Gun
Firearm
noun
The kanji and the word are exactly the same. That means they share meanings as well.
The reading is the one you learned with the kanji.
この銃、弾入ってる？
Does this gun have bullets in it?
こちらの銃は、クレジットカードで買えますか。
Can I buy this firearm with a credit card?
外出中に、部屋に置いてあった銃が母親に見つかってしまったんです。
My mother found my gun in my room while I was out.