Obturate means to block or obstruct.
In conventional English usage, obturation refers specifically to the condition of being obstructed or occluded or the action of blocking, stopping, or filling something up. The mechanism by which an undersized soft metal projectile enlarges to fill the barrel is, for hollow-base bullets, due to expansion from gas pressure within the base cavity and, for solid-base bullets, upsetting - the combined shortening and thickening that occurs when a malleable metal object (e.g. a rivet) is struck forcibly at one end.
The formula used to calculate the pressure required for solid base bullets is:
Bullet's BHN x 1422 = Pounds per square inch
The constant 1422 is an empirically determined factor used throughout the ammunition and reloading industry; it is specifically tailored for calculating the pressure required for expansions that occur in internal ballistics and should not be used for other purposes.
Below is a chart containing various bullet alloys, the BHN, and the PSI required to expand a bullet to the bore:
Material...........BHN..........Pressure (psi)
Pure lead............5...........7,110
1:20 tin/lead.......10..........14,200
1:10 tin/lead.......11.5........16,400
Pure copper.........40..........56,900