[1] A bullet or pellet made of soft material and often with a concave base will flare under the heat and pressure of firing, filling the bore and engaging the barrel's rifling.
[2] The difficulty with leakage was one of the major obstacles to the early adoption of the breech-loading firearm, as it lowered pressures (and hence velocity), and also created danger or irritant to the shooter.
To allow obturation in airguns, pellets use soft alloys or flexible polymers[4] in conjunction with a thin, concave base designed to expand more easily.
Some airgun projectiles are composed of materials harder than the soft alloys traditionally relied upon; these instead commonly make use of elastomer rings capable of providing sufficient obturation under pressure.
To prevent excessive deformation in high pressure rifle and magnum pistol cartridges, lead bullets are often covered in copper or another harder alloy.
The same principles apply to artillery ammunition; guns are traditionally categorized (in English-speaking countries) as "BL" or "QF" (for "Breechloading" or "Quick Firing").