momentum transfer principles which are pretty hard to circumvent.

The following is "in my very limited experience":  I agree that hot loads
accelerate the usual failure modes of frame cracking and underlug shearing
and that sort of stuff.  But the REALLY hot loads wear out the barrel-groove/
slide-groove locking surfaces first.  They just sort of peen over on you and
pretty soon you've got no locking surfaces left - they're not 90 degrees to
the bore anymore.  At this time, you can say you've really screwed up. 

[ many questions in the area of internal ballistics deleted ]

I'm not at all strong in internal ballistics so I should leave it alone. 
(Why should I let _that_ stop me?)  8-)
But a few clues: When discussing fast and slow powders and resultant
velocities, you might think in terms of integral P(V)*dV.  In my experience,
if you compare the exit gas pressure of a load which contains a maximum
charge of fast powder to the exit pressure of another load which uses
a slower powder but has the same resultant muzzle velocity, you'll find
the two exit pressures are very similar.  At least that's what happened
when I did the internal ballistics testing with the 45 ACP cartridge,
results of which I reported here a couple or so weeks back.

I would guess that the amount of noise coming from the muzzle will be some-
what related to the stored energy of the gas which is in the barrel at the 
time the bullet exits.  The stored energy is:
 
U = [(P1*V1)/(k-1)]*[1-(P2/P1)^((k-1)/k)]

where k is Cp/Cv, P1 is the exit pressure, P2 is the ambient pressure
(14.696 psi), and V1 is the volume of the bore and chamber added up.
Only I can't remember the value of k for the products of combustion
of smokeless.  (It's hell getting old and I ain't even old yet.)  Anyone
remember it?  You can bet someone in rec.pyrotechnics does.

This might be controversial, but I would say that there's probably no
such thing as either an underbore or overbore cartridge.  The only reason
that we say that a 50 BMG necked down to a phonograph needle is overbore
is that there is currently no powder made which is slow enough to burn
in it. (Another problem with the 50 BMG/phono needle is that it's a little 
noisy.)  Underbore just means your efficiency of energy transfer from powder
to bullet is getting so darn good that you can't hardly stand it
anymore.  If the bullet starts to slow down in the barrel, the barrel's
too long.

I don't know how to size the case volume to the bore for maximum accuracy.
Dan Powlak did.  Mike Walker does (did? I hope he's still around).
I don't think many people _really_ know.

      JHBercovitz@lbl.gov    (John Bercovitz)