Pistol, Revolver
noun
A short gun is a pistol, because it's not long like a rifle, it's a short gun!
たんじゅう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
しばらく時間はかかったが、ようやく犯行の凶器となった短銃を見つけることができました。
After a long time we eventually succeeded in finding the pistol that was used for the crime.