Penetration

In GURPS one inch of rolled homogeneous armor (RHA) is DR 70.

You can reverse engineer a weapon from this value by assuming each die rolls the exact average roll of 3.5,

You can also calculate the penetration of a given round by "rolling" 3.5 on all the dice and dividing the result by 70.  You also have to multiply the damage by the armor divisor, if any.

For example: The APFSDSDU 6dx33(2) pi++ inc of the T-72's main gun will penetrate 19.8" (503mm) of RHA.

Not enough to punch the turret front of an M1A1(HA) and its 600-680mm RHAe.

The 125mm HEAT (6dx7(10) cr ex linked 6d cr ex) round should penetrate 21" (533.4mm) and still not enough for the 1,080-1,320mm of RHAe.

Composite armor has different effective RHA equivalences depending on if it's a KE or ChemE round.

Our 75mm XM885 APFSDSDU round does 6dx19(2) pi++ inc.  That should punch 11.4" (290mm) of RHA which should NOT penetrate the 380mm RHAe turret front on the T-72.  The T-72B1 and T-72B2 have thicker armor here.  It's buzzing right through the 65-80mm of the sides and back.

The LAV-75 vs a T-72 or better would be looking to implement Sherman v Tiger II tactics.  Speed, accuracy and small silhouette are it's advantages.

But the LAV-75 can punch the frontal armor of the typical T-55.  It can punch the glacis but not the turret front of a T-62 regularly (but a max dice roll CAN penetrate the turret front.