I know the formula for standard/expected obstacle penetration and the way it goes, excluding bullet composition, is:
Mass x Velocity Multiplier
divided by (Density of Target) x Diameter x Diameter
(the velocity multiplier takes into account the velocity, density of the bullet material, tensile strength of the bullet, and the tensile strength of the target material which is a divisor – this is all taken by a Log function so the results are not linear or easy to predict)
(to make that long story short, increasing velocity doesnt increase penetration at a linear rate)
So penetration will be more greatly aided by making the bullet heavier and reducing its velocity. And making it so that it does not deform or fragment (which would increase its diameter on impact).
Making the bullet thinner increases its penetration drastically. But when you reduce caliber size the length of the bullet is normally reduced too, which reduces its mass drastically too, and that makes up for any reduction in diameter.
The 9mm VSSK bullet is an example of a low velocity round that has an immense amount of penetration. Even though it only has about as much firepower as a .45 or .357 maybe, it has as much penetration as an AK-47. Thats because its an armor piercing bullet that weighs 250 grains and is only 9mm wide.
For a subsonic round its also got an effective range of 300-400 yards, because its heavy, and because alot of that weight is in the back as a boat tail.
Thats why a Shotgun wont penetrate like a .308 even though it has just as much firepower. It does get an edge on property destruction because of the damage it causes, but against an armor plate or even a vest its not going to penetrate. It spreads that energy out over a huge amount of area (18.5mm width of projectile vs 8mm width for a .308).