I don't think there really is any hard and fast rule about what
hardness of lead alloy you should use. You have a couple of
variables to play with, hardness is just one. Bullet sizing is
another. I think it depends on the task at hand.
For semi-autos, I keep the alloy as hard as possible because
the rifling on most semi autos is shallow, designed to keep
friction low for jacketed bullets. The harder the alloy, the
more likely it is to grip the rifling. If the lead is soft, high
velocity semi autos may accelerate the bullet at a speed
beyond which the lead can grip the rifling, effectively
stripping the bullet. That will lead your barrel very quickly.
A bullet of Linotype is also likely to survive the journey from
the magazine, up the feed ramp and into the chamber
without deforming. Linotype is the most cost effective alloy
for hard bullets, so I use it.
However, the original question was about revolver bullets.
When casting for revolvers you need to know the dimension
of the cylinder throats, the max diameter of the barrel down
into the groves and to a lesser extent, the speed of the
round. The real objective is to make sure the bullet
effectively seals the barrel as the propellant gas pushes out
of the muzzle.
Let me give an example. I have a revolver whose cylinder
throats measure .450" on all chambers. The barrel measures
.455" down into the groves (a clue to its make). As the round
is fired, the bullet is 'resized' by the cylinder throat to .450"
as it makes the jump across the cylinder/barrel gap. If the
bullet is hard cast, at best it will rattle down the barrel barely
engaging the rifling and display poor accuracy. At worst, it
will still show bad accuracy AND lead the barrel badly. Apart
from lead fragments around a forcing cone, barrel leading can
be caused by hot propellant gas pushing past the bullet,
blowing molten lead ahead of the bullet which, as it cools, is
then smeared onto the barrel by the bullet as it passes by.
I have heard this described as ‘cutting’. For this gun to shoot
well and accurately, a bullet with a hollow base cast from soft
lead is required. The propellant gas expands the soft lead
bullet ‘skirt’ to seal the bore and grip the rifling. The lower
the gas pressure, the softer the lead should be. A hard cast,
hollow base bullet would act the same as a flat base as the
pressure wouldn’t expand it. This is the original ‘Mine ball’
concept.
The Webley is an extreme example, but many S&W 45 Colt
revolvers have cylinder throats of .452” and barrels of .454”.
This is why so many cast bullets for 45 Colt are often sized
to .454" but jacketed bullets are .452".
For those guns, a soft bullet, sized to the diameter of the
cylinder throat, is likely to bump up (or obdurate) and seal
the bore. If you get leading in those guns, the answer is
likely to be softer lead alloy, not harder. With such revolvers
a .452” bullet will grip the rifling in a .454” barrel. Accuracy
won’t be an issue until the leading makes it so.
If you are lucky enough to have a revolver whose cylinder
throats match the largest barrel dimension or (even better)
are about one thou larger than the barrel, then size the cast
bullets to the same diameter as the cylinder throats. My
Ruger GP100 is a good example. Now, the hardness of the
lead matters a little. My rule of thumb is that if the muzzle
velocity is more than 1000 fps, then the harder the lead the
better. I use 20:1 for below 1000 fps, Linotype for above.
That is, unless you are using cast bullets with gas checks.
Then, I always use hard alloy as soft lead doesn’t give the
‘crimp’ on the gas check anything solid to bite into.
Then, there are revolvers that fire rounds designed for
semi-autos. A S&W 625 in 45 ACP is an example. The barrel
is likely to be shallow rifled (to prevent a jacketed bullet from
sticking in the forcing cone and letting the propellant gas
blow out of the cylinder/barrel gap) but the same mismatch
between cylinder throats and barrel dimensions is possible.
Measure them and use hard cast bullets if the throats are
bigger than the barrel. If not, it’s likely the gun won’t shoot
too well and you will need to experiment to find the right
alloy/size/velocity combination.
Cast bullets for lever actions are another story, but in
general, hard alloy sized at one thou over the maximum
barrel dimension is a reasonable guide. I prefer gas check
cast bullets in lever actions.
But anyway, if you have got this far, you are likely to think I am rambling.
Steve.
