In My Brother Sam Is Dead, Tim’s dad, Mr. Eliphalet Meeker, doesn’t want to buy another rifle, because he needs only one, and he doesn’t want Sam to take it in the first place: he doesn't want either of his sons to own a gun. Although he uses a gun himself to hunt deer, as well as to protect himself against robbers while traveling, he sees guns as dangerous instruments of rebellion, and he's against rebellion: he's a Loyalist. Here's how he explains it:
I will not have subversion, I will not have treason in my house. We are Englishmen, we are subjects of the King, this rebellion is the talk of madmen.
Further, Tim and Sam's dad also knows that participating in war brings only heartache and loss. He himself lost his best friend while they were fighting in Louisbourg, in the French and Indian War.
Sam, of course, steals his father's gun, the Brown Bess, in order to join up with the rebellion and fight the "Lobsterbacks." In response, why doesn’t Mr. Meeker just buy another? He can certainly afford it, after all. But that’s not what he cares about: he cares about preventing Sam from taking the family’s gun in the first place and from running off to fight and possibly get wounded or killed in a war that should not, in Mr. Meeker's opinion, even take place. Regardless, the rebels have been disarming Tory families like the Meekers, and so if Mr. Meeker were to purchase another rifle, it would get confiscated anyway.