"bitcoin shells" makes sense since large guns are often referred to as "artillery" and their ammunition is called "shells", only little ammunition is called "bullets" which is derived from the word "ball" i think

ah, yes, "shell" means the whole shebang, like, technically, the lead ball/cone of a "bullet" is the bullet and the assembly with the propellant and casing is a "round" and "shell" refers to a projectile that has explosive inside it