Nails are supposed to separate the powder charge from the projectile. Referring to the topic a bit more extensively, in the case of a black-powder weapon, the buttstock will be used to nail down the powder (as the name itself indicates, of course) - in a breech-loaded weapon, it will be pressed into the barrel after a measured amount of powder has been poured into it. By compressing the charge with the nailed-on one by pressing it a few times, the gunpowder burns better, which in turn translates into giving the bullet more velocity.
It is not only the better combustion of gunpowder that argues in favor of nails, as their use has many more advantages. The combustion of the propelling charge in the barrel, or in this case black powder, involves the product of this combustion, which is gunpowder gases. Their temperature is very high, so separating the lead bullet from these hot gases carries an important advantage, namely, it protects the bullet from over-melting and thus deposition of lead on the barrel walls.
Bullet nails are, as a rule, cork or felt. It is certainly worth getting them if the user of a black-pistol weapon cares about avoiding over-melting of bullets and better combustion of gunpowder, since the nails effectively insulate and seal. As a rule, their diameter is equal to the caliber of the barrel. It is convenient to stock a larger quantity of them.
