But, as a matter of fact, any barrel that has been through
that 'proof-room' will have been subjected to the greatest strain it
will ever have to undergo, for there is no cartridge made that would
have one-half the power in proportion to the size of the barrel."

From the proof-room Hamilton's guide led him through different parts of
the works, where various machines were employed in preparing and
finishing the rough forgings he had seen made and annealed. Thus, for
example, in a receiver for a gun stock, one machine worked a bevel edge
on it, another bored it to the size of the gun barrel, accurate to the
thousandth part of an inch, another pierced the tiny screw holes, and
yet other machines made even the minute screw, done, as was explained to
Hamilton, so that the threads in each should fit with absolute
exactness.

"But do you really mean to say," queried Hamilton in surprise, "that
every one of these fifty or more parts of each gun is inspected and
tested?"

The official led him to a number of long rows of tables.

"Here," he said, "are girls doing nothing else all day long. Here is a
testing die for a part of the ejector of one of our 1911 models. You see
that there are two spaces for all of them. It must fit into this one, it
must not fit into that, which is a thousandth of an inch smaller. If too
big, you see it won't fit into either, if too small, it would fit into
the one where it ought not. Every tiny piece is gauged on all its sides
and in every hole and at all points with this double gauge system."

"That doesn't leave much for guesswork," said Hamilton. "But there is
something that's been puzzling me."

"What is that?" asked his guide.

"I've always heard a lot about gun-metal," Hamilton answered, "and yet
all the way through, these parts have been nothing but steel. And all
the guns I ever saw had that bluish look, as gun-metal has. For example,
my watch is what they call gun-metal," and he took it from his pocket
and showed the back of it.

"Gun-metal," said the other, "is an alloy of copper and tin and once was
used almost exclusively for cannon and big guns generally. But you're
right about all guns having a bluish tinge. That is all steel, but it is
treated by a process called coloring or bluing. I'll show you--both the
old way and the new."

Going down the stairs and crossing the yard, he took Hamilton into a
small building where there were a couple of open charcoal furnaces, in
which th