From: njohnson@nosc.mil (Norman F. Johnson)
Newsgroups: rec.guns
Subject: ruger chambering dimensions
Date: 25 Jun 1996 17:06:02 -0400

# I have slaved for 3 years for a good recipe for a ruger SRH and i 
# am  at a loss!  my pet jacketed load cuts 1.065 of an inch at 75 ft and 
# that is a 12 shot group!  wow or so i said!  but alas i cannot find an 
# idea of a cast load! many moulds  many different lubes cases powders 
# primers many different alloy mixes and sized at .429 .430  but my chamber 
# mouths are on the outside of tolerences and thats why i believe my ruger 
# seems to love hornady at .430 dia.! 

You are on the right path here but simply have not taken it far 
enough.  Cast bullet revolver accuracy is more dependent on 
throat-bullet fit than any other factor, assuming other good 
loading practices.  The reason is that undersized lead bullets 
are subject to gas cutting, which will positively ruin their 
accuracy potential.  Most of the gas cutting takes place while 
the bullet is in the throat.  To give just one example of how 
good fit of bullet to throats pays off, my Redhawk went from a 
best of 2" at 25 yards to a constant (Ransom Rest) average of 
3/4" with a simple bullet diameter change.  Throats in this gun 
are .4345" and I use .434" cast/swaged lead bullets in it.  

Try this: 

Take a SOFT lead bullet and squeeze it gently in a vice until it 
is about .010" oversize (say about .436-.440" for a .44 cal. car-
tridge).  Attempt to push this slug through each throat of your 
cylinder using hand pressure only. If it will not pass through, 
your slug is large enough.  If it will pass you must increase the 
diameter slightly.

Before doing the following, remove the cylinder from the gun, 
clean it of ALL lead or copper fouling, lube it lightly and make 
sure that it is solidly supported on an appropriate wooden block.

With an appropriately sized brass drift, carefully drive the slug 
through each throat from the end of the chamber where the 
cartridge is normally inserted.  What you are attempting here is 
to find the diameter of the smallest throat, so handle the soft 
lead carefully and drive it straight to avoid distorting the 
slug. This may take a couple of practice runs because the 
transition between the area of the chamber where the brass case 
resides and the area into which the bullet projects (throat) 
varies from cylinder to cylinder.  Some cylinders have a sharp 
step while others have a relatively long smooth transition.  If 
the gun that you are working with has a sharp step it may be 
easier to minimize the distortion by driving it from the other 
end of the cylinder.

After you have completed the above, you have the "ideal" bullet 
diameter to be fired in your gun and you will be very fortunate 
if the bullet is the nominal diameter for that caliber.  Usually 
you will find that it is somewhat larger.  For the time being 
don't worry about the difference in the throats in a particular 
cylinder.  I find that even though they are sometimes badly 
oversize, they usually do not vary much more than about .0005".

An exception that would negate this approach would be if you 
encountered a barrel that slugs larger than your throats.  The 
only solution to this unfortunate situation is to either open the 
throats to .001" or .002" larger than the bore or to get a new 
gun.  Fortunately, this occurrence is rare (I haven't yet run into 
the problem, but have read of others who have).  

Another exception is the condition where the oversize bullet is 
so large that when it is loaded into the brass, the cartridge 
cannot be chambered.  In cases where this condition is just 
marginal, a taper crimp will solve the problem.  Where it is more 
extreme, the chamber (but NOT throat) will need to be enlarged to 
accommodate the cartridge, although if the gun is new, return it 
to the manufacturer.

Remember that when shooting cast bullets, the size of the 
expander plug is very important, and if one uses a plug of the 
standard sizes normally supplied by the manufacturer, the 
oversize bullet will be sized down when it is seated in the brass 
case thus ruining all of the above effort.  Your expander plug 
for cast bullets should be .001 to .002 smaller than your new 
bullet diameter.  

The way that I accomplish this is to keep a supply of next-
caliber-up Lyman expander plugs, then when I need a particular 
size, I con a machinist friend to turn one to the proper 
diameter.  The Lyman is a very good design because it has the 
step that makes for an easy way to start the bullet into the case 
with the bullet axis concentric with the case axis, a VERY 
important condition in pistol and rifle ammunition.

Still another problem, although not a cylinder problem, is one 
that few shooters know exists and is directly related.  In most 
revolvers, the area of the bore directly adjacent to the forcing 
cone where the barrel is screwed into the frame is sometimes 
smaller than the rest of the bore. This damages the bullet much 
the same as an undersized throat and, unfortunately, is a very 
common occurrence.  

The solution is to lap the bore with a cast bore lap to remove 
the constriction.  That is not the easiest job for the 
inexperienced.  An alternative is to shoot several hundred rounds 
(200-500) depending on the amount of constriction, charged with a 
proper lapping compound.  This has the added advantages of 
polishing the rest of the bore and providing a very slight taper 
to the bore for excellent cast bullet shooting and will do a 
great job of minimizing leading.  It is important that a PROPER 
lapping compound be used.  Again, Veral Smith of Cast Bullets 
Technology has the proper stuff for about $10.  

You will find, that with properly fit bullets, alloy hardness 
will not be nearly as critical from low velocity target loads to 
the highest velocities that your Super Redhawk will handle.  

Custom molds to fit your gun are available from LBT for little 
more than commercial ones.  

God Bless!

Norm