There are two models of the Byrna, the Byrna SD and the Byrna SD XL.
The Byrna SD (Self Defense) is smaller, lighter, and uses a proprietary 8 gram (g) CO2 (compressed air) cartridge. It can be converted to a Byrna SD XL through addition of a "Byrna Boost" add-on kit.
The Byrna SD XL is slightly larger as it has a longer "barrel" and uses a standard 12g CO2 cartridge, available at many sporting goods stores. Due to the larger 12g CO2 cartridge, the SD XL shoots projectiles at a faster speed and can fire more projectiles at full speed due to the larger CO2 cartridge..
Both the SD and the SD XL fire the same projectiles, both have a capacity of 5 "rounds" (projectiles), and use the same magazines, which are interchangeable.
The Byrna SD XL is available in Black and Orange, while the Byrna SD is available in multiple colors and also has an available "Byrna wrap" to further customize your Byrna.