Not all Walmarts sell guns and ammunition. I live in Louisiana and none of the 4 Walmarts in my town have guns.

However, there is a mom-and-pop (locally owned) grocery store in a small town nearby that is selling one of my favorite rifles (Springfield M1A) along with AR-15s, shotguns, and bolt-action rifles.

No one in their right mind would attempt a purchase a firearm with their groceries. It's time consuming, for one. There are plenty of gun stores in my city. Someone would usually go during the weekend or on their day off. They're not that cheap to buy, you want to look around at the ones in your budget.

Criminals will get a gun from another criminal, if you're wondering. Anyone with a felony is not legally allowed to have them and a legitimate gun store (most of them are) will follow the law to the T, otherwise they could face very stiff penalties (prison time) and fines. Felons can apply for their voting and gun rights after a certain amount of time has passed since they were last convicted. It's up to the state to approve. Felons can also seek a pardon from the governor of the state in which they were convicted or from the President of the US. In some states, you can just ask the sheriff of the town you live in, which the guns cannot leave that town, and you have to notify the sheriff of everything you're planning to do with them, e.g. let them know you're taking them to a gun range. It's up to that town's sheriff to approve.