It depends on the type of massage you're after. Big round head attachments are typically used on larger muscle groups like the quadriceps, hamstrings, and glutes. Its larger surface area makes it more efficient on these larger targets. Meanwhile, a small round head attachment is a great all-around pick that works well across the body. Some massage guns come with a flat head attachment, which works better on smaller surface area targets, such as between the shoulder blades and along the length of the IT bands. A flat head is particularly helpful on denser muscles. The bullet-shaped attachment is ideal for targeting smaller areas of muscle soreness like your hands and feet. Just be mindful of the pressure applied with the bullet attachment because it's quite concentrated to a small surface area, and be extra careful to avoid bone, tendons, and ligaments. Lastly, the two-prong attachment that looks like a vertebra helps bring relief to trigger points along the spine, especially toward the lower back. Again, be careful to never press the attachment itself against the spine.