You know, I've never heard that FMJ deforms or vaporizes on a target. Therefore, hardcast can't penetrate deeper than a standard copper FMJ as the density didn't change the shape...
Standard pressure Buffalo Bore HC-FN are 100-gr bullets; they offer standard pressure FMJ-FN with 95-gr bullets. Thus, the heavier, more dense HC bullets have a greater sectional density.
In Charles Schwartz, 2012,
Quantitative Ammunition Selection, iUniverse, Bloomington, IN, bullet penetration into the human body is well modeled as follows:
Xₜ = Dₛ × Vᵃ
where:
• Xₜ is the bullet terminal penetration depth in inches;
• Dₛ is the bullet sectional density in pounds per square inch;
• V is the bullet impact velocity in feet per second; and,
• a is a unitless bullet form factor.
The respective sectional densities for the HC-FN and FMJ-FN are 0.144 and 0.137 lb/in². Their respective velocities out of a 2.75-in barrel are 910 and 888 ft/s (the HC bullet has less friction with the barrel; thus, it is faster). The form factor for the two bullets is identical at 0.735 (that for a truncated cone).
Crunching the numbers, the predicted penetration for the HC-FN is 21.6 in, and that for the FMJ-FN is 20.1 in. Being heavier and faster the HC-FN should penetrate about 7.2% more than the FMJ-FN, but both have far more penetration potential than is needed to overpenetrate the human body, even in many suboptimal sight pictures. I see no need for premium ammo to assure great penetration when typical FMJ-RN "practice" ammo will also be likely to overpenetrate. But, if penetration is what you're after, HC bullets are one way to get it.