Getting the scope for the Varmint rifle as soon as you start the game can help a lot, but it only randomly appears in Chet's inventory (in Goodsprings). If he doesn't have it on your first try, you have to Wait 24 in-game hours three times for his stock to reset.

Yesterday (in 2022), I was trying a Trait load I hadn't used before and after leaving Doc's and going to the store, got the scope and silencer on the first try.

Today, I started a new game but when I went to the store, Chet wouldn't have mods for the rifle or would have the silencer (a lot of the time) or sometimes the mod that gives you 3 more rounds in the magazine (big whup!). It took about an hour to get the scope by waiting 72 hours in-game between tries..

On the plus side, while trying to get it, I was able to get the Silencer and Magazine mods, although they are not nearly as important. I also buy his 9mm ammo because I'll be using it for a long time. On the first check with Chet, he will have a Salesman Weekly to get, but he never gets more mags after that.

The reason that I am so hung up on getting the scope is that you can get enemies from so far away that they can’t even tell where you are shooting from.

The Walkthrough explains when and how to run up to New Vegas where you can get a sniper rifle in a locked room in a destroyed building near the Molerat Farm (around the corner north of Crimson Caravan).

In this image, the cursor is pointing at the entrance to the building. The marker to the left is the Molerat Farm and southwest of that is Crimson Caravan.

Once you get a sniper rifle, you can sell off the Varmint rifle.

This sniper rifle requires a hard lockpick (75), which you should be able to get if you follow the walkthrough here. If not, after finishing what I came to New Vegas for, I normally go to McCarren next where you can find a sniper rifle on the top bunk in the main tent you are going to, as well as a couple of magazines on the floor.