Which gun was used as the murder weapon and where was it from?

The shotgun, which is the murder weapon described at the beginning of the story, is an important symbol. Its most immediate purpose is obvious: as Holmes points out, it is most definitely an American weapon. The sawed-off shotgun is used by Doyle to clearly underline the American connection. The weapon is also symbolic of the Scowrers themselves, however. It is everything the Scowrers are: cold, brutal, and extremely deadly. As Holmes points out, the triggers are wired together, ensuring both barrels discharge at once, maximizing the impact.

The shotgun embodies the ironic paradox of the Scowrers as well. It is sawn off so it can be carried under a coat—in other words, concealed. However, the loud discharge of the weapon would make a stealthy crime impossible. This is much like the Scowrers: a “secret” society that everyone knows about.