I have played around with milk jugs and believe penetration has to do with velocity and the construction of the bullet. Duh, but was fun doing.
A 158gr. Lead wad cutter from a 38 would go thru half a dozen milk jugs then on into several inches of newspaper. The slug would be complete and the jugs only had a hole thru them.
A 52 gr. hollow point from a .223 at 3000+ fps would not penetrate the second jug. All you could find of the bullet was tiny fragments of copper and maybe a tiny piece of lead. The first jug literally exploded, the outside of the second was broken from the concussing of the firsts violent bursting.
My conclusion after several bullets and calibers is Speed and/or bullet construction about everything.
I use 125 gr. hollow points in a 357 trying to strike a happy medium between penetration and expansion in a self defense bullet, from a under 3 inch barreled revolver