The least expensive rounds can vary depending on several factors such as the caliber, brand, and quantity purchased. However, in general, the most affordable ammunition tends to be target or practice ammunition, often referred to as "range ammo." These rounds are typically produced in bulk and designed for recreational shooting, training, or practice purposes.

Common calibers like 9mm, .223 Remington/5.56 NATO, and .22 LR (Long Rifle) often have a wide availability of affordable range ammo options. These rounds are frequently produced by various manufacturers and are sold in larger quantities, which can help bring down the cost per round.

It's important to note that while range ammo is typically more affordable, it may not possess the same performance characteristics as premium self-defense or specialty ammunition. If you are using ammunition for self-defense or specific applications, it is crucial to select appropriate ammunition that meets your needs and is reliable.

Additionally, ammunition prices can fluctuate based on factors such as market demand, availability, and global events, so it's always a good idea to check with local retailers or online suppliers to get the most accurate and up-to-date pricing information.