"Light" Armor

The Leopard 1 came out at a time when it was thought that HEAT would reign supreme and putting enough armor on to stop such a round was going to be too ungainly for combat because it sacrificed too much mobility.

So the Leo gets "thin" frontal armor.

It translates to DR 385 on the front hull and 358 on the turret front.

To put that in some perspective, a WW2 Tiger E has 400 on the hull and 425 on the turret.  The Panther G has 540 and 600.

An M1A1(HA) has DR 2,000L and DR 2,400L on the front facings.

The L means laminate and that gives double DR vs HEAT and HEDP and cancels the special effects of HESH.  Effectively that changes the armor divisor from 10 to 5.

The Leo's 105mm gun can fire a heat round that, on average, will punch right through 980 points of DR!  The 125mm gun on a T-72 can, likewise, punch 1,470 (or 735 points of laminated armor).

Making the armor 5 times thicker would have made the Leopard 5 times heavier.

A 200 ton tank would not have worked in the late 1950's in Europe.

And then there's high-explosive squash head (HESH) aka high-explosive plastic (HEP) here across the pond.

The 105mm does 6dx16(0.5) pi++ with a linked explosion doing 6dx3 cr ex.

On average the projectile will punch DR 168 with the explosion getting past DR 108.  You'd think the Leo would be safe from that, right?

HESH is designed to cause spalling.  It knocks a hunk of the armor off inside the target to act as a projectile.

That does 1/10 the maximum damage of the HESH's explosion vs 1/100 of the target's DR.  So 11 points against 4 on the Leopard.  While 6 points of cutting damage doesn't seem like a lot, that's what everyone and everything in the turret gets.

You just can't make the armor thick enough when the average hit will punch 1,000 DR!

Laminate armor eliminates this spalling effect.

That leads to the return of kinetic energy to punch holes in tanks and things like the M1A1(HA)'s insanely protective frontal armor.

The T-72's APFSDSDU round does 6dx33(2) pi++ inc.  That will punch 1,386 DR with the average die roll.  That's going to go all the way through a Leopard 1 and (probably) get it burning.